This study investigates the influence of nitrogen incorporation on the high temperature oxidation behaviour of zirconium carbonitrides, with particular emphasis on the effect of carbon-to-nitrogen ratio. Oxidation of zirconium carbonitride consists of two chemical concurrent stages, crosschecked by characterisations. Besides the three compositions compared (ZrC/ZrC0.75N0.25/ZrC0.5N0.5), ZrC0.75N0.25 appears to perform the best from the criteria. ... Read more

Conduction welding offers a fastener-free joining method for next-generation composite fuselage structures, but the fatigue interlaminar behaviour of conduction-welded joints has not previously been characterised. This thesis aims to develop a fatigue characterisation methodology for conduction-welded AS4D/PEKK-FC thermoplastic joints that accounts for fibre bridging, and to determine whether a small ±5° interface offset can control bridging relative to a 0//0 interface configuration. Quasi-static and fatigue Mode I (DCB) and Mode II (ENF) tests were conducted across both configurations and multiple severities, with fatigue crack growth characterised through the strain-energy-dissipation-based dU/dN framework to capture the evolving bridging zone. Results were compared against an autoclave-consolidated baseline to isolate the effect of welding, to establish whether the offset achieves its intended bridging-suppression role, and to provide a basis for fatigue characterisation of welded thermoplastic joints. The results reveal counter-intuitive effects of the interface offset and of welding on both quasi-static and fatigue delamination behaviour. These findings give insight into the damage-tolerant design of welded thermoplastic primary structures ... Read more

Towbarless towing is increasingly used in general-aviation ground handling, yet its fatigue implications for nose-wheel landing gear remain poorly understood. This thesis compares towbar and towbarless towing for representative aft-folding tricycle nose-wheel landing gear configurations in the 4,000–9,000 kg MTOM class. A sequential methodology combined in-field towing measurements, reconstruction of towbarless manoeuvre histories, staged global-local finite-element analysis, and laboratory fatigue testing. For the investigated configuration, this led to substantially higher fatigue severity and materially shorter measured life under towbarless-derived spectra, although overload sequence effects may partly moderate the net penalty in mixed operations. ... Read more

Fatigue life estimation under variable loading has been a central research topic for decades, with spectral methods such as the Tovo–Benasciutti approach gaining popularity due to their computational efficiency compared to time-domain techniques, such as Rainflow Counting. This thesis investigates the application of the Tovo–Benasciutti spectral method for estimating total fatigue damage in critical wind turbine components, including blades and tower structures. In particular, it examines an improved formulation incorporating a modified weighting factor designed to enhance prediction accuracy and robustness under varying loading conditions. This newer formulation is hereafter referred to as the Improved Tovo–Benasciutti (TB) method. The main limitation of the Improved Tovo–Benasciutti method is the absence of a general mathematical expression for the weighting factor used to calculate the total fatigue damage. Therefore, the main objective of this thesis is to develop and evaluate an analytical expression of the weighting factor, that is accurate, robust and applicable across a wide range of different loading spectra, enabling reliable and computationally efficient fatigue damage estimation for offshore wind turbine structures. To achieve this, this thesis starts by introducing all the necessary theoretical framework of fatigue damage estimation in the frequency domain and the Tovo–Benasciutti spectral method as well as its Improved version. The new weighting factor of the Improved TB method is investigated by deriving a new analytical expression for the weighting factor through curve fitting of fatigue data from wind turbine components for different types of loading. The data are prepared and analyzed using polynomial curve fitting of varying orders to identify the best-fit model, with the process implemented and verified using specific statistical tools such as the Goodness of Fit parameters. Additionally, the Damage Equivalent Load (DEL) approach is applied to complement and validate a similar weighting factor. The study found that the weighting factor of the Improved Tovo–Benasciutti method exhibits a clear polynomial behavior, with a second-order polynomial providing the best overall fit. Lower-order polynomials did not achieve satisfactory accuracy, while higher-order polynomials, although potentially better for specific datasets (better results of Goodness of Fit parameters), often performed poorly across different loading spectra. Therefore, the second-order formulation strikes the optimal balance between accuracy and general applicability, making it suitable for reliable fatigue damage estimation across a variety of conditions. These results are significant because they provide a simpler and more robust method to calculate the total fatigue damage or the Damage Equivalent Load of key wind turbine components, such as blades and towers, without relying on manual counting or other complex procedures. In cases where the stress process generating the load is known, estimating the damage requires only a few straightforward calculations. ... Read more

As part of the development of primary adhesively bonded joints, there is a desire for an element-level in-plane shear strength test of such joints. Due to its large gauge size and its theoretical ability to apply a state of uniform pure shear to the specimen, the picture frame shear test is a promising test for this application. However, use of the picture frame test as a strength test of joints was previously unexplored. Therefore, this study serves as an initial investigation and development of such a test, with the aim of developing an experimental testing procedure resulting in reliable and repeatable tests and the ability to detect damage initiation in the joint. Furthermore, accompanying finite element models correlated to the behaviour of the fixture and results of the experimental jointed tests are developed.

The mechanics and adverse effects of the fixture were first characterised. Characteristics of fixture symmetry, mechanism friction, specimen strain state, and fixture deformation were considered. These properties were studied primarily through tests with solid flat plate metal specimens, without a joint, and finite element analysis. Measurements of the experimental tests were made using test bench data, Digital Image Correlation (DIC), and strain gauges. Additionally, a detailed finite element model of the entire picture frame fixture was developed in Abaqus and correlated to the results of the characterisation tests. This model was used to further characterise the mechanics of the fixture.

Secondly, a comprehensive testing procedure was developed aimed at maximising the reliability and repeatability of tests. Minimisation of slip, minimisation of play in the fixture mechanism, and optimalisation of the measurement setup were considered. The developed testing procedure was validated through tests with jointed specimens. In these tests, single lap shear, thick adherend, and foil-defect specimen configurations were included. The results of these test are compared to those of finite element analysis.

Characterisation of the fixture behaviour demonstrates that the test does not succeed in imposing a strain state of uniform pure shear on the specimen, though this strain state is approached at the centre of the test section. It is shown that fixture deformation is the main driver behind this non-idealised fixture behaviour, brought on by the high stiffness of the metal specimens. The resulting peaks of tearing stress at the top and bottom corners greatly reduce shear strain purity in those areas and were correctly predicted by the detailed fixture finite element model.

The developed testing procedure drastically improved repeatability of the test, primarily through the inclusion of a preload cycle and the reduction of slip. The results of the consequent jointed specimen tests demonstrate limited scatter in the results and limited ability to detect the early onset of damage. Strain gauges perform better than DIC at detecting damage, though they remain limited to analysis of a single location. Additionally, analysis of numerical and experimental tests with thick adherend specimens of varying thickness revealed that the thickness of the specimen influences the failure load of the specimen, instead of solely the strength of the adhesive joint.

Given the findings of the present research, the picture frame shear test is concluded to be a fundamentally complex test for the shear strength of adhesively bonded joints. Nevertheless, reliability and repeatability were greatly improved through the development of a comprehensive test procedure. Therefore, the picture frame test remains a promising element-level shear test for larger-scale tests of bonded joints, such as those with foil-defects. ... Read more

Evaluating fracture toughness in composite structures is challenging when material properties are unknown and residual stresses are present, such as aged or repurposed wind turbine blades. This thesis validates stiffness- and curvature-based analytical J-integral formulations for double cantilever beam (DCB) specimens under these conditions. The analytical methods compute the J-integral along the external boundaries, and their accuracy was assessed by comparison with local J-integral values evaluated around the crack tip in finite element models. A surface-based bilinear cohesive zone was implemented in three finite element models: a single-material baseline, a multilayer configuration, and a multilayer model incorporating residual stresses.

The analytical methods showed strong agreement with local J-integral results across a range of load cases, with errors typically below 2% of the fracture toughness. Increased stiffness in the multilayer model lengthened the cohesive zone and improved accuracy by aligning more closely with the analytical assumptions. In the residual stress model, the auxiliary problem, free of residual stresses, accurately reproduced the real problem’s crack plane opening profiles for variations in temperature, stiffness, thermal expansion coefficient, and mechanical loading. Validating the analytical expressions for the auxiliary case--combined with opening profile consistency and the assumption of a potential-function-based J-integral--validated their applicability to the real problem.

These results confirm that the analytical J-integral formulations can accurately estimate the energy release rate when material properties are unknown and residual stresses are present, enabling reliable structural integrity assessments of composite structures. ... Read more

Amid the global energy transition, offshore wind energy has been positioned as a pivotal technology,
utilizing both bottom-fixed (BOWT) and floating offshore wind turbine (FOWT) configurations. Offshore wind turbines face high operation and maintenance (O&M) costs, representing up to 35% of the levelized cost of energy (LCOE), with the drivetrain and especially the main bearing (MB) being among the most failure-prone components, leading to long downtime and reduced energy production. In floating wind turbines, main bearings experience additional challenges due to combined wind and wave loads and the platform motions.

This research investigates the differences in loading and fatigue damage of the upwind main bearing (MB1) in a monopile BOWT and semi-submersible FOWT, both based on the DTU 10 MW reference wind turbine (RWT). Using OpenFAST simulations under Design Load Case (DLC) 1.2, according to Standard IEC 61400-1: 2019, this study calculates the MB1 loads for both wind turbine configurations, at a North Sea location east of Scotland, with a water depth of 60 m. Simulations are conducted across the full operating wind speed range (4.5 - 24.5 m/s), and loads are calculated through an analytical formulation. The axial and radial loads of MB1 are post-processed according to Standard ISO 281:2007, to compute the dynamic equivalent load, which is used to evaluate the fatigue damage of the main bearings. The Load Duration Distribution (LDD) method, which defines the load cycles, and the site-specific Weibull distribution are used to estimate the hourly fatigue damage of MB1, which is then extrapolated to a 20-year design lifetime, yielding the time-dependent Remaining Useful Life (RUL).

A comparative analysis of MB1 load means and standard deviations reveals that, for the examined
drivetrain configuration, radial loads dominate the main bearing loading and fatigue contribution in both configurations. Specifically, for the FOWT, they represent 74 to 90% of the dynamic equivalent load and 39 to 72% of the fatigue damage across all operating wind speeds. For the BOWT configuration, the corresponding ranges are 75 to 90% and 42 to 72%, respectively. MB1 of the FOWT experiences higher axial and radial loads, mainly due to platform surge and pitch motions, which increase rotor thrust and amplify the axial loading, as well as platform sway, roll and yaw motions that mainly amplify the radial loads. The largest load differences between FOWT and BOWT are encountered at around the rated wind speed of 11.4 m/s, and they peak at 13.5 m/s, where the FOWT axial load is 13.17% higher than in the BOWT.

Over the 20-year lifetime, MB1 in the FOWT accumulates 5.03% more fatigue damage, failing at 11.98 years, compared to 12.58 years for the BOWT. BOWT MB1 exhibits higher fatigue damage in the below-rated wind speed region, especially for wind speeds below 8.5 m/s. For aligned wind-wave cases, the reduction in the MB1 damage for both FOWTs and BOWTs is only 0.6% compared to realistic (misaligned) cases, indicating that, the wind-wave misalignment for wind speeds in the above-rated region, has a slight influence on the cumulative damage. A parametric sensitivity analysis shows that turbulence intensity is the most influential environmental factor: a change from Class A to C results in up to 10% reduction in MB1 damage. Among system parameters, platform mass significantly affects MB1 fatigue in FOWTs, with a 20% increase in mass reducing damage by 5.6%, while a 20% decrease increases it by 4.4%.

Overall, the findings of this thesis prove that MB loads in floating wind turbines are subjected to greater complexity and amplification, due to platform motions and aerodynamic, hydrodynamic, structural and control coupling. This results in increased fatigue damage and indicates the importance of advanced fatigue-aware, system-integrated control strategies that consider the platform dynamics to ensure main bearing reliability in future floating wind developments. The study concludes with recommendations for future work, including the validation of the results with field data, the exploration of alternative drivetrain layouts and the implementation of advanced control schemes to mitigate the impact of the platform motion. ... Read more

Carbon-carbon composites are strategic materials for space propulsion and thermal protection, but require oxidation protection. This study explores the feasibility of cold spraying metals and MAX phases onto carbon-carbon substrates for this purpose.

A porous liquid resin infused composite and a CVI substrate were tested to assess the influence of production method. Successful deposition of aluminium and a 25 vol.% MAX phase–Al mixture were achieved on the porous substrates. SEM-EDS analysis identified micro- and macro-scale mechanical bonding. For the CVI substrate, microstructural conditions enabling deposition were found. The discrepancy is attributed to the microstructural anisotropy of graphite and the production methods.

The role of substrate impact toughness was examined by comparing the Charpy impact toughness of five substrates with their degree of deposition, but no clear link was found. Additionally, an adapted analytical model for the critical velocity is presented. ... Read more

This thesis investigates the feasibility of detecting dents on aircraft structures using drones. It was initiated in collaboration with Mainblades, a company specializing in aircraft inspections, and aims to integrate dent detection into its existing platform. Identifying dents for Maintenance, Repair, and Overhaul (MRO) operators is critical because their presence in aircraft structures can compromise structural integrity, negatively impact fatigue and aerodynamic performance, and dents could be accompanied by cracks due to excessive plastic deformation.

The study addresses this goal through a three-part approach. The first part focuses on identifying and adapting suitable sensors for drone-based inspection. Secondly, the thesis investigates the performance of 8tree’s dentCHECK sensor to understand the feasibility of drone-based deployment. The selected sensor, 8tree’s dentCHECK, was tested in controlled experiments using an ABB robotic arm to replicate realistic drone movements and evaluate sensor response under translational and angular disturbances. Finally, a custom machine learning algorithm is developed to classify dents from synthetic point clouds. The algorithm is designed to be drone-agnostic and capable of handling varying data quality from different sensors available on the market.

The results indicate that while structured light sensors, such as 8tree's dentCHECK, show significant promise for drone deployment, their performance is mainly limited by the drone's drift. For the vibrations, linear vibrations in the Z direction (i.e., vertical motion as the drone moves up and down) are critical, while angular vibrations for the roll and pitch were inconclusive. Nevertheless, despite not meeting Boeing's full tolerance criteria (± 0.05 mm for depth and ± 1mm for length and width) when airborne, the 8tree sensor significantly outperforms manual inspection methods in speed and measurement consistency, despite not yet meeting Boeing’s tolerance criteria when airborne. It is a good benchmark for identification purposes that can be complemented and characterized manually with handheld scanners.

At the other end, synthetic point cloud datasets incorporating mathematical and FEM-based dent geometries with Gaussian and robot-collected drone noise profiles were used to train a U-Net-based machine learning model for automatic dent segmentation. The developed machine learning model demonstrated reliable identification for dents larger than 0.5 mm, although further data diversity is needed to enhance length and width predictions. Together, these findings demonstrate that integrating dent detection into autonomous drone platforms is both feasible and promising, laying the groundwork for future refinements in drone stability and data-driven detection methods. ... Read more

The world is facing the challenge of climate change, a problem acknowledged by many governments in the Paris Agreement. All sectors, including the construction industry, must take responsibility for reducing their environmental impact. Using renewable bio-based materials, such as flax fibre composites, offers a promising solution towards reducing the construction sector's environmental footprint. Flax fibre-reinforced polymer composites, which consist of flax fibres embedded in a polymer resin, have gained attention for their potential application in structural elements. However, the lack of standardized long-term performance data, for example, its creep behaviour, limits its broader use in structural applications.

Creep is the gradual deformation of materials under a constant load, which can also affect their strength over extended periods of time. Conventional creep tests are often considered impractical as they are time-consuming and costly. Accelerated creep tests, such as the stepped isostress method (SSM), can offer a faster alternative for predicting long-term creep behaviour. The SSM predicts the long-term creep behaviour of a material by incrementally increasing the applied stress while maintaining constant environmental conditions. This method requires only a single test sample and approximately one day of testing to generate a prediction of the long-term creep master curve for a specific reference stress. However, the SSM is an empirical method that has not yet been standardized in any design codes. This research assesses the reliability of the SSM for flax fibre-reinforced polymer composites through an experimental study. Aiming to support the development of sustainable construction practices and encourage using renewable materials in the building industry.

To validate the SSM predictions, long-term creep tests are conducted using a custom-designed tensile creep test setup, supported by quasi-static tensile tests. Additionally, three different composite laminate configurations are compared, demonstrating the fibres' function as the main load-bearing component in the composite. One of the main findings of this research is the material's sensitivity to temperature. Although the SSM predictions produce creep master curves, these predictions are conservative compared to the long-term creep test results. Moreover, the data handling procedure of the SSM, particularly the horizontal shift, is extremely sensitive and partially subjective. Based on the findings, the current form of the SSM is deemed insufficiently reliable to replace conventional creep tests for structural material characterization. However, its potential as a highly time-efficient testing method should not be overlooked. With further refinement in both the experimental procedure and data processing procedure, SSM could become a more reliable approach, making further research on standardizing its methodology essential.

This research urges the need for further research to improve the reliability of the SSM and to standardize its data processing steps. An essential step in this development is understanding the relationship between the activation volume and the applied stress, which may be material-specific. Clarifying this relationship could help reduce the sensitivity and subjectivity of the horizontal shifting process in the SSM data analysis. Additionally, this research aims to inspire continued investigation into bio-based construction materials, contributing to a more sustainable building industry. ... Read more

The need for better fuel efficiency within the growing aviation sector is increasing every day. Meanwhile conventional aircraft are reaching an asymptote regarding fuel efficiency and therefore the focus switches to unconventional aircraft. These aircraft have unique challenges whereof one is the structural connection between engine and aircraft. In this research an automated design methodology is developed which allows for optimization of such a structure. This design methodology uses a parameterised computer-aided design (CAD) connected to Python, which can automatically alter the parameters and make a finite element (FE) model of the design and subsequently run the desired simulations. The
results of the simulations are extracted and post-processed into margins of safety (MoS) using the direct results or (semi-)analytical equations for each individual component. In total five failure modes are assessed, which are Von Mises stress, displacement of engine, crippling, panel buckling and column buckling. The most critical MoS and the relation to the parameters is used in combination with an adaptive damping parameter to optimize each individual part. This design methodology is tested using the Flying-V, which is a novel aircraft configuration in the shape of a V, as a reference aircraft. Therefore, first the requirements, critical loadcases, loads and initial design have been developed based on regulations and previous research regarding the Flying-V and have been used as input for the CAD model and simulations. The design methodology has proven not be perfect as regions with stress concentration cause the minimum MoS to be below zero for all iterations and could not be solved within the time frame of this research. However, the design methodology has proven to be significantly faster compared to a design of experiments used in previous research. Overall it should be noted that there are still many other limitations regarding this method. These limitations are related to the limited amount of failure modes used and the fact that it still requires some kind of Design of Experiment, which is time consuming. The final mass of an optimized engine mounting structure is estimated to be approximately 40% higher than the design methodology used in previous research, which is due to the higher amount of details in the design as well as the significant higher estimated mass for the engine. This means that the current estimated mass for the full engine mounting structure including landing gear and engine is approximately 22150 kg compared to the estimated 17360 kg before.

Furthermore, an assessment of the influence of the developed design on the crashworthiness of the Flying-V has been done. This assessment focuses on one of the four main criteria regarding occupant protection, namely the guarantee of acceptable acceleration and loads sustained by the occupants. This is done by connecting the engine mounting structure to the developed wing-fuselage structure of the Flying-V with a tie connection. In order to compare different cases a new variable has been created, which is based on empirical equations developed by the U.S. Air Force. In addition, a new method based on moments of inertia has been developed to take the rest of the aircraft into account without increasing the size and thus computational effort of the simulation. To validate this method, the wing-fuselage structure has first been extended to fit the structure and results are compared to previous research. This has shown large, unexpected, differences which could not be explained even with thorough investigation. Applying the inertia method to the extended fuselage has shown small but explainable and expected differences, validating that the method has no unforeseen effects on further results. Simulations using this inertia method and the wing-fuselage with full engine mounting structure shows an improved performance in crashworthiness as the engine mount also absorbs some of the energy. Similar simulation without engine shows an even more improved performance, meaning that engine separation is of interest for the Flying-V. Initial further-reaching investigation into engine separation, using a point-mass model, shows that engine separation might be possible without impacting the Flying-V for crashes without any initial yaw, pitch or roll angle. ... Read more

This thesis project focuses on the evaluation of the Extended Finite Element Model (XFEM) for modelling skin-stringer separation in thermoplastic composite stiffened panels. The study uses test results from a reference study in the literature on a PEKK-FC carbon composite panel with three stringers joined to the skin using a short-fibre reinforced butt joint. The modelling strategy was developed using Double Cantilever Beam (DCB) specimens. The XFEM-based panel models were then developed using two different damage initiation criteria: Quadratic Stress Criterion (QUADS) and Maximum Principal Stress Criterion (MAXPS). These models were compared with the Virtual Crack Closure Technique (VCCT) model from the reference study. The project also addresses the challenges posed by the crack tip impingement problem in the XFEM-based model with MAXPS and tests different strategies to overcome it. A proof of concept is provided for using the UDMGINI subroutine to address these challenges, setting the stage for future studies and algorithm development. ... Read more

In this thesis project, the out-of-autoclave process for the manufacturing of a thermoplastic wing rib was analyzed, with the goal of improving the robustness of the manufacturing process. The C/PEKK wing rib was manufactured by assembling the constituent laminates in an out-of-autoclave tool subjected to two-side heating.

The quality of the manufactured rib was evaluated using dimensional and non-destructive inspection (NDI) criteria to assess the consistency and robustness of the manufacturing process. Thermal behavior of the consolidation tool was analyzed to identify any temperature outliers that could negatively impact the rib quality. Additionally, the movement of the tool block in contact with the rib foot was monitored to determine if there was any correlation between the tool block movement and the quality of the rib foot.

For the T-joint configuration on the rib, two types of fillers were tested under tension loading: a lay-up filler and a 3D printed filler. The large radius of the web flanges in the rib made the filler a non-inspectable region in the rib, which could not be evaluated for defects using conventional NDI techniques. Therefore, the role of defects around the filler in the wing rib was examined by simulating defects in the filler/flange radius junction. Mechanical testing under tension loading was performed on samples made from the manufactured ribs to assess the severity of performance reduction due to simulated defects.

The research concluded that temperature distribution in the tool and part was not responsible for non-compliant parts; however, the pressure application required optimization. No significant correlation between the movement of the foot tool block and the achieved part quality was identified. The presence of defects around the filler resulted in a reduction in performance under tension loading, highlighting the importance of minimizing defects at the radius/filler interface.
... Read more

An analytical model for the onset and growth of delaminations induced by matrix cracks under static loading is proposed. The model introduces four distinct configurations of delaminations originating from crack tips, which are applicable to cross-ply and general symmetrical laminates subjected to tension or tension + shear.

The delamination onset method employs Kassapoglou & Socci’s analytical crack propagation model to describe matrix cracks within the laminate. This leads to closed-form expressions for the Strain Energy Release Rate associated with both crack propagation and the initiation of delaminations at crack tips. Both expressions are used to predict the delamination onset load and crack spacing. Predictions show excellent agreement with test results for cross-plies under
tension.

Regarding the delamination growth model, it assumes a constant distance between cracks during growth (crack saturation). Experimentally-obtained Mode II interlaminar fracture toughness equation and closed-form expressions for Strain Energy Release Rate for delamination growth are employed for predictions. The model overpredicts the initial delamination length but exhibits satisfactory agreement with test results for the delamination growth in cross-ply laminates under tension.

Combining all 3 models allows for a comprehensive prediction of crack propagation, delamination onset and delamination growth for general symmetrical laminates. This comprehensive approach enables the visualization of all relevant information in a single figure, providing a concise and informative representation of the damage processes. Moreover, the analytical model facilitates the construction of design curves to investigate delamination onset in detail. ... Read more

In composite structures, delamination damage is typically the most common failure mechanism. Accurately characterizing the delamination behaviour of composite laminates is therefore crucial for predicting the fatigue safe-life of such structures. To this end, double-cantilever beam (DCB) composite specimens are used to measure the interlaminar fracture toughness and delamination growth rate under cyclic Mode I loading.

In unidirectional (UD) composite laminates, delamination planes may exhibit fibre nesting, leading to the development of the fibre bridging effect during delamination growth. This effect, which resists delamination, significantly increases the apparent fracture toughness of the laminate. However, fibre bridging is usually insignificant in multidirectional (MD) laminates, where delamination occurs between plies with different fibre orientations. Nesting typically does not happen in MD laminates. As a result, MD laminates should not be designed using fatigue resistance data obtained from UD specimens without first accounting for the fibre bridging effect. Neglecting fibre bridging exclusion can result in an overestimation of delamination resistance, leading to unsafe failure predictions.

This research investigated methods to exclude the fibre bridging effect in cyclic Mode I experiments with UD composite specimens. Existing literature suggested different approaches to account for this effect, aiming to create a "zero-bridging" fatigue delamination resistance curve. The study examined methods such as cutting bridging fibres in-situ, constant-SERR experiments, specimen-specific extrapolation, and utilizing the Hartman-Schijve equation to describe fatigue delamination.

By examining different exclusion methods and understanding their limitations, this work contributed to enhancing the reliability of fatigue delamination predictions in composite specimens under laboratory conditions. This study compared methods to exclude the fibre bridging effect and assessed their merits in terms of ease of use, accuracy, and conservative predictions of delamination resistance. The results of this study suggest that a specimen-specific extrapolation method is a suitable approach to account for fibre bridging. ... Read more

A Helically wrapped structure is a hollow pipe structure that is made from a metal strip and the overlapping interfaces are adhesively joined. It is yet unknown how to determine the quasi-static strength and predict the fatigue life of this structure. A method was developed to determine whether either the adherend or the adhesive is critical to determine the strength and fatigue life. A machine was made to create the helically wrapped structure and tested for validation. It was found that the adherend is critical instead of the adhesive. ... Read more

To reduce greenhouse gas emissions, electrification is the biggest trend in the current-day automotive industry. In this transition, important topics are efficiency and weight reduction. This study focuses on the application of composite materials for the undershield of an automotive rechargeable energy storage system (REESS). In this application, the undershield forms the structural floor of a battery enclosure. The goal of the research was to determine if the homologation and safety demands for the undershield can be met with a composite-dominant design. Additionally, the goal was to determine if such a design can result in mass, cost, and emissions improvements compared to current metallic designs. Based on European and Chinese homologation documents, SAE standards, and demands by Volvo Cars, a set of relevant requirements was constructed. Based on these requirements, a sandwich design proposal was done and potentially suitable materials were identified. Multiple material configurations for the design were then verified based on the driving requirements. Lastly, a performance comparison was done of the mass, cost, and CO¬2 emissions of each configuration. The results indicate that it is feasible to meet the homologations and safety demands for the undershield of an automotive REESS with a composite sandwich design. Using polyester or phenolic glass fiber SMC for the top face of the sandwich can provide a successful fire protection barrier. PET foam was identified as a low emissions core material for improved impact protection in combination with steel or PP GMT bottom plate. Additionally, lower costs and emissions can be offered with steel configurations compared to current metallic designs. Alternatively, PP GMT configurations offer mass and emissions reductions. All in all, the research has succeeded in demonstrating the potential of composite or hybrid designs for the application in an automotive REESS for the improvement of mass, cost, and/or greenhouse gas emissions. ... Read more

The IsoTruss is an interesting continuous fibre reinforced polymer composite design which is closely related to open lattice composite structures, although it is currently aimed mostly at civil applications. While it currently lacks applicability to the aerospace sector, its production process is easier to automate (or change the shape input parameters on the fly) than open lattice structures, which rely on a fixed mandrel. Although automated compaction of these structures is a challenging problem, the structure can be optimised for any (static) load case if their production process could be automated, providing a range of applications where aerodynamic contour is not a requirement. To bridge the gap between the IsoTruss and other composite lattice structures, a conceptual design for an IsoTruss derivative, termed the "iso-truss", was envisioned. This raised the following research question: Can composite continuous fibre reinforced iso-truss structures be produced using a scalable and cost-effective automated manufacturing method? Through this thesis project, an affirmative answer to this question was sought. The focus of this thesis was on the investigation of all the aspects necessary for a production of this iso-truss structure, focusing on a process suited for later automation. Initial concepts using thermoset composites like the original IsoTruss could not solve the issue of automated compaction, as these would always rely on vacuum bagging to minimise the void content within the composite members. Automated vacuum bagging of the finished iso-truss structure was considered to be an infeasible solution due to the product's complexity. Through the use of thermoplastic composite materials however, the process steps could be split up and compaction could be moved to the very beginning of the process in the form of pultrusions. The shape of the resulting iso-truss was designed to make use of this process, providing a parameterised structure which could easily be optimised. The structure would make use of straight pultrusion rods, acting as the longitudinal members in the truss, and curved pultrusion rods, which would act as helical members. The iso-truss structure would be created by joining these members together at intersections by means of thermoplastic welding, forming a continuously produced open lattice tubular structure. With this, conceptual production of an iso-truss was based on three key steps: pultrusion, forming, and intersection welding. The pultrusion process was outsourced to an outside company, vDijk Pultrusion Products (DPP), due to its high required equipment cost. It was not the aim of this thesis to re-invent this process, as it has already reached a sufficient state of maturity. Nevertheless, the process has not yet been optimised for thermoplastic based composites, leaving room for improvement in this area. Materials for the concept were based on availability. Elium resin (a PMMA based thermoplastic polymer developed by Arkema) was chosen for its similarity to existing process materials and its direct availability from DPP. As reinforcement, a Toray T700SC type carbon fibre was used. By taking pultrusion out of the research scope, forming and welding were left as processes to demonstrate. A separate welding test was isolated to provide a way to perform standardisable tests. Simultaneously, a forming process demonstrator was designed, as well as a welding jig to demonstrate assembly of the iso-truss. For the execution of the welding and forming experiments, a single batch of material was ordered from DPP. The batch was the result of a first successful attempt to make rods of this diameter with this combination of materials. Upon arrival of the samples, several tests including SEM, TGA and DSC were performed to estimate important composite material parameters. From these tests, the fibre volume fraction could be estimated to be around 70%, while the onset of T_g was estimated to be 95degC by definition of maximum loss modulus. Additionally, several interesting phenomena were observed at elevated temperatures in the form of fibre kinking when bent and circumferential decompaction when twisted, indicative of a weak fibre-matrix interface. More indications of this limited fibre-matrix interface were obtained from SEM images, some of which showed small voids around fibres. DMA tests showed a steady decline of the material's (shear) stiffness over a large temperature range, including before the onset of glass transition. This could likely be attributed to the amorphous nature of the polymer. For the execution of joining experiments, a standard intersection layout was established, after which several joining techniques were considered to create these intersections. From these techniques, heated mould welding was selected as the best currently feasible option. After performing numerous experiments to fine-tune the method of heated mould welding, a standard setup could be designed around this technique. This setup addressed methods for heating, alignment of members to be welded, a compaction mechanism, physical test preparation and physical testing. Simultaneously, a detailed design of a helical winder demonstrator was made, which was prepared for production by DEMO. The design focused on the demonstration of the mechanical aspect, leaving heating as a secondary concern. Similarly, a more detailed concept of an assembly demonstrator was discussed and designed, although a production-ready design did not prove feasible within the time frame of this project. The main goal of the intersection welding experiments was to optimise the joint strength by optimising the processing conditions. Due to the limited resin volume content, PMMA foil and epoxy adhesive were used to locally increase resin volume, greatly improving the bond strength compared to initial samples. Using mould temperatures in between 180 to 200degC, a large degree of deformation could be achieved to maximise the joint area, creating mostly consistent intersection samples. Physical tests proved that the PMMA-based joints were very fragile, while epoxy based joints performed significantly better. Estimated shear strengths for the samples varied from 3MPa for some of the lower performing PMMA-based intersections which were believed to have sustained prior damage, to as high as 13MPa for the epoxy based intersections. For the buckling tests, maximum loads in between 2.1 to 3.2kN were observed. In the final buckling test setup, diagonal members did not increase the buckling load by forcing higher-mode buckling. Instead, they offered some support in post-buckling by resisting out-of-plane deformation. Failure modes in both shear and buckling tests were within the adherend, occurring as a combination of fibre break-out, member kinking and member splitting, more indications of a poor fibre-matrix interface. In summary, the experiments proved that the investigated welding method is feasible if the base material can be improved. As of writing, such improvements have already been reported by DPP through increased process stability and (likely) better fibre sizing. For the helical winding experiments, the main goal was to demonstrate the production process of heated forming of initially straight pultrusion samples into helices. The physical test setup was designed to offer continuous support of the members over their full length, realise a gradual increase in radius of curvature and allow torsional and axial forces to be introduced independently. For the resulting setup, heating of the central cylinder and introduction of heat through both contact and a heated air chamber proved most feasible within the time frame of this project. It did not prove feasible to produce carbon-Elium composite helices due to the current material limitations, that is the fibre kinking observed in previous tests. Instead, the process was demonstrated using pure PMMA rods at forming temperatures of around 90degC, which proved highly effective. The process showed the potential to create constant curvature helices with controllable helical angle, although more testing should be performed to achieve accurate and repeatable results. It is concluded that it is theoretically possible to produce continuous fibre reinforced iso-truss structures using a scalable and cost-effective automated manufacturing method. The process would rely on existing production capabilities which can easily be scaled up or down, without greatly affecting the cost of equipment. The pultrusion, helical shaping and intersection welding processes can all be easily automated using existing techniques to be able to produce an iso-truss tube of indefinite length. The only limitation of this manufacturing method would be the variation of process parameters within the same iso-truss sample. While it is theoretically possible to vary the radius of curvature of pultrusion dies during the pultrusion process, enabling the possibility of varying the outer diameter and the intersection node-to-node distance along the length of a tube, the current state-of-the-art does not accommodate such a design. While it would also be possible to vary the cross-sectional area of each member along their length, it is considered undesirable as it would negatively affect the fibre volume content for a pultrusion-based process. Instead, it is recommended to construct an iso-truss structure in stages, being connected by a structure that is to be envisioned and produced in future research. ... Read more

Mechanical air entrapment, void compression and void dissolution are the main mechanisms behind void formation in liquid composite molding. Mechanical air entrapment is induced by the highly non-uniform geometry at the meso- and micro-scale and the heterogeneous properties of the reinforcing material. This results in differences in the velocity profile through the porous- and free flow domains, which can lead to air being entrapped in slow-flowing domains, by resin flowing through the faster flowing domains. Depending on the competition between the viscous flow through the free flow domains and the capillary flow through the porous domains, voids occur either in the intra-bundle domain or in the inter-bundle domain, where the competition between the two flow types can be quantified by the capillary number. Once the voids are entrapped, they can change in size due to void compression and -dissolution, all which occur on considerably different timescales. An experimental set-up was developed which uses fast radiation curing to almost instantly cure the resin during injection and thus detach the different stages of void formation from each other. In this research the relationship between the mesoscale structure of a woven fabric and mesoscale void formation was investigated. The mesoscale structure is strongly related to void formation, as viscous flow is related to the inter-bundle domain size in the through-thickness direction of the preform and capillary flow is related to the mesoscale bundle porosity. As the bundle porosity is related to both the bundle permeability, and the capillary pressure, its influences on void formation were considered to be large, and thus bundle porosity became the main research topic. A 2D semi-analytical model based on mechanical air entrapment was constructed which was capable of determining the filling times of the intra- and inter-bundle domains on the mesoscale. A parameter called the competitive number was introduced which was related to these filling times. Once the competitive number was larger than 1, spherical inter-bundle voids should be formed, once it was equal to 1 no voids should be formed and once it was lower than 1 ellipsoidal intra-bundle voids should be formed. This model was validated experimentally. Flow behavior predicted by the model was compared to video footage of the flow front propagation at the surface of the preform at the macro- and mesoscale during multiple injections. Void volumes, types and locations obtained from the model were compared to voids observed in micro-CT scanned samples and were in agreement with each other. The validated model was eventually used to investigate the effects of bundle porosity on void formation. It was found that increased bundle porosity at a constant global preform porosity, leads to increased intra-bundle flow and decreased viscous flow, thus considerably influencing void formation by mechanical air entrapment. In short injection cycles with high macroscopic flow velocities, which can lead to intra-bundle voids, it could be beneficial to switch a fabric with a higher bundle porosity, as that would effectively reduce the intra-bundle void size. For injections with low macroscopic flow velocities, the opposite would apply, thus usage of fabrics with lower bundle porosities could help to reduce the inter-bundle void volumes. ... Read more