D.M.J. Peeters
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1
Composite casings for solid rocket motors offer significant mass savings over metallic alternatives, but their manufacture using Out-Of-Autoclave (OOA) filament winding processes remains significantly less mature than autoclave-based methods. This thesis investigates how the process parameters involved in the winding and curing operations affect the quality and hoop-direction mechanical response of OOA carbon-towpreg overwraps co-cured onto paper–phenolic liners.
The experimental campaign ran in three tranches: preliminary microscopy specimens, a broad screening tranche, and a directed low/medium/high-tension by tape/no-tape study informed by the results of the screening tranche. Split-D testing was completed for 49 coupons from Tranches 2 and 3, and optical microscopy on fifteen sections supplied the cross-sectional area, the detected void area fraction, the void-size distribution, and a descriptive interface metric.
Maximum force was treated as the primary terminal-capacity endpoint and proved comparatively stable across configurations. Nominal stress was treated only as its fixed rescaling. The stress per unit of measured cross-section varied materially because the cross-section itself varied between configurations, though no statistically significant relationship between processing and cross-sectional area could be resolved. Functional shrink tape at high tension reduced the detected optical void area fraction from approximately 12 % to 9 %, the most consistent processing signal observed in this project. It also produced a significantly improved interface appearance compared to the same configuration without tape, though it was found that this did not produce a statistically resolved improvement in ultimate force. The tape may also have slightly reduced the threshold for first damage to occur. Higher recorded tension was associated with a higher apparent system stiffness but a matching change in the intrinsic modulus of the overwrap was only suggestive. First detected force discontinuities were more sensitive to configuration than terminal force, which indicates that the onset of detectable damage responds to processing choices more strongly than the final load capacity does.
Early testing identified an issue with liner blistering. Drying a full-length phenolic liner removed 0.59 % of its initial mass, consistent with a removable volatile. However, the treatment did not prevent blistering in the directed tranche, indicating that an alternative material or a modified cure cycle may be required to avoid this defect.
The work identifies a promising process window and establishes which measurements are sensitive enough to detect manufacturing effects, but it does not establish robust statistical confidence in the observed trends. The results are therefore suitable for informing the next stage of development, but not for establishing design allowables or production readiness. The detected void area fractions of 9 % to 12 % are significantly higher than would typically be achieved with a standard autoclave processing technique, but it is expected that with the use of better manufacturing processes and equipment it may be possible to reduce the void content to acceptable levels for single-use rocket motors for unmanned systems, though not for human-rated systems or high-cycle fatigue environments. ...
The experimental campaign ran in three tranches: preliminary microscopy specimens, a broad screening tranche, and a directed low/medium/high-tension by tape/no-tape study informed by the results of the screening tranche. Split-D testing was completed for 49 coupons from Tranches 2 and 3, and optical microscopy on fifteen sections supplied the cross-sectional area, the detected void area fraction, the void-size distribution, and a descriptive interface metric.
Maximum force was treated as the primary terminal-capacity endpoint and proved comparatively stable across configurations. Nominal stress was treated only as its fixed rescaling. The stress per unit of measured cross-section varied materially because the cross-section itself varied between configurations, though no statistically significant relationship between processing and cross-sectional area could be resolved. Functional shrink tape at high tension reduced the detected optical void area fraction from approximately 12 % to 9 %, the most consistent processing signal observed in this project. It also produced a significantly improved interface appearance compared to the same configuration without tape, though it was found that this did not produce a statistically resolved improvement in ultimate force. The tape may also have slightly reduced the threshold for first damage to occur. Higher recorded tension was associated with a higher apparent system stiffness but a matching change in the intrinsic modulus of the overwrap was only suggestive. First detected force discontinuities were more sensitive to configuration than terminal force, which indicates that the onset of detectable damage responds to processing choices more strongly than the final load capacity does.
Early testing identified an issue with liner blistering. Drying a full-length phenolic liner removed 0.59 % of its initial mass, consistent with a removable volatile. However, the treatment did not prevent blistering in the directed tranche, indicating that an alternative material or a modified cure cycle may be required to avoid this defect.
The work identifies a promising process window and establishes which measurements are sensitive enough to detect manufacturing effects, but it does not establish robust statistical confidence in the observed trends. The results are therefore suitable for informing the next stage of development, but not for establishing design allowables or production readiness. The detected void area fractions of 9 % to 12 % are significantly higher than would typically be achieved with a standard autoclave processing technique, but it is expected that with the use of better manufacturing processes and equipment it may be possible to reduce the void content to acceptable levels for single-use rocket motors for unmanned systems, though not for human-rated systems or high-cycle fatigue environments. ...
Composite casings for solid rocket motors offer significant mass savings over metallic alternatives, but their manufacture using Out-Of-Autoclave (OOA) filament winding processes remains significantly less mature than autoclave-based methods. This thesis investigates how the process parameters involved in the winding and curing operations affect the quality and hoop-direction mechanical response of OOA carbon-towpreg overwraps co-cured onto paper–phenolic liners.
The experimental campaign ran in three tranches: preliminary microscopy specimens, a broad screening tranche, and a directed low/medium/high-tension by tape/no-tape study informed by the results of the screening tranche. Split-D testing was completed for 49 coupons from Tranches 2 and 3, and optical microscopy on fifteen sections supplied the cross-sectional area, the detected void area fraction, the void-size distribution, and a descriptive interface metric.
Maximum force was treated as the primary terminal-capacity endpoint and proved comparatively stable across configurations. Nominal stress was treated only as its fixed rescaling. The stress per unit of measured cross-section varied materially because the cross-section itself varied between configurations, though no statistically significant relationship between processing and cross-sectional area could be resolved. Functional shrink tape at high tension reduced the detected optical void area fraction from approximately 12 % to 9 %, the most consistent processing signal observed in this project. It also produced a significantly improved interface appearance compared to the same configuration without tape, though it was found that this did not produce a statistically resolved improvement in ultimate force. The tape may also have slightly reduced the threshold for first damage to occur. Higher recorded tension was associated with a higher apparent system stiffness but a matching change in the intrinsic modulus of the overwrap was only suggestive. First detected force discontinuities were more sensitive to configuration than terminal force, which indicates that the onset of detectable damage responds to processing choices more strongly than the final load capacity does.
Early testing identified an issue with liner blistering. Drying a full-length phenolic liner removed 0.59 % of its initial mass, consistent with a removable volatile. However, the treatment did not prevent blistering in the directed tranche, indicating that an alternative material or a modified cure cycle may be required to avoid this defect.
The work identifies a promising process window and establishes which measurements are sensitive enough to detect manufacturing effects, but it does not establish robust statistical confidence in the observed trends. The results are therefore suitable for informing the next stage of development, but not for establishing design allowables or production readiness. The detected void area fractions of 9 % to 12 % are significantly higher than would typically be achieved with a standard autoclave processing technique, but it is expected that with the use of better manufacturing processes and equipment it may be possible to reduce the void content to acceptable levels for single-use rocket motors for unmanned systems, though not for human-rated systems or high-cycle fatigue environments.
The experimental campaign ran in three tranches: preliminary microscopy specimens, a broad screening tranche, and a directed low/medium/high-tension by tape/no-tape study informed by the results of the screening tranche. Split-D testing was completed for 49 coupons from Tranches 2 and 3, and optical microscopy on fifteen sections supplied the cross-sectional area, the detected void area fraction, the void-size distribution, and a descriptive interface metric.
Maximum force was treated as the primary terminal-capacity endpoint and proved comparatively stable across configurations. Nominal stress was treated only as its fixed rescaling. The stress per unit of measured cross-section varied materially because the cross-section itself varied between configurations, though no statistically significant relationship between processing and cross-sectional area could be resolved. Functional shrink tape at high tension reduced the detected optical void area fraction from approximately 12 % to 9 %, the most consistent processing signal observed in this project. It also produced a significantly improved interface appearance compared to the same configuration without tape, though it was found that this did not produce a statistically resolved improvement in ultimate force. The tape may also have slightly reduced the threshold for first damage to occur. Higher recorded tension was associated with a higher apparent system stiffness but a matching change in the intrinsic modulus of the overwrap was only suggestive. First detected force discontinuities were more sensitive to configuration than terminal force, which indicates that the onset of detectable damage responds to processing choices more strongly than the final load capacity does.
Early testing identified an issue with liner blistering. Drying a full-length phenolic liner removed 0.59 % of its initial mass, consistent with a removable volatile. However, the treatment did not prevent blistering in the directed tranche, indicating that an alternative material or a modified cure cycle may be required to avoid this defect.
The work identifies a promising process window and establishes which measurements are sensitive enough to detect manufacturing effects, but it does not establish robust statistical confidence in the observed trends. The results are therefore suitable for informing the next stage of development, but not for establishing design allowables or production readiness. The detected void area fractions of 9 % to 12 % are significantly higher than would typically be achieved with a standard autoclave processing technique, but it is expected that with the use of better manufacturing processes and equipment it may be possible to reduce the void content to acceptable levels for single-use rocket motors for unmanned systems, though not for human-rated systems or high-cycle fatigue environments.
Automated Fiber Placement (AFP) enables the efficient manufacture of high-performance composite structures but inevitably introduces tow-drop gaps that create resin-rich regions, fibre waviness, and local strain concentrations, resulting in structural knockdown. This research investigates the feasibility of mitigating these effects by co-curing PEI into tow-drop gaps of 8552/AS4 carbon/epoxy laminates using two concepts: a continuous PEI film and discrete PEI inserts. An experimental approach comprising microstructural characterization, tensile testing, and Digital Image Correlation (DIC) was employed to evaluate manufacturing compatibility and mechanical performance. DIC analysis showed that PEI modified the local strain distribution and reduced principal strain localization, with the insert configuration exhibiting the most consistent behavior. The results demonstrate that PEI gap filling is a feasible local strain-management strategy rather than a complete solution for AFP tow-drop defects.
...
Automated Fiber Placement (AFP) enables the efficient manufacture of high-performance composite structures but inevitably introduces tow-drop gaps that create resin-rich regions, fibre waviness, and local strain concentrations, resulting in structural knockdown. This research investigates the feasibility of mitigating these effects by co-curing PEI into tow-drop gaps of 8552/AS4 carbon/epoxy laminates using two concepts: a continuous PEI film and discrete PEI inserts. An experimental approach comprising microstructural characterization, tensile testing, and Digital Image Correlation (DIC) was employed to evaluate manufacturing compatibility and mechanical performance. DIC analysis showed that PEI modified the local strain distribution and reduced principal strain localization, with the insert configuration exhibiting the most consistent behavior. The results demonstrate that PEI gap filling is a feasible local strain-management strategy rather than a complete solution for AFP tow-drop defects.
A portable, camera-based fiducial positioning approach intended for general contact NDT is presented and experimentally assessed, with phasedarray ultrasonic testing (PAUT) used as the demonstrator application. The positioning method employs inside-out tracking, with a camera rigidly mounted to the probe and a 3D fiducial reference object fixed in the inspection environment. The probe 6-DoF pose is obtained by estimating the camera pose and applying a pivot-based co-calibration to express the phased-array measurement in the fiducial frame. A holistic assessment is performed by (i) quantifying in-plane and out-of-plane translation repeatability and rotational repeatability over relevant working distances and viewing angles, (ii) characterising the camera-to-probe co-calibration accuracy, and (iii) propagating these contributions to an end-toend defect localisation error. Experiments show submillimeter translation repeatability within a practical operating range of 1.5–2.0 m, degrading to a few millimeters at∼3 m and for larger off-axis viewing angles. The pivot co-calibration achieves an RMS error of approximately 1.5 mm relative to a calibration jig. End-to-end validation on a GFRP plate demonstrates millimeter-scale defect localisation and sizing, with sub-millimetre in-plane shift relative to a reference C-scan. These results indicate that fiducial-based positioning provides accuracy sufficient for contact PAUT and constitutes a viable alternative to encoder-based tracking for portable NDT.
...
A portable, camera-based fiducial positioning approach intended for general contact NDT is presented and experimentally assessed, with phasedarray ultrasonic testing (PAUT) used as the demonstrator application. The positioning method employs inside-out tracking, with a camera rigidly mounted to the probe and a 3D fiducial reference object fixed in the inspection environment. The probe 6-DoF pose is obtained by estimating the camera pose and applying a pivot-based co-calibration to express the phased-array measurement in the fiducial frame. A holistic assessment is performed by (i) quantifying in-plane and out-of-plane translation repeatability and rotational repeatability over relevant working distances and viewing angles, (ii) characterising the camera-to-probe co-calibration accuracy, and (iii) propagating these contributions to an end-toend defect localisation error. Experiments show submillimeter translation repeatability within a practical operating range of 1.5–2.0 m, degrading to a few millimeters at∼3 m and for larger off-axis viewing angles. The pivot co-calibration achieves an RMS error of approximately 1.5 mm relative to a calibration jig. End-to-end validation on a GFRP plate demonstrates millimeter-scale defect localisation and sizing, with sub-millimetre in-plane shift relative to a reference C-scan. These results indicate that fiducial-based positioning provides accuracy sufficient for contact PAUT and constitutes a viable alternative to encoder-based tracking for portable NDT.
Gaps and Overlaps in Automated Fiber Placement Composites
Causes, Prediction and Process Design
Manufacturing variations in the automated fiber placement (AFP) process are one of the causes of gaps and overlaps. These manufacturing variations can be due to robot inaccuracy, tow lateral movement on the roller, tow width variation or tow compaction. An experimental setup was built to measure and investigate these various sources of manufacturing variations and their relative contributions to gap and overlap defects. This setup consisted of a commercial AFP head instrumented with additional sensors. Among all the measured sources of variations, lateral movement of the tow on the compaction roller was the biggest contributor to gaps and overlaps. The distributions of these sources of variations were fit with probability density functions. Random samples from these fits were used to simulate adjacent tows and predict the occurrence of gap and overlap defects. The distribution of predicted gaps correlated closely with the distribution of experimentally measured gaps. Thus, this approach of using statistical information about the sources of manufacturing variations to predict the frequency and magnitude of defects in a layup was validated.
Gap and overlap defects are formed during automated fiber placement (AFP) layup due to stochastic variations in tow position and geometry from the aforementioned sources. There is utility in predicting the size and frequency of these defects since they affect the manufacturing rate and structural performance. Two methods which reflect the measured process variability were implemented to simulate realistic tow geometry. A Monte Carlo (MC) simulation used independent random sampling from the distributions of the sources of variation. A Markov Chain Monte Carlo (MCMC) simulation used the Random Walk Metropolis algorithm which samples these distributions while considering the neighborhood of past samples thus preserving spatial continuity. Simulated tows were assembled into virtual laminae. Tow-level validation using spatial-frequency Fast Fourier Transform (FFT) analysis demonstrated that the MCMC method reproduces the dominant wavelengths and spectral characteristics of experimentally measured tow waviness, whereas the MC method introduces unrealistic high frequency variations. Lamina-level comparisons showed that while both methods predict similar gap and overlap area percentages, only the MCMC method accurately captures defect length distributions. The proposed framework provides realistic virtual layup geometries suitable for use in the modeling of the mesoscale aspects of AFP layups. The framework is also useful in determining optimal process parameters for manufacturing process design
Position variations are twice as effective as geometry variations in creating gap and overlaps defects. Tow lateral movement which is one of the causes of position variation was found to be a major contributor to gap and overlap defects during AFP layup in a straight line. In an experiment, to investigate this phenomenon under steering conditions, tow lateral movement was recorded during curvilinear AFP layup. This layup involved two different tow materials at steering radii varying from 1000 mm to 2000 mm. A shift of the tow in the radially outward direction of the curvilinear path was observed for both materials. The tows were observed to gradually shift to reach and stabilize at a position away from the roller center. The shift of the stabilized position was observed to become higher when the steering radius was reduced gradually from 2000 mm to 1000 mm. The rate of shift and the noise in the tow lateral movement differed for both materials. An analytical model was developed to predict and explain these tow lateral movement behaviors. A sensitivity analysis showed that the steering radius and the tow feed system chute geometry parameters had the most significant effect on the predicted tow shift magnitude while the coefficients of friction and compaction force parameters had the most significant effect on the predicted range of tow lateral movement. Further, the utility of the model predictions in offsetting the planned path to achieve layup trajectories with reduced gap and overlap defects was explained. ...
Gap and overlap defects are formed during automated fiber placement (AFP) layup due to stochastic variations in tow position and geometry from the aforementioned sources. There is utility in predicting the size and frequency of these defects since they affect the manufacturing rate and structural performance. Two methods which reflect the measured process variability were implemented to simulate realistic tow geometry. A Monte Carlo (MC) simulation used independent random sampling from the distributions of the sources of variation. A Markov Chain Monte Carlo (MCMC) simulation used the Random Walk Metropolis algorithm which samples these distributions while considering the neighborhood of past samples thus preserving spatial continuity. Simulated tows were assembled into virtual laminae. Tow-level validation using spatial-frequency Fast Fourier Transform (FFT) analysis demonstrated that the MCMC method reproduces the dominant wavelengths and spectral characteristics of experimentally measured tow waviness, whereas the MC method introduces unrealistic high frequency variations. Lamina-level comparisons showed that while both methods predict similar gap and overlap area percentages, only the MCMC method accurately captures defect length distributions. The proposed framework provides realistic virtual layup geometries suitable for use in the modeling of the mesoscale aspects of AFP layups. The framework is also useful in determining optimal process parameters for manufacturing process design
Position variations are twice as effective as geometry variations in creating gap and overlaps defects. Tow lateral movement which is one of the causes of position variation was found to be a major contributor to gap and overlap defects during AFP layup in a straight line. In an experiment, to investigate this phenomenon under steering conditions, tow lateral movement was recorded during curvilinear AFP layup. This layup involved two different tow materials at steering radii varying from 1000 mm to 2000 mm. A shift of the tow in the radially outward direction of the curvilinear path was observed for both materials. The tows were observed to gradually shift to reach and stabilize at a position away from the roller center. The shift of the stabilized position was observed to become higher when the steering radius was reduced gradually from 2000 mm to 1000 mm. The rate of shift and the noise in the tow lateral movement differed for both materials. An analytical model was developed to predict and explain these tow lateral movement behaviors. A sensitivity analysis showed that the steering radius and the tow feed system chute geometry parameters had the most significant effect on the predicted tow shift magnitude while the coefficients of friction and compaction force parameters had the most significant effect on the predicted range of tow lateral movement. Further, the utility of the model predictions in offsetting the planned path to achieve layup trajectories with reduced gap and overlap defects was explained. ...
Manufacturing variations in the automated fiber placement (AFP) process are one of the causes of gaps and overlaps. These manufacturing variations can be due to robot inaccuracy, tow lateral movement on the roller, tow width variation or tow compaction. An experimental setup was built to measure and investigate these various sources of manufacturing variations and their relative contributions to gap and overlap defects. This setup consisted of a commercial AFP head instrumented with additional sensors. Among all the measured sources of variations, lateral movement of the tow on the compaction roller was the biggest contributor to gaps and overlaps. The distributions of these sources of variations were fit with probability density functions. Random samples from these fits were used to simulate adjacent tows and predict the occurrence of gap and overlap defects. The distribution of predicted gaps correlated closely with the distribution of experimentally measured gaps. Thus, this approach of using statistical information about the sources of manufacturing variations to predict the frequency and magnitude of defects in a layup was validated.
Gap and overlap defects are formed during automated fiber placement (AFP) layup due to stochastic variations in tow position and geometry from the aforementioned sources. There is utility in predicting the size and frequency of these defects since they affect the manufacturing rate and structural performance. Two methods which reflect the measured process variability were implemented to simulate realistic tow geometry. A Monte Carlo (MC) simulation used independent random sampling from the distributions of the sources of variation. A Markov Chain Monte Carlo (MCMC) simulation used the Random Walk Metropolis algorithm which samples these distributions while considering the neighborhood of past samples thus preserving spatial continuity. Simulated tows were assembled into virtual laminae. Tow-level validation using spatial-frequency Fast Fourier Transform (FFT) analysis demonstrated that the MCMC method reproduces the dominant wavelengths and spectral characteristics of experimentally measured tow waviness, whereas the MC method introduces unrealistic high frequency variations. Lamina-level comparisons showed that while both methods predict similar gap and overlap area percentages, only the MCMC method accurately captures defect length distributions. The proposed framework provides realistic virtual layup geometries suitable for use in the modeling of the mesoscale aspects of AFP layups. The framework is also useful in determining optimal process parameters for manufacturing process design
Position variations are twice as effective as geometry variations in creating gap and overlaps defects. Tow lateral movement which is one of the causes of position variation was found to be a major contributor to gap and overlap defects during AFP layup in a straight line. In an experiment, to investigate this phenomenon under steering conditions, tow lateral movement was recorded during curvilinear AFP layup. This layup involved two different tow materials at steering radii varying from 1000 mm to 2000 mm. A shift of the tow in the radially outward direction of the curvilinear path was observed for both materials. The tows were observed to gradually shift to reach and stabilize at a position away from the roller center. The shift of the stabilized position was observed to become higher when the steering radius was reduced gradually from 2000 mm to 1000 mm. The rate of shift and the noise in the tow lateral movement differed for both materials. An analytical model was developed to predict and explain these tow lateral movement behaviors. A sensitivity analysis showed that the steering radius and the tow feed system chute geometry parameters had the most significant effect on the predicted tow shift magnitude while the coefficients of friction and compaction force parameters had the most significant effect on the predicted range of tow lateral movement. Further, the utility of the model predictions in offsetting the planned path to achieve layup trajectories with reduced gap and overlap defects was explained.
Gap and overlap defects are formed during automated fiber placement (AFP) layup due to stochastic variations in tow position and geometry from the aforementioned sources. There is utility in predicting the size and frequency of these defects since they affect the manufacturing rate and structural performance. Two methods which reflect the measured process variability were implemented to simulate realistic tow geometry. A Monte Carlo (MC) simulation used independent random sampling from the distributions of the sources of variation. A Markov Chain Monte Carlo (MCMC) simulation used the Random Walk Metropolis algorithm which samples these distributions while considering the neighborhood of past samples thus preserving spatial continuity. Simulated tows were assembled into virtual laminae. Tow-level validation using spatial-frequency Fast Fourier Transform (FFT) analysis demonstrated that the MCMC method reproduces the dominant wavelengths and spectral characteristics of experimentally measured tow waviness, whereas the MC method introduces unrealistic high frequency variations. Lamina-level comparisons showed that while both methods predict similar gap and overlap area percentages, only the MCMC method accurately captures defect length distributions. The proposed framework provides realistic virtual layup geometries suitable for use in the modeling of the mesoscale aspects of AFP layups. The framework is also useful in determining optimal process parameters for manufacturing process design
Position variations are twice as effective as geometry variations in creating gap and overlaps defects. Tow lateral movement which is one of the causes of position variation was found to be a major contributor to gap and overlap defects during AFP layup in a straight line. In an experiment, to investigate this phenomenon under steering conditions, tow lateral movement was recorded during curvilinear AFP layup. This layup involved two different tow materials at steering radii varying from 1000 mm to 2000 mm. A shift of the tow in the radially outward direction of the curvilinear path was observed for both materials. The tows were observed to gradually shift to reach and stabilize at a position away from the roller center. The shift of the stabilized position was observed to become higher when the steering radius was reduced gradually from 2000 mm to 1000 mm. The rate of shift and the noise in the tow lateral movement differed for both materials. An analytical model was developed to predict and explain these tow lateral movement behaviors. A sensitivity analysis showed that the steering radius and the tow feed system chute geometry parameters had the most significant effect on the predicted tow shift magnitude while the coefficients of friction and compaction force parameters had the most significant effect on the predicted range of tow lateral movement. Further, the utility of the model predictions in offsetting the planned path to achieve layup trajectories with reduced gap and overlap defects was explained.
Designing a Telescope for Laser Communication with Ceramic Matrix Composites
Evaluating the viability of C/C-SiC for thermally stable space optics
Carbon–carbon silicon carbide (C/C–SiC) is a ceramic matrix composite combining low density with high stiffness, damage tolerance, and excellent thermal properties. A feature of this material is its low (0.1𝜇m/mK) and tunable coefficient of thermal expansion (CTE), making it highly suitable for precision structures that must remain dimensionally stable under varying thermal loads. This thesis investigates its applicability to a 100 mm diameter Cassegrain telescope structure as a candidate for space-based laser communication terminals. A parametric thermo-elastic finite element model was developed in COMSOL Multiphysics. Rigid body motions and surface form errors were quantified using a Zernike-polynomial-based Python post-processing pipeline, enabling evaluation of optical performance. Results show that by targeting a near-zero in-plane CTE, C/C–SiC can maintain deformations within acceptable limits, supporting its feasibility for lightweight and stable free-space optical communication
systems. Using an opto-thermo-mechanical workflow, the model couples radiative and conductive heat transfer with the structural response to assess wavefront stability under representative orbital thermal load cases. Within the evaluated steady-state cases, tuning the in-plane CTE to approximately 0.1–0.5 𝜇m/mK kept rigid-body motions and surface-form errors within a 𝜆/30 ≈ 50 nm RMS budget, leaving margin for other effects treated as out of scope. From a production perspective, however, current cleanliness and manufacturing-consistency challenges for large continuous-fiber C/C–SiC constrain near-term applicability to off-the-shelf terminals, suggesting more viable deployment in other use cases until maturity improves.
http://10.4121/e139a134-6bd2-4298-932d-975356efd964
Location of the models and post-processing scripts ...
systems. Using an opto-thermo-mechanical workflow, the model couples radiative and conductive heat transfer with the structural response to assess wavefront stability under representative orbital thermal load cases. Within the evaluated steady-state cases, tuning the in-plane CTE to approximately 0.1–0.5 𝜇m/mK kept rigid-body motions and surface-form errors within a 𝜆/30 ≈ 50 nm RMS budget, leaving margin for other effects treated as out of scope. From a production perspective, however, current cleanliness and manufacturing-consistency challenges for large continuous-fiber C/C–SiC constrain near-term applicability to off-the-shelf terminals, suggesting more viable deployment in other use cases until maturity improves.
http://10.4121/e139a134-6bd2-4298-932d-975356efd964
Location of the models and post-processing scripts ...
Carbon–carbon silicon carbide (C/C–SiC) is a ceramic matrix composite combining low density with high stiffness, damage tolerance, and excellent thermal properties. A feature of this material is its low (0.1𝜇m/mK) and tunable coefficient of thermal expansion (CTE), making it highly suitable for precision structures that must remain dimensionally stable under varying thermal loads. This thesis investigates its applicability to a 100 mm diameter Cassegrain telescope structure as a candidate for space-based laser communication terminals. A parametric thermo-elastic finite element model was developed in COMSOL Multiphysics. Rigid body motions and surface form errors were quantified using a Zernike-polynomial-based Python post-processing pipeline, enabling evaluation of optical performance. Results show that by targeting a near-zero in-plane CTE, C/C–SiC can maintain deformations within acceptable limits, supporting its feasibility for lightweight and stable free-space optical communication
systems. Using an opto-thermo-mechanical workflow, the model couples radiative and conductive heat transfer with the structural response to assess wavefront stability under representative orbital thermal load cases. Within the evaluated steady-state cases, tuning the in-plane CTE to approximately 0.1–0.5 𝜇m/mK kept rigid-body motions and surface-form errors within a 𝜆/30 ≈ 50 nm RMS budget, leaving margin for other effects treated as out of scope. From a production perspective, however, current cleanliness and manufacturing-consistency challenges for large continuous-fiber C/C–SiC constrain near-term applicability to off-the-shelf terminals, suggesting more viable deployment in other use cases until maturity improves.
http://10.4121/e139a134-6bd2-4298-932d-975356efd964
Location of the models and post-processing scripts
systems. Using an opto-thermo-mechanical workflow, the model couples radiative and conductive heat transfer with the structural response to assess wavefront stability under representative orbital thermal load cases. Within the evaluated steady-state cases, tuning the in-plane CTE to approximately 0.1–0.5 𝜇m/mK kept rigid-body motions and surface-form errors within a 𝜆/30 ≈ 50 nm RMS budget, leaving margin for other effects treated as out of scope. From a production perspective, however, current cleanliness and manufacturing-consistency challenges for large continuous-fiber C/C–SiC constrain near-term applicability to off-the-shelf terminals, suggesting more viable deployment in other use cases until maturity improves.
http://10.4121/e139a134-6bd2-4298-932d-975356efd964
Location of the models and post-processing scripts
Global warming drives innovation in aerospace materials, with thermoplastic composites offering recyclability and compatibility with automated manufacturing. This study proposes a methodology to translate optimized variable stiffness laminate (VSL) designs into manufacturable geometries. A two-level clustering process based on thickness and principal stresses, followed by genetic algorithm optimization, is applied to a beam structure. Results show feasibility of automatic zone definition but reveal misalignment with load paths and increased weight in final designs. While limitations remain, the approach provides a foundation for refining VSL design methodologies and advancing their practical use in aerospace applications.
...
Global warming drives innovation in aerospace materials, with thermoplastic composites offering recyclability and compatibility with automated manufacturing. This study proposes a methodology to translate optimized variable stiffness laminate (VSL) designs into manufacturable geometries. A two-level clustering process based on thickness and principal stresses, followed by genetic algorithm optimization, is applied to a beam structure. Results show feasibility of automatic zone definition but reveal misalignment with load paths and increased weight in final designs. While limitations remain, the approach provides a foundation for refining VSL design methodologies and advancing their practical use in aerospace applications.
The increasing demand for lightweight and cost-effective structures in aerospace and wind energy applications has led to widespread adoption of fibre-reinforced polymers (FRPs). While carbon fibre reinforced polymers (CFRPs) offer excellent stiffness-to-weight ratios, their high cost and poor damage tolerance pose challenges for structural applications such as wind turbine and helicopter rotor blades. Hybrid laminates combining CFRPs and low-cost, higher failure strain (compared to CFRP) Glass fibre reinforced polymers (GFRPs) present a promising alternative, balancing competing objectives of cost and stiffness while providing higher damage tolerance. Most existing optimisation studies of hybrid composite plates treat material selection and ply orientation as decoupled problems, lacking a unified formulation that can capture their interaction holistically. Furthermore, existing approaches prevent the application of gradient-based optimisation via lamination parameters(LP), efficient for stiffness-targeted optimisation, when attempting to unify material assignment and ply orientation within a single design framework. This thesis develops a novel method to enable the use of lamination parameters for hybrid composite laminates by introducing material-dependent dispersion parameters(DP), unifying material selection and ply orientation within a single design formulation. With the material properties of the laminate as a function of volume fraction, the dispersion parameters describe the through-thickness material distribution in a continuous, geometry-independent manner. The combined LP–DP formulation defines a continuous, convex design region amenable to gradient-based optimisation frameworks. The DPs and feasible region are further explored to create a combinatorics-based and deterministic method to constrain the region, creating a relation between LPs and DPs. Further, different optimisation strategies have been discussed to achieve an optimum solution for design problems. A two-level optimisation strategy is roposed, combining a gradient-based optimisation for target parameter identification and a genetic algorithm to retrieve manufacturable, symmetric, and balanced stacking sequences that comply with standard layup rules.
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The increasing demand for lightweight and cost-effective structures in aerospace and wind energy applications has led to widespread adoption of fibre-reinforced polymers (FRPs). While carbon fibre reinforced polymers (CFRPs) offer excellent stiffness-to-weight ratios, their high cost and poor damage tolerance pose challenges for structural applications such as wind turbine and helicopter rotor blades. Hybrid laminates combining CFRPs and low-cost, higher failure strain (compared to CFRP) Glass fibre reinforced polymers (GFRPs) present a promising alternative, balancing competing objectives of cost and stiffness while providing higher damage tolerance. Most existing optimisation studies of hybrid composite plates treat material selection and ply orientation as decoupled problems, lacking a unified formulation that can capture their interaction holistically. Furthermore, existing approaches prevent the application of gradient-based optimisation via lamination parameters(LP), efficient for stiffness-targeted optimisation, when attempting to unify material assignment and ply orientation within a single design framework. This thesis develops a novel method to enable the use of lamination parameters for hybrid composite laminates by introducing material-dependent dispersion parameters(DP), unifying material selection and ply orientation within a single design formulation. With the material properties of the laminate as a function of volume fraction, the dispersion parameters describe the through-thickness material distribution in a continuous, geometry-independent manner. The combined LP–DP formulation defines a continuous, convex design region amenable to gradient-based optimisation frameworks. The DPs and feasible region are further explored to create a combinatorics-based and deterministic method to constrain the region, creating a relation between LPs and DPs. Further, different optimisation strategies have been discussed to achieve an optimum solution for design problems. A two-level optimisation strategy is roposed, combining a gradient-based optimisation for target parameter identification and a genetic algorithm to retrieve manufacturable, symmetric, and balanced stacking sequences that comply with standard layup rules.
This thesis investigated the morphology of gaps formed during the in-situ consolidated thermoplastic automated fibre placement (ISC-TP AFP) process. Through two experimental trials, gap features were parametrised using geometric metrics such as arc radii and void width. Several distinct morphological types were identified and consistently quantified. The effects of process parameters like consolidation force and layup speed were analysed, though some were obscured due to overshoot. The findings offer a robust framework for gap classification and pave the way for future studies linking morphological features to mechanical performance in ISC-TP AFP composite laminates.
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This thesis investigated the morphology of gaps formed during the in-situ consolidated thermoplastic automated fibre placement (ISC-TP AFP) process. Through two experimental trials, gap features were parametrised using geometric metrics such as arc radii and void width. Several distinct morphological types were identified and consistently quantified. The effects of process parameters like consolidation force and layup speed were analysed, though some were obscured due to overshoot. The findings offer a robust framework for gap classification and pave the way for future studies linking morphological features to mechanical performance in ISC-TP AFP composite laminates.
Investigating the Effect of Tow-Drop Gaps in AFP Produced Laminates
A Theoretical and Numerical Study
Master thesis
(2025)
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A. Giotas, D.M.J. Peeters, S. Giovani Pereira Castro, Jaap Dekker, B. Çağlar, R.C. Alderliesten
Unidirectional composite tapes are extensively used in automated manufacturing to produce high-performance composite parts with significant design flexibility. Their superior mechanical properties stem from the alignment of continuous fibres in a single direction, making them ideal for advanced engineering applications. However, the performance of these components is closely linked to their microstructure, and accurately predicting their behaviour requires a detailed understanding of their microstructural characteristics....
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Unidirectional composite tapes are extensively used in automated manufacturing to produce high-performance composite parts with significant design flexibility. Their superior mechanical properties stem from the alignment of continuous fibres in a single direction, making them ideal for advanced engineering applications. However, the performance of these components is closely linked to their microstructure, and accurately predicting their behaviour requires a detailed understanding of their microstructural characteristics....
Enhanced characterization of tow gaps in fiber steered laminates
Advanced specimen design for better representation of defects in structures
The thesis aimed to characterize the effects of manufacturing-induced defects, particularly gaps in variable stiffness laminates. The primary focus centered on designing a specimen capable of accurately characterizing gap effects. A refined, element-level specimen design featuring a controlled gauge zone was developed, based on pre-defined requirements to represent common variable stiffness structures accurately. The specimens were manufactured and tested. The experimental work featured Digital Image Correlation, Dye Penetrant Testing, and Optical Microscopy. Experimental observations revealed deformation peaks induced by gaps, along with trends in void formation and ply deformation during curing processes. Furthermore, digital image correlation (DIC) analysis unveiled distinct out-of-plane tendencies associated with gap locations. Discrepancies between finite element method (FEM) results and experimental strain distributions highlighted the need for improved modeling, particularly accounting for ply waviness. The main hypothesis presumes that the effects of gaps are influencing off-axis plies through matrix failures. The hypothesis was not proven true based on insufficient proof.
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The thesis aimed to characterize the effects of manufacturing-induced defects, particularly gaps in variable stiffness laminates. The primary focus centered on designing a specimen capable of accurately characterizing gap effects. A refined, element-level specimen design featuring a controlled gauge zone was developed, based on pre-defined requirements to represent common variable stiffness structures accurately. The specimens were manufactured and tested. The experimental work featured Digital Image Correlation, Dye Penetrant Testing, and Optical Microscopy. Experimental observations revealed deformation peaks induced by gaps, along with trends in void formation and ply deformation during curing processes. Furthermore, digital image correlation (DIC) analysis unveiled distinct out-of-plane tendencies associated with gap locations. Discrepancies between finite element method (FEM) results and experimental strain distributions highlighted the need for improved modeling, particularly accounting for ply waviness. The main hypothesis presumes that the effects of gaps are influencing off-axis plies through matrix failures. The hypothesis was not proven true based on insufficient proof.
Influence of Process Parameters on the Tack Behaviour of Dry Fibre Materials in the Context of Automated Dry Fibre Placement
An Experimental investigation into the influence of process parameters influencing tack of ADFP deposited dry fibre materials
With the increasing demand for manufacturing sustainable aerospace parts, along with cost effectiveness and preciseness. Automated Dry Fibre Placement(ADFP) has emerged as a promising solution. ADFP combines these goals and enables the production of precise and cost-effective aerospace components with the potential for reduced defects. Tack, which is the adhesion between fibre layers during deposition, plays an important role in achieving high-quality preforms. Tack is essential for securely holding down the plies. Insufficient tack leads to final part defects like wrinkling contributing to the stability of the final part. This study investigates the influence of nip point temperature, layup speed, and compaction force on the tack behaviour of Hexcel HiTape® dry fibre material during the Automated Dry Fibre Placement (ADFP) process. It also assesses the applicability of an existing tack model, developed for Solvay TX1100 dry fibre material, to the Hexcel HiTape®, possessing a different binder distribution and different fibre architecture. Tack was evaluated using 90- degree peel tests for specimens manufactured using different parameter combinations. Initial statistical analysis of the main parameter effects and interactions used a 2-level factorial design of experiments. To investigate the non-linear behaviour of the parameters, this design was extended to a 3-level face-centred central composite design of experiments. The results show that temperature is the most dominant factor affecting tack, with higher temperatures significantly increasing tack forces due to binder activation and polymer diffusion at the interface. Layup speed also had a notable influence, with higher speeds leading to increased tack forces, which contrasts with the inverse relationship predicted by the existing model. This is likely due to the material architecture of Hexcel HiTape ®, where faster speeds reduce binder seepage through perforations, allowing more binder to remain at the interface. Compaction force had a minor impact on tack behaviour and the results of this analysis deemed the effect to be statistically insignificant. Additionally, while interaction effects between the parameters were analysed, they were not statistically significant at the 95% confidence level. The findings suggest that the independent contributions of temperature and velocity are key, but deviations from expected behaviour may be due to material-specific characteristics such as perforations and fibre structure. The behaviour of the material in relation to the statistically significant parameters shows that there is non-linearity.To test the generalisability of the model, the existing tack model for Solvay TX1100 dry fibre was applied to Hexcel HiTape ®. This model did not fit the data of Hexcel HiTape ® material. The observed positive relationship between speed and tack, contrary to the Solvay TX1100 model’s inverse relationship, shows the need to adapt the model to Hexcel HiTape ® and indicates limitations in generalising tack models across dry fibre materials. Additionally, fibre fraying of Hexcel HiTape® was also observed during peel tests, marking a finding that warrants further investigation to help quantify the effect, aiding in understanding the dry fibre material behaviour.
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With the increasing demand for manufacturing sustainable aerospace parts, along with cost effectiveness and preciseness. Automated Dry Fibre Placement(ADFP) has emerged as a promising solution. ADFP combines these goals and enables the production of precise and cost-effective aerospace components with the potential for reduced defects. Tack, which is the adhesion between fibre layers during deposition, plays an important role in achieving high-quality preforms. Tack is essential for securely holding down the plies. Insufficient tack leads to final part defects like wrinkling contributing to the stability of the final part. This study investigates the influence of nip point temperature, layup speed, and compaction force on the tack behaviour of Hexcel HiTape® dry fibre material during the Automated Dry Fibre Placement (ADFP) process. It also assesses the applicability of an existing tack model, developed for Solvay TX1100 dry fibre material, to the Hexcel HiTape®, possessing a different binder distribution and different fibre architecture. Tack was evaluated using 90- degree peel tests for specimens manufactured using different parameter combinations. Initial statistical analysis of the main parameter effects and interactions used a 2-level factorial design of experiments. To investigate the non-linear behaviour of the parameters, this design was extended to a 3-level face-centred central composite design of experiments. The results show that temperature is the most dominant factor affecting tack, with higher temperatures significantly increasing tack forces due to binder activation and polymer diffusion at the interface. Layup speed also had a notable influence, with higher speeds leading to increased tack forces, which contrasts with the inverse relationship predicted by the existing model. This is likely due to the material architecture of Hexcel HiTape ®, where faster speeds reduce binder seepage through perforations, allowing more binder to remain at the interface. Compaction force had a minor impact on tack behaviour and the results of this analysis deemed the effect to be statistically insignificant. Additionally, while interaction effects between the parameters were analysed, they were not statistically significant at the 95% confidence level. The findings suggest that the independent contributions of temperature and velocity are key, but deviations from expected behaviour may be due to material-specific characteristics such as perforations and fibre structure. The behaviour of the material in relation to the statistically significant parameters shows that there is non-linearity.To test the generalisability of the model, the existing tack model for Solvay TX1100 dry fibre was applied to Hexcel HiTape ®. This model did not fit the data of Hexcel HiTape ® material. The observed positive relationship between speed and tack, contrary to the Solvay TX1100 model’s inverse relationship, shows the need to adapt the model to Hexcel HiTape ® and indicates limitations in generalising tack models across dry fibre materials. Additionally, fibre fraying of Hexcel HiTape® was also observed during peel tests, marking a finding that warrants further investigation to help quantify the effect, aiding in understanding the dry fibre material behaviour.
Master thesis
(2024)
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R.I. Ciobotia, S. Giovani Pereira Castro, D.M.J. Peeters, A.H. van der Laan, D. Zarouchas, B. Chen
The present study, which was carried out in collaboration with GKN Fokker, focuses on incorporating bird strike crashworthiness requirements within a multidisciplinary optimization (MDO) framework. During the preceding three-month internship in the same company, a pivotal contribution to this project was the development of an Abaqus interface for the Multidisciplinary Modeller, MDM, created within the Center of Competence in Design department. MDM is a Python/ParaPy-based automated generator of wings, moveables and flaps, starting from a set of user-specified parameters. The generation of ready-to-run input files thus lays the foundation for the subsequent optimization process, as any changes in materials or geometry can be easily accommodated.
The core objective of the research is to minimize the weight of an aircraft wing while taking into account additional requirements related to the extent of damage caused by bird strikes. Unfortunately, such events occur more frequently than one would be comfortable with, and stringent requirements are set in place to guarantee the safety of the passengers. Among these requirements, the aircraft must be capable of landing safely after such an event, being subject to loads associated with get-home conditions.
As a consequence, two critical constraints are formulated within the optimization framework, addressing the residual strength of the damaged front spar following a bird strike, coupled with a requirement based on a maximum penetration depth. The last constraint has also been included due to the rising popularity of the electric vertical take-off and landing aircraft, which not only fly at low altitudes, thus increasing the risk of bird strike, but may also contain battery packs in the leading edge, for instance, which can pose a significant risk if damaged. To tackle the complexity of this highly-dimensional optimization challenge, a methodology based on Bayesian optimization is proposed, employing surrogate models coupled with a preliminary variable ranking procedure.
The Kriging metamodel is identified as a suitable candidate, thanks to its error prediction capabilities, which are paramount in Bayesian optimization. A variance-based dimensionality reduction method is proposed, which makes use of an initial surrogate to estimate the main and interaction effects of the variables. The quantification of the significance of a variable is expressed as its percentage contribution to the total variance, thus allowing for an intuitive selection of the most important parameters. After the screening procedure is complete, the optimization procedure is carried out in the reduced design space, which uses the constrained expected improvement as an acquisition function. The proposed methodology is then applied on a case study problem, involving a five-bay metallic wing segment subject to the constraints aforementioned, involving 19 design variables representing the thicknesses of various components.
Remarkable weight savings have been achieved, the final result being 40\% lighter than the lightest feasible design among the initial data points. A significant dimensional reduction has also been attained for the maximum depth constraint, which is expected due to the local nature of the impact. Not only did the number of variables greatly decrease from 19 to just 3, but a considerable increase in the accuracy of the corresponding metamodel has also been registered, thanks to an increase in sampling density in the reduced space. However, the variable screening procedure revealed intricate interaction effects with respect to the residual strength of the front spar, emphasizing the nuanced complexity inherent in crashworthiness considerations. Nevertheless, a moderate dimensional reduction has been achieved for this constraint as well, reducing the number of variables to 8, thus proving the efficacy of the proposed variable screening procedure.
In conclusion, the utilization of Kriging models, variable ranking procedures, and Bayesian optimization collectively contributed to the success of achieving remarkable weight savings, proving the efficiency of the proposed methodology. Moreover, it has been shown that the integration of a residual strength requirement is necessary, as many cases were uncovered where no significant penetration occurred, although the application of the considered load case, which is not from critical to an undamaged wing, resulted in high stresses to the front spar of the damaged structure. ...
The core objective of the research is to minimize the weight of an aircraft wing while taking into account additional requirements related to the extent of damage caused by bird strikes. Unfortunately, such events occur more frequently than one would be comfortable with, and stringent requirements are set in place to guarantee the safety of the passengers. Among these requirements, the aircraft must be capable of landing safely after such an event, being subject to loads associated with get-home conditions.
As a consequence, two critical constraints are formulated within the optimization framework, addressing the residual strength of the damaged front spar following a bird strike, coupled with a requirement based on a maximum penetration depth. The last constraint has also been included due to the rising popularity of the electric vertical take-off and landing aircraft, which not only fly at low altitudes, thus increasing the risk of bird strike, but may also contain battery packs in the leading edge, for instance, which can pose a significant risk if damaged. To tackle the complexity of this highly-dimensional optimization challenge, a methodology based on Bayesian optimization is proposed, employing surrogate models coupled with a preliminary variable ranking procedure.
The Kriging metamodel is identified as a suitable candidate, thanks to its error prediction capabilities, which are paramount in Bayesian optimization. A variance-based dimensionality reduction method is proposed, which makes use of an initial surrogate to estimate the main and interaction effects of the variables. The quantification of the significance of a variable is expressed as its percentage contribution to the total variance, thus allowing for an intuitive selection of the most important parameters. After the screening procedure is complete, the optimization procedure is carried out in the reduced design space, which uses the constrained expected improvement as an acquisition function. The proposed methodology is then applied on a case study problem, involving a five-bay metallic wing segment subject to the constraints aforementioned, involving 19 design variables representing the thicknesses of various components.
Remarkable weight savings have been achieved, the final result being 40\% lighter than the lightest feasible design among the initial data points. A significant dimensional reduction has also been attained for the maximum depth constraint, which is expected due to the local nature of the impact. Not only did the number of variables greatly decrease from 19 to just 3, but a considerable increase in the accuracy of the corresponding metamodel has also been registered, thanks to an increase in sampling density in the reduced space. However, the variable screening procedure revealed intricate interaction effects with respect to the residual strength of the front spar, emphasizing the nuanced complexity inherent in crashworthiness considerations. Nevertheless, a moderate dimensional reduction has been achieved for this constraint as well, reducing the number of variables to 8, thus proving the efficacy of the proposed variable screening procedure.
In conclusion, the utilization of Kriging models, variable ranking procedures, and Bayesian optimization collectively contributed to the success of achieving remarkable weight savings, proving the efficiency of the proposed methodology. Moreover, it has been shown that the integration of a residual strength requirement is necessary, as many cases were uncovered where no significant penetration occurred, although the application of the considered load case, which is not from critical to an undamaged wing, resulted in high stresses to the front spar of the damaged structure. ...
The present study, which was carried out in collaboration with GKN Fokker, focuses on incorporating bird strike crashworthiness requirements within a multidisciplinary optimization (MDO) framework. During the preceding three-month internship in the same company, a pivotal contribution to this project was the development of an Abaqus interface for the Multidisciplinary Modeller, MDM, created within the Center of Competence in Design department. MDM is a Python/ParaPy-based automated generator of wings, moveables and flaps, starting from a set of user-specified parameters. The generation of ready-to-run input files thus lays the foundation for the subsequent optimization process, as any changes in materials or geometry can be easily accommodated.
The core objective of the research is to minimize the weight of an aircraft wing while taking into account additional requirements related to the extent of damage caused by bird strikes. Unfortunately, such events occur more frequently than one would be comfortable with, and stringent requirements are set in place to guarantee the safety of the passengers. Among these requirements, the aircraft must be capable of landing safely after such an event, being subject to loads associated with get-home conditions.
As a consequence, two critical constraints are formulated within the optimization framework, addressing the residual strength of the damaged front spar following a bird strike, coupled with a requirement based on a maximum penetration depth. The last constraint has also been included due to the rising popularity of the electric vertical take-off and landing aircraft, which not only fly at low altitudes, thus increasing the risk of bird strike, but may also contain battery packs in the leading edge, for instance, which can pose a significant risk if damaged. To tackle the complexity of this highly-dimensional optimization challenge, a methodology based on Bayesian optimization is proposed, employing surrogate models coupled with a preliminary variable ranking procedure.
The Kriging metamodel is identified as a suitable candidate, thanks to its error prediction capabilities, which are paramount in Bayesian optimization. A variance-based dimensionality reduction method is proposed, which makes use of an initial surrogate to estimate the main and interaction effects of the variables. The quantification of the significance of a variable is expressed as its percentage contribution to the total variance, thus allowing for an intuitive selection of the most important parameters. After the screening procedure is complete, the optimization procedure is carried out in the reduced design space, which uses the constrained expected improvement as an acquisition function. The proposed methodology is then applied on a case study problem, involving a five-bay metallic wing segment subject to the constraints aforementioned, involving 19 design variables representing the thicknesses of various components.
Remarkable weight savings have been achieved, the final result being 40\% lighter than the lightest feasible design among the initial data points. A significant dimensional reduction has also been attained for the maximum depth constraint, which is expected due to the local nature of the impact. Not only did the number of variables greatly decrease from 19 to just 3, but a considerable increase in the accuracy of the corresponding metamodel has also been registered, thanks to an increase in sampling density in the reduced space. However, the variable screening procedure revealed intricate interaction effects with respect to the residual strength of the front spar, emphasizing the nuanced complexity inherent in crashworthiness considerations. Nevertheless, a moderate dimensional reduction has been achieved for this constraint as well, reducing the number of variables to 8, thus proving the efficacy of the proposed variable screening procedure.
In conclusion, the utilization of Kriging models, variable ranking procedures, and Bayesian optimization collectively contributed to the success of achieving remarkable weight savings, proving the efficiency of the proposed methodology. Moreover, it has been shown that the integration of a residual strength requirement is necessary, as many cases were uncovered where no significant penetration occurred, although the application of the considered load case, which is not from critical to an undamaged wing, resulted in high stresses to the front spar of the damaged structure.
The core objective of the research is to minimize the weight of an aircraft wing while taking into account additional requirements related to the extent of damage caused by bird strikes. Unfortunately, such events occur more frequently than one would be comfortable with, and stringent requirements are set in place to guarantee the safety of the passengers. Among these requirements, the aircraft must be capable of landing safely after such an event, being subject to loads associated with get-home conditions.
As a consequence, two critical constraints are formulated within the optimization framework, addressing the residual strength of the damaged front spar following a bird strike, coupled with a requirement based on a maximum penetration depth. The last constraint has also been included due to the rising popularity of the electric vertical take-off and landing aircraft, which not only fly at low altitudes, thus increasing the risk of bird strike, but may also contain battery packs in the leading edge, for instance, which can pose a significant risk if damaged. To tackle the complexity of this highly-dimensional optimization challenge, a methodology based on Bayesian optimization is proposed, employing surrogate models coupled with a preliminary variable ranking procedure.
The Kriging metamodel is identified as a suitable candidate, thanks to its error prediction capabilities, which are paramount in Bayesian optimization. A variance-based dimensionality reduction method is proposed, which makes use of an initial surrogate to estimate the main and interaction effects of the variables. The quantification of the significance of a variable is expressed as its percentage contribution to the total variance, thus allowing for an intuitive selection of the most important parameters. After the screening procedure is complete, the optimization procedure is carried out in the reduced design space, which uses the constrained expected improvement as an acquisition function. The proposed methodology is then applied on a case study problem, involving a five-bay metallic wing segment subject to the constraints aforementioned, involving 19 design variables representing the thicknesses of various components.
Remarkable weight savings have been achieved, the final result being 40\% lighter than the lightest feasible design among the initial data points. A significant dimensional reduction has also been attained for the maximum depth constraint, which is expected due to the local nature of the impact. Not only did the number of variables greatly decrease from 19 to just 3, but a considerable increase in the accuracy of the corresponding metamodel has also been registered, thanks to an increase in sampling density in the reduced space. However, the variable screening procedure revealed intricate interaction effects with respect to the residual strength of the front spar, emphasizing the nuanced complexity inherent in crashworthiness considerations. Nevertheless, a moderate dimensional reduction has been achieved for this constraint as well, reducing the number of variables to 8, thus proving the efficacy of the proposed variable screening procedure.
In conclusion, the utilization of Kriging models, variable ranking procedures, and Bayesian optimization collectively contributed to the success of achieving remarkable weight savings, proving the efficiency of the proposed methodology. Moreover, it has been shown that the integration of a residual strength requirement is necessary, as many cases were uncovered where no significant penetration occurred, although the application of the considered load case, which is not from critical to an undamaged wing, resulted in high stresses to the front spar of the damaged structure.
Width deformation of thermoplastic prepreg tapes during in-situ Automated Fiber Placement
An experimental investigation into the width deformation mechanism and influence of processing parameters
Increasing demands for composite structures and sustainability goals have resulted in the growth of thermoplastic polymer matrix composites and automated manufacturing technologies. The in-situ Automated Fiber Placement (AFP) manufacturing of thermoplastic prepreg tapes has the potential to provide a fast and cost-effective manufacturing solution for large composite structures. However, it is still under development because of the complex mechanisms and short processing times involved which leads to several defect formations, especially gaps and overlaps. One of the primary reasons for the formation of these gaps and overlaps is the tape width deformation during placement.
Current literature on tape width deformation shows that the resulting tape width is influenced by several processing parameters such as temperature, pressure and placement speed. However, results from different studies do not agree with each other, indicating that the tape temperature distribution might be at play. Additionally, the conventionally considered tape width deformation mechanism i.e., transverse squeeze flow has been suggested to be incorrect for the AFP process as the experimental deformations do not agree with the results of the transverse squeeze flow model. Therefore, the research objective for this study was to experimentally investigate the width deformation mechanism and the influence of processing parameters for thermoplastic prepreg tapes using in-situ AFP manufacturing and humm3® (from Heraeus) as the heating device.
The specimens were manufactured according to a full-factorial Design of Experiments (DoE) with two settings (high and low) for the following processing parameters: heated length, nip-point temperature and compaction force. The tape width was measured for all the specimens to investigate the influence of the different processing parameters and some post-processing analyses were carried out to understand the tape width deformation mechanism. This included width measurement in the heating phase of the process, surface roughness analysis, tape cross-section profile inspection and fiber-resin content analysis.
From the post-processing analyses and investigations, it was found that the tape width deforms in the heating as well as the consolidation phase of the process. Additionally, the cross-section images show that the conformable roller led the tape edge profile to have a gradual decrease in thickness with a clear slope and the tape edges show a clear indication of spreading of the fiber-resin mixture due to the presence of both fibers and resin. Moreover, the surface roughness data show an indication of the role of temperature distribution because the as-received tape surface roughness was achieved for the higher temperature and longer heated length settings which are assumed to promote better heat distribution in the material.
The influence of the processing parameters on the tape width deformation did not show clear trends for all specimen configurations. However, the exceptions pointed towards the role of temperature distribution in the tape that led to the overshadowing of the effect of other processing parameters. Considering this, it was observed that the change in heated length did not have a significant effect on the tape width except for one configuration i.e., 300 N, 370 °C, wherein a clear increase with no overlap in data was seen. For the effect of temperature and compaction force, it was found that they have an influence on the tape width for temperatures lower than the melting temperature of the polymer resin (Tm). Additionally, the fiber straining effect on the tape width deformation with compaction force was suggested for the higher temperature specimens. ...
Current literature on tape width deformation shows that the resulting tape width is influenced by several processing parameters such as temperature, pressure and placement speed. However, results from different studies do not agree with each other, indicating that the tape temperature distribution might be at play. Additionally, the conventionally considered tape width deformation mechanism i.e., transverse squeeze flow has been suggested to be incorrect for the AFP process as the experimental deformations do not agree with the results of the transverse squeeze flow model. Therefore, the research objective for this study was to experimentally investigate the width deformation mechanism and the influence of processing parameters for thermoplastic prepreg tapes using in-situ AFP manufacturing and humm3® (from Heraeus) as the heating device.
The specimens were manufactured according to a full-factorial Design of Experiments (DoE) with two settings (high and low) for the following processing parameters: heated length, nip-point temperature and compaction force. The tape width was measured for all the specimens to investigate the influence of the different processing parameters and some post-processing analyses were carried out to understand the tape width deformation mechanism. This included width measurement in the heating phase of the process, surface roughness analysis, tape cross-section profile inspection and fiber-resin content analysis.
From the post-processing analyses and investigations, it was found that the tape width deforms in the heating as well as the consolidation phase of the process. Additionally, the cross-section images show that the conformable roller led the tape edge profile to have a gradual decrease in thickness with a clear slope and the tape edges show a clear indication of spreading of the fiber-resin mixture due to the presence of both fibers and resin. Moreover, the surface roughness data show an indication of the role of temperature distribution because the as-received tape surface roughness was achieved for the higher temperature and longer heated length settings which are assumed to promote better heat distribution in the material.
The influence of the processing parameters on the tape width deformation did not show clear trends for all specimen configurations. However, the exceptions pointed towards the role of temperature distribution in the tape that led to the overshadowing of the effect of other processing parameters. Considering this, it was observed that the change in heated length did not have a significant effect on the tape width except for one configuration i.e., 300 N, 370 °C, wherein a clear increase with no overlap in data was seen. For the effect of temperature and compaction force, it was found that they have an influence on the tape width for temperatures lower than the melting temperature of the polymer resin (Tm). Additionally, the fiber straining effect on the tape width deformation with compaction force was suggested for the higher temperature specimens. ...
Increasing demands for composite structures and sustainability goals have resulted in the growth of thermoplastic polymer matrix composites and automated manufacturing technologies. The in-situ Automated Fiber Placement (AFP) manufacturing of thermoplastic prepreg tapes has the potential to provide a fast and cost-effective manufacturing solution for large composite structures. However, it is still under development because of the complex mechanisms and short processing times involved which leads to several defect formations, especially gaps and overlaps. One of the primary reasons for the formation of these gaps and overlaps is the tape width deformation during placement.
Current literature on tape width deformation shows that the resulting tape width is influenced by several processing parameters such as temperature, pressure and placement speed. However, results from different studies do not agree with each other, indicating that the tape temperature distribution might be at play. Additionally, the conventionally considered tape width deformation mechanism i.e., transverse squeeze flow has been suggested to be incorrect for the AFP process as the experimental deformations do not agree with the results of the transverse squeeze flow model. Therefore, the research objective for this study was to experimentally investigate the width deformation mechanism and the influence of processing parameters for thermoplastic prepreg tapes using in-situ AFP manufacturing and humm3® (from Heraeus) as the heating device.
The specimens were manufactured according to a full-factorial Design of Experiments (DoE) with two settings (high and low) for the following processing parameters: heated length, nip-point temperature and compaction force. The tape width was measured for all the specimens to investigate the influence of the different processing parameters and some post-processing analyses were carried out to understand the tape width deformation mechanism. This included width measurement in the heating phase of the process, surface roughness analysis, tape cross-section profile inspection and fiber-resin content analysis.
From the post-processing analyses and investigations, it was found that the tape width deforms in the heating as well as the consolidation phase of the process. Additionally, the cross-section images show that the conformable roller led the tape edge profile to have a gradual decrease in thickness with a clear slope and the tape edges show a clear indication of spreading of the fiber-resin mixture due to the presence of both fibers and resin. Moreover, the surface roughness data show an indication of the role of temperature distribution because the as-received tape surface roughness was achieved for the higher temperature and longer heated length settings which are assumed to promote better heat distribution in the material.
The influence of the processing parameters on the tape width deformation did not show clear trends for all specimen configurations. However, the exceptions pointed towards the role of temperature distribution in the tape that led to the overshadowing of the effect of other processing parameters. Considering this, it was observed that the change in heated length did not have a significant effect on the tape width except for one configuration i.e., 300 N, 370 °C, wherein a clear increase with no overlap in data was seen. For the effect of temperature and compaction force, it was found that they have an influence on the tape width for temperatures lower than the melting temperature of the polymer resin (Tm). Additionally, the fiber straining effect on the tape width deformation with compaction force was suggested for the higher temperature specimens.
Current literature on tape width deformation shows that the resulting tape width is influenced by several processing parameters such as temperature, pressure and placement speed. However, results from different studies do not agree with each other, indicating that the tape temperature distribution might be at play. Additionally, the conventionally considered tape width deformation mechanism i.e., transverse squeeze flow has been suggested to be incorrect for the AFP process as the experimental deformations do not agree with the results of the transverse squeeze flow model. Therefore, the research objective for this study was to experimentally investigate the width deformation mechanism and the influence of processing parameters for thermoplastic prepreg tapes using in-situ AFP manufacturing and humm3® (from Heraeus) as the heating device.
The specimens were manufactured according to a full-factorial Design of Experiments (DoE) with two settings (high and low) for the following processing parameters: heated length, nip-point temperature and compaction force. The tape width was measured for all the specimens to investigate the influence of the different processing parameters and some post-processing analyses were carried out to understand the tape width deformation mechanism. This included width measurement in the heating phase of the process, surface roughness analysis, tape cross-section profile inspection and fiber-resin content analysis.
From the post-processing analyses and investigations, it was found that the tape width deforms in the heating as well as the consolidation phase of the process. Additionally, the cross-section images show that the conformable roller led the tape edge profile to have a gradual decrease in thickness with a clear slope and the tape edges show a clear indication of spreading of the fiber-resin mixture due to the presence of both fibers and resin. Moreover, the surface roughness data show an indication of the role of temperature distribution because the as-received tape surface roughness was achieved for the higher temperature and longer heated length settings which are assumed to promote better heat distribution in the material.
The influence of the processing parameters on the tape width deformation did not show clear trends for all specimen configurations. However, the exceptions pointed towards the role of temperature distribution in the tape that led to the overshadowing of the effect of other processing parameters. Considering this, it was observed that the change in heated length did not have a significant effect on the tape width except for one configuration i.e., 300 N, 370 °C, wherein a clear increase with no overlap in data was seen. For the effect of temperature and compaction force, it was found that they have an influence on the tape width for temperatures lower than the melting temperature of the polymer resin (Tm). Additionally, the fiber straining effect on the tape width deformation with compaction force was suggested for the higher temperature specimens.
Laser-assisted fiber placement (LAFP) is an automated manufacturing technique with promising potential to produce larger aerospace structural components using thermoplastic composites. However, during the rapid heating phase, deconsolidation of composite tapes can occur, resulting in negative effects on bond forming ability. This study aimed to investigate the effect of LAFP process parameters on the rapid laser heating deconsolidation forms of CF/PEEK tape samples by developing an experimental setup. Six different combinations of placement speed and consolidation pressure were tested, and the specimens were characterized for roughness, degree of effective intimate contact (DEIC), void content, and thickness. The results showed that an increase in placement speed and pressure can yield a positive effect on the resolvement of deconsolidation. The nip-point temperature was found to have a significant influence, with expected higher temperatures resulting in better re-compaction of the fiber bed and flow of the matrix material. The developed experimental setup provides a foundation for further investigation into the in-situ consolidation of LAFP for meeting aerospace standards.
...
Laser-assisted fiber placement (LAFP) is an automated manufacturing technique with promising potential to produce larger aerospace structural components using thermoplastic composites. However, during the rapid heating phase, deconsolidation of composite tapes can occur, resulting in negative effects on bond forming ability. This study aimed to investigate the effect of LAFP process parameters on the rapid laser heating deconsolidation forms of CF/PEEK tape samples by developing an experimental setup. Six different combinations of placement speed and consolidation pressure were tested, and the specimens were characterized for roughness, degree of effective intimate contact (DEIC), void content, and thickness. The results showed that an increase in placement speed and pressure can yield a positive effect on the resolvement of deconsolidation. The nip-point temperature was found to have a significant influence, with expected higher temperatures resulting in better re-compaction of the fiber bed and flow of the matrix material. The developed experimental setup provides a foundation for further investigation into the in-situ consolidation of LAFP for meeting aerospace standards.
Master thesis
(2023)
-
L. Bărbulescu, T. Keviczky, D.M.J. Peeters, A.M. Mendes Florindo, G.A. van der Hoorn, M. Guo
Robot manipulators are significantly more accurate than their human counterparts and enhance the repeatability of various tasks. However, manufactures still provide reduced information regarding the robot controller functions, typically affecting robots' predictability. Additionally, industrial examples show that system transparency could be improved, helping users to understand the process and apply corrective actions. One step in the direction of improved predictability is the identification of the Limiter block associated with the KUKA robot controllers that limit the acceleration and jerk obtained during motions. This work aims to provide dynamically feasible trajectories with reduced performance deterioration. Specifically, the Limiter's behaviour was analyzed and later predicted in a iterative manner, aiming for the maximum velocity, acceleration and jerk that can be achieved, starting from the current robot state and given specific robot information. Using the predicted bounds, the trajectory generator will determine the maximum achievable point that fulfills all the constraints, running an optimization problem in an iterative fashion. Nonetheless, the practical experiments represent a significant component allowing data collection and results validation.
...
Robot manipulators are significantly more accurate than their human counterparts and enhance the repeatability of various tasks. However, manufactures still provide reduced information regarding the robot controller functions, typically affecting robots' predictability. Additionally, industrial examples show that system transparency could be improved, helping users to understand the process and apply corrective actions. One step in the direction of improved predictability is the identification of the Limiter block associated with the KUKA robot controllers that limit the acceleration and jerk obtained during motions. This work aims to provide dynamically feasible trajectories with reduced performance deterioration. Specifically, the Limiter's behaviour was analyzed and later predicted in a iterative manner, aiming for the maximum velocity, acceleration and jerk that can be achieved, starting from the current robot state and given specific robot information. Using the predicted bounds, the trajectory generator will determine the maximum achievable point that fulfills all the constraints, running an optimization problem in an iterative fashion. Nonetheless, the practical experiments represent a significant component allowing data collection and results validation.
Stiffness Design of Laminated Composites
Efficient Conversion of Lamination Parameters into Stacking Sequences
Master thesis
(2023)
-
R. Manikandan, J.M.J.F. van Campen, D.M.J. Peeters, S. Giovani Pereira Castro, N. Yue
Fibre-reinforced laminated composites are constructed layer-by-layer, allowing their material properties to be easily tailored relative to their isotropic counterparts (metallic alloys). Moreover, a composite structure with variable stiffness (VS) can be made by spatially tailoring stiffness across a laminate. This permits better load distribution within a structure, efficiently using a given material, and allow for lighter construction. Typically, the design of VS structures follows a bi-level procedure: first, the structure’s stiffness distribution is optimised using lamination parameters (LPs), and second, the stacking sequences (SS) of differently oriented fibres are designed to achieve the desired stiffness distribution. However, the designs envisioned using LPs are only sometimes exactly matched by the designed SS. The shortcomings faced during this conversion are called the ’Inverse Problem’. Upon reviewing the existing techniques in literature to handle the inverse problem, it was understood that converting LPs into SS is challenging without incurring substantial computational costs or imposing significant restrictions on allowable fibre orientations in the design. Such restrictions tend to under-utilise the directional properties of the fibres. Thus, there was a need to explore and develop better stiffness design methods for laminated composites by bridging the gap between LPs and SS in a computationally efficient way.
In response to this challenge, a hierarchical design framework was proposed to handle the Inverse Problem. Diverging from conventional methods that directly design the SS and try to match a given set of LPs, this framework divides the problem into two distinct stages. Initially, the focus is to use the In-Plane LPs and determine the number of layers in each orientation within a laminate, also called the Fibre Angle Distribution (FAD). Subsequently, the FAD serves as an interim solution, and the SS can be designed by using the Out-of-Plane LPs along with it. This problem partitioning enhances computational efficiency and offers potential benefits in solving the Inverse Problem more effectively. Given the time constraints inherent to a master thesis, the primary undertaking of this study was to efficiently design the FAD while accommodating a wide range of possible fibre orientations. To address this, a novel method using the Fast-Fourier Transform (FFT) was developed to facilitate FAD design with ply angle multiples of 15° (or [∆15° ] = [0, ±15, ±30, ±45, ±60, ±75, 90]). Moreover, empirical guidelines for SS design, such as the Symmetry and Balancing rule, were incorporated into the design step.
The primary contribution of this report is introducing an FFT-based method to design FADs, a novel addition to the existing body of research. Upon extensive testing, it was shown that this implementation could convert LPs into multiple unique FAD solutions in less than 0.3 seconds on a regular office laptop, outperforming other methods in literature by at least ten times. Furthermore, the implementation was also used to demonstrate the benefits of designing laminated composites using [∆15°] over the conventional [∆45°] orientations (or [0, ±45, 90]). In light of the positive results, it is pointed out that the In-Plane LPs (and consequently the In-Plane stiffness) are known to be sensitive only to the FAD and not their SS. As such, the readers of this thesis are presented with a very computationally efficient approach for designing laminated composites for In-Plane Stiffness...
...
In response to this challenge, a hierarchical design framework was proposed to handle the Inverse Problem. Diverging from conventional methods that directly design the SS and try to match a given set of LPs, this framework divides the problem into two distinct stages. Initially, the focus is to use the In-Plane LPs and determine the number of layers in each orientation within a laminate, also called the Fibre Angle Distribution (FAD). Subsequently, the FAD serves as an interim solution, and the SS can be designed by using the Out-of-Plane LPs along with it. This problem partitioning enhances computational efficiency and offers potential benefits in solving the Inverse Problem more effectively. Given the time constraints inherent to a master thesis, the primary undertaking of this study was to efficiently design the FAD while accommodating a wide range of possible fibre orientations. To address this, a novel method using the Fast-Fourier Transform (FFT) was developed to facilitate FAD design with ply angle multiples of 15° (or [∆15° ] = [0, ±15, ±30, ±45, ±60, ±75, 90]). Moreover, empirical guidelines for SS design, such as the Symmetry and Balancing rule, were incorporated into the design step.
The primary contribution of this report is introducing an FFT-based method to design FADs, a novel addition to the existing body of research. Upon extensive testing, it was shown that this implementation could convert LPs into multiple unique FAD solutions in less than 0.3 seconds on a regular office laptop, outperforming other methods in literature by at least ten times. Furthermore, the implementation was also used to demonstrate the benefits of designing laminated composites using [∆15°] over the conventional [∆45°] orientations (or [0, ±45, 90]). In light of the positive results, it is pointed out that the In-Plane LPs (and consequently the In-Plane stiffness) are known to be sensitive only to the FAD and not their SS. As such, the readers of this thesis are presented with a very computationally efficient approach for designing laminated composites for In-Plane Stiffness...
...
Fibre-reinforced laminated composites are constructed layer-by-layer, allowing their material properties to be easily tailored relative to their isotropic counterparts (metallic alloys). Moreover, a composite structure with variable stiffness (VS) can be made by spatially tailoring stiffness across a laminate. This permits better load distribution within a structure, efficiently using a given material, and allow for lighter construction. Typically, the design of VS structures follows a bi-level procedure: first, the structure’s stiffness distribution is optimised using lamination parameters (LPs), and second, the stacking sequences (SS) of differently oriented fibres are designed to achieve the desired stiffness distribution. However, the designs envisioned using LPs are only sometimes exactly matched by the designed SS. The shortcomings faced during this conversion are called the ’Inverse Problem’. Upon reviewing the existing techniques in literature to handle the inverse problem, it was understood that converting LPs into SS is challenging without incurring substantial computational costs or imposing significant restrictions on allowable fibre orientations in the design. Such restrictions tend to under-utilise the directional properties of the fibres. Thus, there was a need to explore and develop better stiffness design methods for laminated composites by bridging the gap between LPs and SS in a computationally efficient way.
In response to this challenge, a hierarchical design framework was proposed to handle the Inverse Problem. Diverging from conventional methods that directly design the SS and try to match a given set of LPs, this framework divides the problem into two distinct stages. Initially, the focus is to use the In-Plane LPs and determine the number of layers in each orientation within a laminate, also called the Fibre Angle Distribution (FAD). Subsequently, the FAD serves as an interim solution, and the SS can be designed by using the Out-of-Plane LPs along with it. This problem partitioning enhances computational efficiency and offers potential benefits in solving the Inverse Problem more effectively. Given the time constraints inherent to a master thesis, the primary undertaking of this study was to efficiently design the FAD while accommodating a wide range of possible fibre orientations. To address this, a novel method using the Fast-Fourier Transform (FFT) was developed to facilitate FAD design with ply angle multiples of 15° (or [∆15° ] = [0, ±15, ±30, ±45, ±60, ±75, 90]). Moreover, empirical guidelines for SS design, such as the Symmetry and Balancing rule, were incorporated into the design step.
The primary contribution of this report is introducing an FFT-based method to design FADs, a novel addition to the existing body of research. Upon extensive testing, it was shown that this implementation could convert LPs into multiple unique FAD solutions in less than 0.3 seconds on a regular office laptop, outperforming other methods in literature by at least ten times. Furthermore, the implementation was also used to demonstrate the benefits of designing laminated composites using [∆15°] over the conventional [∆45°] orientations (or [0, ±45, 90]). In light of the positive results, it is pointed out that the In-Plane LPs (and consequently the In-Plane stiffness) are known to be sensitive only to the FAD and not their SS. As such, the readers of this thesis are presented with a very computationally efficient approach for designing laminated composites for In-Plane Stiffness...
In response to this challenge, a hierarchical design framework was proposed to handle the Inverse Problem. Diverging from conventional methods that directly design the SS and try to match a given set of LPs, this framework divides the problem into two distinct stages. Initially, the focus is to use the In-Plane LPs and determine the number of layers in each orientation within a laminate, also called the Fibre Angle Distribution (FAD). Subsequently, the FAD serves as an interim solution, and the SS can be designed by using the Out-of-Plane LPs along with it. This problem partitioning enhances computational efficiency and offers potential benefits in solving the Inverse Problem more effectively. Given the time constraints inherent to a master thesis, the primary undertaking of this study was to efficiently design the FAD while accommodating a wide range of possible fibre orientations. To address this, a novel method using the Fast-Fourier Transform (FFT) was developed to facilitate FAD design with ply angle multiples of 15° (or [∆15° ] = [0, ±15, ±30, ±45, ±60, ±75, 90]). Moreover, empirical guidelines for SS design, such as the Symmetry and Balancing rule, were incorporated into the design step.
The primary contribution of this report is introducing an FFT-based method to design FADs, a novel addition to the existing body of research. Upon extensive testing, it was shown that this implementation could convert LPs into multiple unique FAD solutions in less than 0.3 seconds on a regular office laptop, outperforming other methods in literature by at least ten times. Furthermore, the implementation was also used to demonstrate the benefits of designing laminated composites using [∆15°] over the conventional [∆45°] orientations (or [0, ±45, 90]). In light of the positive results, it is pointed out that the In-Plane LPs (and consequently the In-Plane stiffness) are known to be sensitive only to the FAD and not their SS. As such, the readers of this thesis are presented with a very computationally efficient approach for designing laminated composites for In-Plane Stiffness...
The use of Grid-stiffened structures has been increasing in the space industry over the last decade. Thanks to it is high specific mechanical properties and excellent damage tolerance characteristics, researchers also started looking into the potential of using them for aeronautical applications. However, not much has been done to investigate the reduction in strength after barely visible impact damage and visible impact damage, and how much energy is needed to cause such damage, which is an important requirement for aircraft certification. In this project, a grid-stiffened replica of an existing reference cargo drone is created, and the design is tested to check its compliance with the requirements. This involved modelling of the design, manufacturing of test coupons, impact and mechanical tests, and model validation. The results showed a possible weight reduction, with no major reduction in strength observed with impacting using the cut-off energy level set by the manufacturer.
...
The use of Grid-stiffened structures has been increasing in the space industry over the last decade. Thanks to it is high specific mechanical properties and excellent damage tolerance characteristics, researchers also started looking into the potential of using them for aeronautical applications. However, not much has been done to investigate the reduction in strength after barely visible impact damage and visible impact damage, and how much energy is needed to cause such damage, which is an important requirement for aircraft certification. In this project, a grid-stiffened replica of an existing reference cargo drone is created, and the design is tested to check its compliance with the requirements. This involved modelling of the design, manufacturing of test coupons, impact and mechanical tests, and model validation. The results showed a possible weight reduction, with no major reduction in strength observed with impacting using the cut-off energy level set by the manufacturer.
Deconsolidation of thermoplastic prepreg tapes during the heating phase of LAFP
An experimental investigation into the effect of a resin-rich surface and tape pre-tension
Laser-assisted automated fiber placement (LAFP) is a promising additive manufacturing technique for the production of large aerospace components. Thermoplastic prepreg tapes will be placed ply-by-ply on top of a mould and in-situ consolidated. This is beneficial for higher production rates. The mechanical performance of thermoplastic composite laminates is highly dependent on the consolidation quality. One of the quality indicators is the maximum allowable void content after LAFP-manufacturing, which still exceeds the 1% limit for aerospace standards. Challenges remain before LAFP can be completely industrialized with the desired throughput and still obtaining the minimum required quality.
During the heating phase the material is heated to a processing temperature of around 400 ±C within a very short heating time (0.2-0.8s) and no pressure application. As a result of rapid heating, interconnected mechanisms can occur which affect the final consolidation quality. These mechanisms and their relation with processing parameters need to be understood to achieve a high quality laminate manufactured by LAFP. Previous research at Delft University of Technology has shown that deconsolidation phenomena, such as decompaction of the fiber reinforcement network, waviness formation and void thermal growth occur during the heating phase. This will give rise to an increase in void content, surface roughness, out-of-plane deformation and dimensional changes. However, this research did not include tape pre-tension in the experimental setup which is the main component of a LAFP tape placement head. Also, thermoplastic prepreg tapes with a resin-rich surface have been suggested in literature to contribute to a higher consolidation quality of the final laminate. The effect on deconsolidation of thermoplastic prepreg tapes with resin-rich surface during the rapid heating phase is not known yet.
Therefore, the focus of this study is on the effect of a resin-rich surface and tape pre-tension on the deconsolidation response during the heating phase of LAFP. Deconsolidation was quantified through the following response variables (output): surface roughness, maximum out-of-plane deformation, void content, thickness increase and arc-length increase. The results showed that the processing parameters (heating time, heated spot length, resin-richness and tape pre-tension) affect the deconsolidation response through similar interlinked mechanisms as were observed before. However, it has been demonstrated that the mechanisms affecting the deconsolidation response are different due to the presence of a resin-rich surface and as a result of tape pre-tension.
Suprem resin-rich thermoplastic prepreg tapes have a great potential to be used together with the LAFP- process. It has been shown that a resin-rich surface contributes to significantly less decompaction of the fiber reinforcement network. This resulted in less surface roughness and less out-of-plane deformation after the heating phase. Because the fibers are surrounded by resin, no dry fibers are popping-out of the heated surface. The smoother and more resin-rich surface are beneficial for intimate contact development during the consolidation phase of LAFP. It is therefore expected that a higher degree of effective intimate contact can be reached with Suprem resin-rich tapes which is favourable for a higher final quality of a LAFP-manufactured laminate.
Laser heating experiments were performed with three levels of tape pre-tension: 5N, 10N and 15N. Increasing the level of tape pre-tension improved the contact between the tape and the surface of the tool. Since the tool worked as a heat sink in this case, it was shown that local heat absorption occurred only at locations where fiber clusters were present. Increasing the tape pre-tension level towards 10N and 15N seem to be disadvantageous for the LAFP-process. Large out-of-plane deformation was observed and the temperature data showed a highly non-uniform temperature across the simulated nip-point (for 10N and 15N) which is unfavourable for intimate contact development during the consolidation phase of the LAFP-process. It is therefore expected that the optimum pre-tension level lies around 5N. It is assumed that global out-of- plane decompaction and global surface roughness increase will remain lower if a low (5N) pre-tension force is applied.
...
During the heating phase the material is heated to a processing temperature of around 400 ±C within a very short heating time (0.2-0.8s) and no pressure application. As a result of rapid heating, interconnected mechanisms can occur which affect the final consolidation quality. These mechanisms and their relation with processing parameters need to be understood to achieve a high quality laminate manufactured by LAFP. Previous research at Delft University of Technology has shown that deconsolidation phenomena, such as decompaction of the fiber reinforcement network, waviness formation and void thermal growth occur during the heating phase. This will give rise to an increase in void content, surface roughness, out-of-plane deformation and dimensional changes. However, this research did not include tape pre-tension in the experimental setup which is the main component of a LAFP tape placement head. Also, thermoplastic prepreg tapes with a resin-rich surface have been suggested in literature to contribute to a higher consolidation quality of the final laminate. The effect on deconsolidation of thermoplastic prepreg tapes with resin-rich surface during the rapid heating phase is not known yet.
Therefore, the focus of this study is on the effect of a resin-rich surface and tape pre-tension on the deconsolidation response during the heating phase of LAFP. Deconsolidation was quantified through the following response variables (output): surface roughness, maximum out-of-plane deformation, void content, thickness increase and arc-length increase. The results showed that the processing parameters (heating time, heated spot length, resin-richness and tape pre-tension) affect the deconsolidation response through similar interlinked mechanisms as were observed before. However, it has been demonstrated that the mechanisms affecting the deconsolidation response are different due to the presence of a resin-rich surface and as a result of tape pre-tension.
Suprem resin-rich thermoplastic prepreg tapes have a great potential to be used together with the LAFP- process. It has been shown that a resin-rich surface contributes to significantly less decompaction of the fiber reinforcement network. This resulted in less surface roughness and less out-of-plane deformation after the heating phase. Because the fibers are surrounded by resin, no dry fibers are popping-out of the heated surface. The smoother and more resin-rich surface are beneficial for intimate contact development during the consolidation phase of LAFP. It is therefore expected that a higher degree of effective intimate contact can be reached with Suprem resin-rich tapes which is favourable for a higher final quality of a LAFP-manufactured laminate.
Laser heating experiments were performed with three levels of tape pre-tension: 5N, 10N and 15N. Increasing the level of tape pre-tension improved the contact between the tape and the surface of the tool. Since the tool worked as a heat sink in this case, it was shown that local heat absorption occurred only at locations where fiber clusters were present. Increasing the tape pre-tension level towards 10N and 15N seem to be disadvantageous for the LAFP-process. Large out-of-plane deformation was observed and the temperature data showed a highly non-uniform temperature across the simulated nip-point (for 10N and 15N) which is unfavourable for intimate contact development during the consolidation phase of the LAFP-process. It is therefore expected that the optimum pre-tension level lies around 5N. It is assumed that global out-of- plane decompaction and global surface roughness increase will remain lower if a low (5N) pre-tension force is applied.
...
Laser-assisted automated fiber placement (LAFP) is a promising additive manufacturing technique for the production of large aerospace components. Thermoplastic prepreg tapes will be placed ply-by-ply on top of a mould and in-situ consolidated. This is beneficial for higher production rates. The mechanical performance of thermoplastic composite laminates is highly dependent on the consolidation quality. One of the quality indicators is the maximum allowable void content after LAFP-manufacturing, which still exceeds the 1% limit for aerospace standards. Challenges remain before LAFP can be completely industrialized with the desired throughput and still obtaining the minimum required quality.
During the heating phase the material is heated to a processing temperature of around 400 ±C within a very short heating time (0.2-0.8s) and no pressure application. As a result of rapid heating, interconnected mechanisms can occur which affect the final consolidation quality. These mechanisms and their relation with processing parameters need to be understood to achieve a high quality laminate manufactured by LAFP. Previous research at Delft University of Technology has shown that deconsolidation phenomena, such as decompaction of the fiber reinforcement network, waviness formation and void thermal growth occur during the heating phase. This will give rise to an increase in void content, surface roughness, out-of-plane deformation and dimensional changes. However, this research did not include tape pre-tension in the experimental setup which is the main component of a LAFP tape placement head. Also, thermoplastic prepreg tapes with a resin-rich surface have been suggested in literature to contribute to a higher consolidation quality of the final laminate. The effect on deconsolidation of thermoplastic prepreg tapes with resin-rich surface during the rapid heating phase is not known yet.
Therefore, the focus of this study is on the effect of a resin-rich surface and tape pre-tension on the deconsolidation response during the heating phase of LAFP. Deconsolidation was quantified through the following response variables (output): surface roughness, maximum out-of-plane deformation, void content, thickness increase and arc-length increase. The results showed that the processing parameters (heating time, heated spot length, resin-richness and tape pre-tension) affect the deconsolidation response through similar interlinked mechanisms as were observed before. However, it has been demonstrated that the mechanisms affecting the deconsolidation response are different due to the presence of a resin-rich surface and as a result of tape pre-tension.
Suprem resin-rich thermoplastic prepreg tapes have a great potential to be used together with the LAFP- process. It has been shown that a resin-rich surface contributes to significantly less decompaction of the fiber reinforcement network. This resulted in less surface roughness and less out-of-plane deformation after the heating phase. Because the fibers are surrounded by resin, no dry fibers are popping-out of the heated surface. The smoother and more resin-rich surface are beneficial for intimate contact development during the consolidation phase of LAFP. It is therefore expected that a higher degree of effective intimate contact can be reached with Suprem resin-rich tapes which is favourable for a higher final quality of a LAFP-manufactured laminate.
Laser heating experiments were performed with three levels of tape pre-tension: 5N, 10N and 15N. Increasing the level of tape pre-tension improved the contact between the tape and the surface of the tool. Since the tool worked as a heat sink in this case, it was shown that local heat absorption occurred only at locations where fiber clusters were present. Increasing the tape pre-tension level towards 10N and 15N seem to be disadvantageous for the LAFP-process. Large out-of-plane deformation was observed and the temperature data showed a highly non-uniform temperature across the simulated nip-point (for 10N and 15N) which is unfavourable for intimate contact development during the consolidation phase of the LAFP-process. It is therefore expected that the optimum pre-tension level lies around 5N. It is assumed that global out-of- plane decompaction and global surface roughness increase will remain lower if a low (5N) pre-tension force is applied.
During the heating phase the material is heated to a processing temperature of around 400 ±C within a very short heating time (0.2-0.8s) and no pressure application. As a result of rapid heating, interconnected mechanisms can occur which affect the final consolidation quality. These mechanisms and their relation with processing parameters need to be understood to achieve a high quality laminate manufactured by LAFP. Previous research at Delft University of Technology has shown that deconsolidation phenomena, such as decompaction of the fiber reinforcement network, waviness formation and void thermal growth occur during the heating phase. This will give rise to an increase in void content, surface roughness, out-of-plane deformation and dimensional changes. However, this research did not include tape pre-tension in the experimental setup which is the main component of a LAFP tape placement head. Also, thermoplastic prepreg tapes with a resin-rich surface have been suggested in literature to contribute to a higher consolidation quality of the final laminate. The effect on deconsolidation of thermoplastic prepreg tapes with resin-rich surface during the rapid heating phase is not known yet.
Therefore, the focus of this study is on the effect of a resin-rich surface and tape pre-tension on the deconsolidation response during the heating phase of LAFP. Deconsolidation was quantified through the following response variables (output): surface roughness, maximum out-of-plane deformation, void content, thickness increase and arc-length increase. The results showed that the processing parameters (heating time, heated spot length, resin-richness and tape pre-tension) affect the deconsolidation response through similar interlinked mechanisms as were observed before. However, it has been demonstrated that the mechanisms affecting the deconsolidation response are different due to the presence of a resin-rich surface and as a result of tape pre-tension.
Suprem resin-rich thermoplastic prepreg tapes have a great potential to be used together with the LAFP- process. It has been shown that a resin-rich surface contributes to significantly less decompaction of the fiber reinforcement network. This resulted in less surface roughness and less out-of-plane deformation after the heating phase. Because the fibers are surrounded by resin, no dry fibers are popping-out of the heated surface. The smoother and more resin-rich surface are beneficial for intimate contact development during the consolidation phase of LAFP. It is therefore expected that a higher degree of effective intimate contact can be reached with Suprem resin-rich tapes which is favourable for a higher final quality of a LAFP-manufactured laminate.
Laser heating experiments were performed with three levels of tape pre-tension: 5N, 10N and 15N. Increasing the level of tape pre-tension improved the contact between the tape and the surface of the tool. Since the tool worked as a heat sink in this case, it was shown that local heat absorption occurred only at locations where fiber clusters were present. Increasing the tape pre-tension level towards 10N and 15N seem to be disadvantageous for the LAFP-process. Large out-of-plane deformation was observed and the temperature data showed a highly non-uniform temperature across the simulated nip-point (for 10N and 15N) which is unfavourable for intimate contact development during the consolidation phase of the LAFP-process. It is therefore expected that the optimum pre-tension level lies around 5N. It is assumed that global out-of- plane decompaction and global surface roughness increase will remain lower if a low (5N) pre-tension force is applied.