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. ... Read more

Lightweight wind turbine blades play a crucial role in improving wind turbine technol- ogy. They enforce lower loads on the overall system, making material and cost sav- ings possible. Also, increasingly longer blades can be used, enabling more efficient, economically viable turbines. This thesis investigates how the mass of a rotor blade can be reduced without compromising its structural integrity. The work is based on the OLW934 blade from Olsen Wings A/S. Its mass is reduced using a comprehensive optimization framework .

A structural and aeroelastic optimization process was applied using the gradient-based AESOpt algorithm, finite element analysis (FEM), and aeroelastic checks with HAWC2S. The framework reduced the blade’s mass by 35% while keeping its structural integrity. The redesigned blade was found with improved internal structure and layup. Since the optimization mainly focused on the structure, the aeroelastic performance was checked manually throughout the process. Due to stiffness assumptions in the tip re- gion, no definite validation on the aeroelastic stability of the turbine with the redesigned blade has been found yet, but the results indicate sufficiency.

This behaviour should be verified. Also, the internal structure setup, especially the spar caps and shear webs, should be studied in more detail, as only initial design con- siderations were explored. Additionally, varying initial layup regions and evaluating alternative material options could yield further improvements. Still, the results show that the structural optimization framework used can significantly reduce blade mass and can be applied to other designs. This supports the development of lighter blades and better turbine performance. ... Read more

This thesis explores the structural implications of utilising natural fibre-reinforced polymer composites (NFRCs) in wind turbine blades through a numerical investigation. To perform the research, the DTU 12.6 m research wind turbine blade is taken as case study. A sensitivity study is performed, investigating the impact of different material engineering constants on the blade's performance. A structural redesign of the DTU 12.6m blade with NFRCs followed to further investigate the implications of utilising bio-based materials. The findings reveal that the reduced mechanical properties of NFRCs restrict the direct replacement of traditional glass fibre composites. The structural redesign process showed that the torsional rigidity and material strengths are the primary drivers for the bio-based blade design. This differs from the conventional blade design where the tip deflection and therefore specific stiffnesses are the main drivers. In order to meet the design requirements the structural blade design will need to be rethought. ... Read more

Wind turbines suffer significantly from leading edge erosion due to water droplet impacts, leading to significant loss in annual energy production. This work studies the possibility of using structured surfaces inspired by erosion resistant organisms. This is done using a combination of experiments using pulsating jet erosion testing and simulations using a coupled SPH-FEM model. Conclusions are then drawn on the impact of the various studied parameters. ... Read more

This thesis investigates the use of Tow-Based Discontinuous Composite (TBDC) interleaves to enhance the mode I fracture toughness of adhesively bonded joints with Carbon Fiber Reinforced Polymer (CFRP) substrates. Aiming to improve joint safety by slowing crack propagation and facilitating less sudden failure, this study focuses on integrating interlaminar-toughened substrates to resist crack growth and enhance the fracture behavior in the joint's substrate. Two main research questions are addressed: the influence of TBDC interleaves on mode I fracture toughness of CFRP laminates and their subsequent effect when used in CFRP-based bonded joints.

For CFRP laminates, Double-Cantilever Beam (DCB) samples were tested across three configurations: a non-toughened baseline and two TBDC-toughened variants. Based on previous research, three DCB configurations identified as the most promising for leveraging TBDC toughening in adhesive joints were tested. The [90/45/-45/TBDC/0]s and [90/60/90/-60/TBDC/0]s laminate substrates were bonded with the low-toughness adhesive Araldite 2015-1, while the [0/TBDC/90_2/0]s substrate was bonded with AF 163-2U, a high-toughness adhesive.

TBDC-toughened CFRP laminates demonstrated up to 130% higher fracture toughness compared to non-toughened counterparts. This was due to TBDC material crack propagation mechanisms such as crack branching, deflection, and fiber bridging.

In adhesively bonded joints, TBDC interleaves in CFRP substrates enhanced the decay of fracture toughness in specimens where cracks deflected from the bond line into the substrate, leading to a less abrupt reduction after reaching peak values. Joints with low-toughness adhesive exhibited more than a 100% increase in crack length from peak fracture toughness to the final value compared to non-TBDC-toughened substrate joints. Meanwhile, joints with high-toughness adhesive demonstrated toughness values 150% to 750% greater than those observed in non-toughened configurations at comparable crack lengths.

These findings highlight the potential of TBDC interleaves to enhance joint toughness, presenting new pathways to improve the safety of composite bonded structures.
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The increasing focus on sustainable living and the need to reduce dependency on fossil fuels has led to a growing interest in renewable energy sources. Among these, the wind energy sector has not only experienced significant growth in terms of numbers but also in size. Larger turbines lead to more severe leading-edge erosion and further increase operation and maintenance costs. To mitigate this problem, the new advanced leading-edge protection has become vital in the wind energy sector. This thesis focuses on the evaluation of two polymer coating materials (PA and PD) using a Pulsating Jet Erosion Test (PJET) setup.

The first aim of this thesis is to propose a novel analysis method to address the issue of volume interdependence in the PJET. To achieve this, a concept called "equivalent velocity" is introduced. The equivalent velocity represents the velocity at which a spherical droplet should impact a surface to exert the same kinetic energy per impingement as the actual water slug moving at the impact velocity. By utilizing this concept, the velocity-number of impacts plot takes into account the volume interdependence in erosion experiments.

The second aim is to utilize the PJET to analyze the erosion behavior of PA and PD coatings. The investigation focuses on understanding the relationship between impact velocity and the number of impacts until the incubation period and the breakthrough. The incubation period refers to the interval until the damage is visible and the breakthrough is the moment until the filler underneath the coating is exposed. Additionally, the erosion damage progression of the coatings was analyzed, and the lifetime prediction was evaluated using an existing long-term leading-edge rain erosion model.

The experimental results revealed that the ductile material (PD) exhibits a longer resistance to erosion compared to the stiff material (PA), with the mean number of impacts until breakthrough being 2 to 3 times higher for PD. Moreover, the long-term leading-edge rain erosion model highlights the importance of the accurate measurement of material properties, as lifetime prediction is very sensitive to ultimate tensile strength and Poisson’s ratio.

However, it is crucial to validate the equivalent velocity method through experiments and numerical modeling, while also improving the experimental method to allow for continuous observation of the erosion process in a controlled environment with temperature and humidity regulation. Conducting tests in a wider range of velocities is also recommended. Additionally, improvements for the rain erosion model are necessary to accommodate the utilization of the equivalent velocity. ... Read more

Laser-assisted fiber placement (LAFP) is a promising manufacturing process for thermoplastic composites (TPCs), which enables out-of-autoclave production and eliminates post-consolidation steps. In LAFP, thermoplastic carbon fiber tapes are heated with a laser heat source and placed into the desired layup with the help of a robot, while being compressed by a pressure roller to achieve intimate contact between layers. Despite its advantages, the technology is not yet ready for commercial use due to limitations in laminate quality and consolidation. A crucial factor affecting these is the degree of crystallinity in the thermoplastic matrix, which has received limited attention in previous research. The crystallization behavior in rapidly laser-heated TPCs is unknown, making it difficult to predict matrix-fiber viscosity and intimate contact. This study investigates the impact of rapid laser heating on the crystallization of TPCs. ... Read more

In cold regions, the formation and accumulation of ice can cause safety hazards and impede proper operation of equipment. By example, ice accumulation on aircraft wings can increase drag, increase weight, reduce upward force and decrease aircraft speed.

For these reasons, proper anti- or de-icing techniques need to be developed. These techniques can be divided in passive and active systems. Active systems require a supply of external energy. On the other hand, by physical or chemical surface modification, passive systems inherently possess anti-/de-icing characteristics without the requirement of external energy. For this reason, this master thesis focuses on the development of passive anti-icing coatings.

One possible new approach to develop passive anti-icing coatings could be to modify surfaces with ice-binding proteins, more specifically anti-freeze proteins (AFPs). These proteins can be found in organisms living in cold climates. They are able to inhibit freezing, thus making life in cold environments possible. Currently, limited research has been performed regarding these AFPs as anti-icing coating material. As a result, this thesis delves deeper into the effect of different environments on the behaviour of AFPs. This is fundamental knowledge that needs to be uncovered before AFPs can be used as an anti-icing material.

In a first step, AFPs are directly attached to the surface in various concentrations using a polyethylene glycol (PEG) chain with a specific chain length. From the freezing data, an unexpected phenomenon was observed. It appears that AFP-surfaces freeze faster with increasing AFP concentration. As such, they act as ice promoter instead of the expected ice inhibitor. It is hypothesized that this phenomenon could be largely attributed to the limited protein mobility on the surface. To further test this theory, AFPs with various linker chain lengths were attached to the surface. Indeed, freezing was detected at later time points with increasing linker chain length meaning that AFPs with higher mobility are able to inhibit ice growth.

In addition, the incorporationof AFPs showed another interesting phenomenon as well. Ice dendrites on the AFP surfaces appeared to grow more straight compared to their silane-treated counterpart. Because of this, dendrites on the AFP-surfaces were also more easy to blow away.

Except for attaching AFPs directly to the surface, their behaviour within a polymeric environment is also studied. For this purpose, various concentrations of AFPs were incorporated within a PEG hydrogel. DSC was used to study the different types of water within the different AFP hydrogels. Interestingly, the amount of freezable bound water increased with increasing AFP concentration. No clear trend could be found between the amount of non-freezing water and the AFP concentration. In addition, freezing tests showed that hydrogels with increasing AFP concentration inhibited ice growth. This behaviour is opposite to the behaviour that was detected for AFPs attached directly to the surface.

In a final test, the hydrogels are dehydrated and again subjected to freezing tests. Now, the freezing behaviour follows a similar trend as the AFP-surfaces, meaning that, with increasing AFP concentration, ice formation is promoted.

From these results, it is clear that the environment of the AFPs plays a crucial role to the AFP behaviour. Depending on the type of environment in which they are introduced, AFPs can either act as ice inhibitor or ice promotor. ... Read more

Leading Edge Erosion is a major concern for the wind energy industry as it affects the aerodynamic efficiency of wind turbine blades and, therefore, the power generated. Hail impact on the leading edge could significantly damage the protective layer and the composite substrate. The focus of this research study is to investigate the effect of kinetic energy on the damage mode in gelcoated glass fibre reinforced composite in terms of the velocity and sizes of the hailstone during hail impact. The gelcoated glass fibre composite samples were tested using a gas cannon using parameters based on conditions experienced in real life but with slight modifications to enable accelerated testing. The gelcoated samples were impacted at various velocities for different hailstone sizes, 15mm hailstone (140 m/s, 150 m/s and 160 m/s), 18mm hailstone (100 m/s, 110 m/s, 120 m/s and 140 m/s) and 20mm hailstone (90 m/s) to determine the Failure Threshold Energy (FTE) for each hailstone size considered. This threshold was determined to be the limit below which no damage would be initiated. The kinetic energy of each hailstone is estimated from the mass of the hailstone, and impact velocity, which is measured using a high-speed camera. Using non-contact profilometry (optical microscopy), cross-sectional damage analysis, and ultrasonic C-scan, the damage on gelcoated fibre-reinforced polymer composite samples were analysed.
The coatings did not exhibit any damage or signs of delamination in the coating-substrate interface. However, the substrate showed signs of impact damage. The damage mode was highly influenced by the kinetic energy imparted by hail impact. The damage mode of matrix cracks in the transverse and longitudinal directions was observed in the substrate for most of the impact parameters. The damage mode remained the same for the different hailstone sizes. The impact tests confirmed that a Failure Threshold Energy existed for each hailstone size. It was observed that FTE was increasing with the hailstone size, and it could be stated that multiple impacts had no effect on damage initiation for a limited number of impacts.
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The growing demand for renewable energy has led to significant developments in wind turbine technology. The ever increasing size of turbine blades and their exposure to a variety of environmental factors can affect their annual energy production and service life. Erosion caused by rainfall and hailstones is identified as two of the most detrimental types of environmental factors to the life of a turbine blade. Hailstone impact in particular is expected to affect the aerodynamic profile of the leading edge as well as cause significant damage to the composite substrate.

The aim of this research study is to investigate the effect of varying hailstone sizes on the damage mode in leading edge polyurethane coated composites subjected to hail impact. The coated glass fibre composite samples were experimentally tested using an impact gas cannon. The impact parameters were determined based on real-life scenarios of blade tip speeds and hailstone sizes. Simulated hail ice (SHI) were manufactured using de-ionized water to form monolithic ice spheres. SHI of 15 mm and 20 mm diameter were used in the research for conducting the hail impact experiments. The coated composite samples were evaluated using non-contact profilometry (optical microscopy) and non-destructive testing (ultrasonic c-scan). Observations revealed that the polyurethane coatings remain largely intact throughout the hail impacts and no visible sign of damage or delamination between the coating and substrate was noticed during damage analysis. The damage mode of matrix cracks in the substrate for the impact parameters used, remained the same for both hailstone sizes. Further, it was seen over the experiments that there exists a failure threshold energy (FTE) for each hailstone size and sample thickness, below which no surface/sub-surface damage is visible. It is hypothesized based on observations in literature that a smaller hailstone will have a lower FTE compared to a larger hailstone and will be more lethal, owing to the concentrated area of contact. Future research to develop further awareness of damage evolution in the coated composites is recommended and discussed. ... Read more

Operating turbines suffering from leading edge erosion (LEE) experience significant (up to 5%) annual energy production (AEP) loss and require expensive maintenance campaigns. Evaluation of state-of-the-art coating systems relies on rain erosion testing (RET) with no accepted method for the computational evaluation of the coatings lifetime. In this work, the computation framework for rain erosion developed in the DURALEDGE research project of DTU Wind Energy is subjected to analyses and used for the comparison of erosion performance of four novel leading-edge coating systems. Improvements for the computation workflow are proposed and implemented into the workflow. Furthermore, relevant conclusions are drawn linking the material’s visco-elastic properties to erosion performance.
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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.

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While aramid fibre composites are generally said to have excellent impact properties, this has not yet been translated to the aerospace sector, where impact prone secondary structures remain to be made from metals or glass fibre reinforced polymer. Aramid fibre reinforced engineering thermoplastics could combine the stiffness of thermoset resins with the ductility of commodity thermoplastics in a novel composite with both high structural and impact properties. In the following thesis work, Twaron and Technora fibres are evaluated for their suitability in such a material.

Processing aramid reinforced materials is a trade-off between the composite consolidation quality and tensile strength retention in the fiber. Good consolidation is required to achieve the required bending properties in structural laminates and requires a low polymer melt viscosity. Strength loss in the yarn is caused by the high temperatures needed to achieve such levels of viscosity, which induce a number of degradation mechanisms if no countermeasures are taken. Oxidation, hydrolysis and a purely thermal component are identified as the main mechanisms causing deterioration. Using a Kapton vacuum bag as well as drying the reinforcement and matrix material greatly improves strength retention during processing. Purely thermal degradation cannot be counteracted and makes Twaron or other para-aramid homopolymers unsuitable for reinforcing polymers with processing temperatures above 250°C. Technora is shown to be more resistant against both hydrolysis and thermal degradation, with only minor strength losses well beyond 320°C. The high toughness of Technora attributes these fibres a high energy absorption potential.

Various Twaron and Technora reinforced engineering thermoplastics are produced to evaluate the influence of the fibre-matrix interface and matrix material on the low-velocity impact response. Higher interfacial adhesion is achieved by using epoxy sizing on Twaron yarn and appears to improve the energy absorbed during perforation through increased fracture toughness. Technora reinforced PPS and PEI appear to have a high energy absorption potential but relatively low out-of plane performance. The eventual feasibility of the novel material will eventually depend on its high impact performance with respect to other, conventional laminates such as aramid-epoxy. ... Read more

Carbon fibre reinforced plastics offer various options to improve structures made from conventional materials such as metal. Carbon fibre reinforced plastics are used increasingly in various sectors such as the aerospace, automotive or sports industry. As carbon fibre reinforced plastics are less damage tolerant as metals due to the brittle epoxy resin matrix, this project dealt with improving the damage tolerance of the CFRP-steel hybrid structures. An extensive literature review highlights the possible toughening mechanisms of epoxy resin matrices by using additives made from polymeric and inorganic materials. Furthermore, the manufacturing process resin transfer moulding, but also potential test setups were researched. Apart from toughening the matrix, the interface between the carbon fibre body and a thin steel sheet attached on top of the specimen was researched as well. For that, different interface materials such as short fibre veils, weaves and polymeric materials were reviewed. Based on the literature review a hypothesis was defined. Increasing additive concentrations, which were core-shell rubber particles and silica nanoparticles in particular, should yield an increasing impact performance. However, the influence of the manufacturing process and its limitations had to be considered as well. Furthermore, the influence of different interface materials can improve the bond strength of metal-CFRP hybrid structures. All test specimens were produced with resin transfer moulding. The matrix toughening was researched by doing an impact testing series, which was analysed with microscopy, ultrasonic testing and differential scanning calorimetry. A variety of different additive combinations were included in impact specimens. An air pressured impact cannon was used to impact the specimens. For the interface toughening, a mandrel peel test setup was developed to compare the required peel forces for different interface materials. ... Read more

Rain erosion of leading edges in wind turbines caused by high-velocity droplet impacts is one of the current problems in wind energy that affects their energy production and maintenance costs. The most widespread solution for this problem is the application of viscoelastic coatings (generally, polyurethane) along the leading edge of the wind blade, the area that first meets the raindrop. The focus of the present study is to assess experimentally the effect of the frequency of droplet impacts and the performance of dry intervals (this is, intervals of time at which the test stops) on the incubation times of industrial viscoelastic polyurethane PU coatings. The coated samples were tested in a Pulsating Jet Erosion Tester (PJET) facility. The incubation times, defined as the period of time until damage is observed in the material, were measured in number of impacts and in global testing time (time that the test was kept running) until damage. The two effects that are aimed to investigate, the droplet impact frequency and the dry intervals, represent different relaxation times that the coating may undergo under real-life rainfall conditions: time intervals between individual raindrops (between raindrops) and between rain events (between rainfalls). Higher relaxation times are expected to have a beneficial effect on the rain erosion performance, as the viscoelastic coating would have time to recover itself to the original stress and strain state. The effects of test conditions that may influence the PJET, such as the water cushion effect, temperature and humidity were considered and evaluated. It was found that droplet impact frequency has an effect on the number of impacts until incubation: the lower the impact frequency, the lower the number of impacts until incubation. This effect has been linked to the longer and more energetic droplets produced by the PJET for these low frequencies and is not related to the viscoelastic behaviour of the coating. The existence of dry intervals when testing significantly increased the incubation times of PU coatings. This increase was larger when the dry intervals had a longer duration and when they were performed earlier during the tests. The effects of dry intervals in incubation times have been linked to the viscoelastic behaviour of the polyurethane coating. These results are expected to shed light on the effects of testing variables (impact frequency and dry intervals) that have historically been ignored in rain erosion tests and will help the design of new test parameters for erosion testing. The conclusions of the work will help to understand the importance of relaxation times and viscoelasticity in the incubation time of PU coatings and reveal that these effects will help to refine models that aim to predict the lifetime of polyurethane coatings in wind blade turbines based on real-life rainfall data. While investigating the influence of impact frequency, effects that may affect the PJET results were investigated. The effect of the water cushion was evaluated. This phenomenon consists of a water film that remains on the coating and that affects the posterior droplet impact. Water cushion effects were found to slightly increase incubation times for the highest impact frequencies. Their effects were counteracted with an air supplier aimed at the tested sample which removed the water cushion. Humidity and temperature were found to have an effect on the incubation times: in general, the higher the temperature and the relative humidity, the shorter the incubation times. The incubation times of saturated samples which went through humidity treatments were tested: their incubation times were lower than for those samples which did not undergo that treatment. DMA analysis was performed on PU samples, but the change in mechanical properties of the coating could not be directly linked to the drop in incubation times. It is suspected that plasticization is affecting the mechanical properties of the coating. The change in the environmental conditions may also be affecting the adhesion between the coating and substrate, resulting in lower incubation times. ... Read more

L-AFP is able to place composite tapes in very precise directions, thereby allowing tailored stiffness designs. The laser is used to heat the thermoplastic matrix material to temperatures above its melting point. CF/PEEK is a popular choice for aerospace grade structures due to the high structural performance and high glass transition and melting temperatures. Current L-AFP research is driven by the desire to produce OOA laminates with a performance comparable to autoclave level. This will remove manual, labor intensive, and energy consuming steps, leaving a repeatable and automated process that is more predictable. A key phase for the performance of a laminate during L-AFP is the development of intimate contact. Recent research showed the effects of thermal deconsolidation due to rapid laser heating and questioned the current practice of characterizing the surface of the composite tape material for the intimate contact development models: instead of characterizing the pristine tape surface, the laser deconsolidated surface should be characterized. Additionally, another dominant physical mechanism, driving the intimate contact development was proposed based on the observations of thermal deconsolidation. These ideas are novel and require more extensive understanding of what thermal deconsolidation due to rapid laser heating is, how it affects intimate contact development. The effects of rapid laser deconsolidation were studied by performing (1) a rapid laser deconsolidation experiment where CF/PEEK samples were prepared and (2) an intimate contact development experiment where the specimens were compressed under constant pressure while being subjected to a temperature profile. Three degrees of deconsolidation were manufactured with the laser set-up: (1) zero deconsolidation by using the as-received tape directly as specimens, (2) slightly laser deconsolidated tapes which experienced maximum temperatures between the glass transition and melting region, and (3) highly laser deconsolidated tapes that experienced temperatures above the melting point. All samples were characterized on void content, roughness, and waviness; all of which significantly increased which showed that the state of the tape right before the nip-point during L-AFP is significantly different than the pristine state of the CF/PEEK tape. The intimate contact development experiment was carried out with pressure levels of 10, 50, 100, and 300 kPa reaching maximum temperatures of 363°C. Decreasing the degree of deconsolidation or increasing the pressure resulted in better consolidation. At 10 kPa, most of the characterization showed very little difference from a pristine tape, but a significant amount of DEIC was developed with 36% for the highly laser deconsolidated tape to 46% for the as-received degree of deconsolidation. At the highest pressure of 300 kPa, no significant difference between the degrees of deconsolidation was observed as the average DEIC values are between 85 to 87%. Additional intimate contact development temperature settings below melt showed that no intimate contact was developed below the melting temperature. During the glass transition, a significant compaction occurred which eliminated most of the void content. Between the glass transition and melting temperature, however, void content, thickness, and roughness all remained constant. At the melting point, a second thickness compaction occurred together with the development of DEIC and decrease of roughness which appeared to also relates with resin percolation. ... Read more

With a growing demand for longer blades in the wind turbine industry for higher rated power per turbine, a structurally sound blade-root connection is of commercial importance. Bushing connections have been a commercially favoured design in the past few years, replacing the commonly used T-bolt connection as the joining method of choice. The new design replaces the barrel nut in the T-bolt with an axial bushing that the bolt connects to and can be assembled with the laminate during the lay-up stage of the blade skin. It has been theorised that it can result in a reliable connection due to the elimination of laminate stress concentrations. However, literature outlining the performance of a blade-root connection with bushings is lacking in the current body of knowledge. While several patents for bushing designs exist, they don’t provide verifiable results on their efficacy due to trademark laws. The objective of this project is to design and conduct a numerical study of a blade-root connection with bushings with an aim to replace the T-bolt connection, along with providing evidence of the effect of various parameters on the structural performance of the blade root. The design is to be based on a Suzlon Energy-make blade with a pitch circle diameter of 3m and a blade length of 63m. The project has been planned in three phases: (1) Design of the root; (2) Validation of the design; (3) Comparative analysis. Modelling and FE analysis has been carried out in the ANSYS environment. Parameters of the bolted connection have been determined according to industry standards provided by VDI and GL. Design validation was conducted based on structural constraints; the key design constraint relevant to the blade-root as a sub-component of the wind turbine is the accumulated fatigue damage. For the final phase of the study, various parameters associated with the assembled root were identified and tested in iterations and their effect on the structural performance, weight, and cost of the assembly were studied. The results confirm the hypothesis of reduction of stress concentrations within the laminate; this eliminates several failure modes associated with composite laminates at the blade root. As is, the bushing connection can be considered a viable alternative to the T-bolt joint. Within the connection, higher absolute stresses and stress gradients were developed in the bolt joining the blade to the hub. Hence, this was the area of focus for fatigue damage evaluations. Conservative estimates of the accumulated damage show values well within the acceptable range. The connection has been designed keeping several concurrent variables in mind. Given the commercial applicability of the design, a rigid optmized design is not feasible due to unpredictable parameters like certification costs, total assembly times, and procurement costs. Therefore, an effort has been made to understand the effect of varying component parameters. The design lends itself to flexibility of dimensioning and material choice within the sub-components; parameters can be optimised according to cost, manufacturability, and performance requirements. While the base design configuration for the bushing connection is heavier than the T-bolt design, improved fatigue performance can be seen as a favourable trade-off. ... Read more

The aim of this study is to analyse the influence of ply drop-off regions and related parameters on the VARTM cure-induced residual stress generation and related distortions within thick thermoset composites. These parameters include the resin pocket material properties and geometry, ply orientations and cure cycle parameters. By means of a coupled thermal-mechanical cure model it is observed that during the initial part of the cure process, both the chemical shrinkage and thermal expansion of the resin pocket evolve in counteracting manner. The resin pocket geometry and orientation of the surrounding plies influence the out-of-plane direct tensile stress within these plies whenever either the aspect ratio of the resin pocket or the in-plane stiffness of these plies is increased. The orientation of the terminated ply proved to be of influence as well, where higher in-plane shrinkage and thermal contraction of this ply increases the out-of-plane stress around the drop-off region. Implementing multiple drop-off regions introduce stress interactions caused by the different ply orientations used. These regions influence the local out-of-plane stress levels around the drop-off regions, where the stagger distance between these drop-offs generally lowers this effect. As result of this study, existing drop-off design guidelines are reconsidered and new guidelines are proposed for implementing ply drop-off regions within composite laminates. ... 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