The thesis aimed to characterize the effects of manufacturing-induced defects, particularly gaps in variable stiffness laminates. The primary focus centered on designing a specimen capable of accurately characterizing gap effects. A refined, element-level specimen design featuring a controlled gauge zone was developed, based on pre-defined requirements to represent common variable stiffness structures accurately. The specimens were manufactured and tested. The experimental work featured Digital Image Correlation, Dye Penetrant Testing, and Optical Microscopy. Experimental observations revealed deformation peaks induced by gaps, along with trends in void formation and ply deformation during curing processes. Furthermore, digital image correlation (DIC) analysis unveiled distinct out-of-plane tendencies associated with gap locations. Discrepancies between finite element method (FEM) results and experimental strain distributions highlighted the need for improved modeling, particularly accounting for ply waviness. The main hypothesis presumes that the effects of gaps are influencing off-axis plies through matrix failures. The hypothesis was not proven true based on insufficient proof.

With the increasing demand for manufacturing sustainable aerospace parts, along with cost effectiveness and preciseness. Automated Dry Fibre Placement(ADFP) has emerged as a
promising solution. ADFP combines these goals and enables the production of precise and
cost-effective aerospace components with the potential for reduced defects. Tack, which is
the adhesion between fibre layers during deposition, plays an important role in achieving
high-quality preforms. Tack is essential for securely holding down the plies. Insufficient tack
leads to final part defects like wrinkling contributing to the stability of the final part.
This study investigates the influence of nip point temperature, layup speed, and compaction
force on the tack behaviour of Hexcel HiTape® dry fibre material during the Automated
Dry Fibre Placement (ADFP) process. It also assesses the applicability of an existing tack
model, developed for Solvay TX1100 dry fibre material, to the Hexcel HiTape®, possessing
a different binder distribution and different fibre architecture. Tack was evaluated using 90-
degree peel tests for specimens manufactured using different parameter combinations. Initial
statistical analysis of the main parameter effects and interactions used a 2-level factorial
design of experiments. To investigate the non-linear behaviour of the parameters, this design
was extended to a 3-level face-centred central composite design of experiments.
The results show that temperature is the most dominant factor affecting tack, with higher
temperatures significantly increasing tack forces due to binder activation and polymer diffusion at the interface. Layup speed also had a notable influence, with higher speeds leading
to increased tack forces, which contrasts with the inverse relationship predicted by the existing model. This is likely due to the material architecture of Hexcel HiTape ®, where faster
speeds reduce binder seepage through perforations, allowing more binder to remain at the
interface. Compaction force had a minor impact on tack behaviour and the results of this
analysis deemed the effect to be statistically insignificant. Additionally, while interaction
effects between the parameters were analysed, they were not statistically significant at the
95% confidence level. The findings suggest that the independent contributions of temperature
and velocity are key, but deviations from expected behaviour may be due to material-specific
characteristics such as perforations and fibre structure. The behaviour of the material in
relation to the statistically significant parameters shows that there is non-linearity.To test
the generalisability of the model, the existing tack model for Solvay TX1100 dry fibre was
applied to Hexcel HiTape ®. This model did not fit the data of Hexcel HiTape ® material.
The observed positive relationship between speed and tack, contrary to the Solvay TX1100
ix
model’s inverse relationship, shows the need to adapt the model to Hexcel HiTape ® and
indicates limitations in generalising tack models across dry fibre materials. Additionally, fibre fraying of Hexcel HiTape® was also observed during peel tests, marking a finding that
warrants further investigation to help quantify the effect, aiding in understanding the dry
fibre material behaviour.

The present study, which was carried out in collaboration with GKN Fokker, focuses on incorporating bird strike crashworthiness requirements within a multidisciplinary optimization (MDO) framework. During the preceding three-month internship in the same company, a pivotal contribution to this project was the development of an Abaqus interface for the Multidisciplinary Modeller, MDM, created within the Center of Competence in Design department. MDM is a Python/ParaPy-based automated generator of wings, moveables and flaps, starting from a set of user-specified parameters. The generation of ready-to-run input files thus lays the foundation for the subsequent optimization process, as any changes in materials or geometry can be easily accommodated.

The core objective of the research is to minimize the weight of an aircraft wing while taking into account additional requirements related to the extent of damage caused by bird strikes. Unfortunately, such events occur more frequently than one would be comfortable with, and stringent requirements are set in place to guarantee the safety of the passengers. Among these requirements, the aircraft must be capable of landing safely after such an event, being subject to loads associated with get-home conditions.

As a consequence, two critical constraints are formulated within the optimization framework, addressing the residual strength of the damaged front spar following a bird strike, coupled with a requirement based on a maximum penetration depth. The last constraint has also been included due to the rising popularity of the electric vertical take-off and landing aircraft, which not only fly at low altitudes, thus increasing the risk of bird strike, but may also contain battery packs in the leading edge, for instance, which can pose a significant risk if damaged. To tackle the complexity of this highly-dimensional optimization challenge, a methodology based on Bayesian optimization is proposed, employing surrogate models coupled with a preliminary variable ranking procedure.

The Kriging metamodel is identified as a suitable candidate, thanks to its error prediction capabilities, which are paramount in Bayesian optimization. A variance-based dimensionality reduction method is proposed, which makes use of an initial surrogate to estimate the main and interaction effects of the variables. The quantification of the significance of a variable is expressed as its percentage contribution to the total variance, thus allowing for an intuitive selection of the most important parameters. After the screening procedure is complete, the optimization procedure is carried out in the reduced design space, which uses the constrained expected improvement as an acquisition function. The proposed methodology is then applied on a case study problem, involving a five-bay metallic wing segment subject to the constraints aforementioned, involving 19 design variables representing the thicknesses of various components.

Remarkable weight savings have been achieved, the final result being 40\% lighter than the lightest feasible design among the initial data points. A significant dimensional reduction has also been attained for the maximum depth constraint, which is expected due to the local nature of the impact. Not only did the number of variables greatly decrease from 19 to just 3, but a considerable increase in the accuracy of the corresponding metamodel has also been registered, thanks to an increase in sampling density in the reduced space. However, the variable screening procedure revealed intricate interaction effects with respect to the residual strength of the front spar, emphasizing the nuanced complexity inherent in crashworthiness considerations. Nevertheless, a moderate dimensional reduction has been achieved for this constraint as well, reducing the number of variables to 8, thus proving the efficacy of the proposed variable screening procedure.

In conclusion, the utilization of Kriging models, variable ranking procedures, and Bayesian optimization collectively contributed to the success of achieving remarkable weight savings, proving the efficiency of the proposed methodology. Moreover, it has been shown that the integration of a residual strength requirement is necessary, as many cases were uncovered where no significant penetration occurred, although the application of the considered load case, which is not from critical to an undamaged wing, resulted in high stresses to the front spar of the damaged structure.

Increasing demands for composite structures and sustainability goals have resulted in the growth of thermoplastic polymer matrix composites and automated manufacturing technologies. The in-situ Automated Fiber Placement (AFP) manufacturing of thermoplastic prepreg tapes has the potential to provide a fast and cost-effective manufacturing solution for large composite structures. However, it is still under development because of the complex mechanisms and short processing times involved which leads to several defect formations, especially gaps and overlaps. One of the primary reasons for the formation of these gaps and overlaps is the tape width deformation during placement.

Current literature on tape width deformation shows that the resulting tape width is influenced by several processing parameters such as temperature, pressure and placement speed. However, results from different studies do not agree with each other, indicating that the tape temperature distribution might be at play. Additionally, the conventionally considered tape width deformation mechanism i.e., transverse squeeze flow has been suggested to be incorrect for the AFP process as the experimental deformations do not agree with the results of the transverse squeeze flow model. Therefore, the research objective for this study was to experimentally investigate the width deformation mechanism and the influence of processing parameters for thermoplastic prepreg tapes using in-situ AFP manufacturing and humm3® (from Heraeus) as the heating device.

The specimens were manufactured according to a full-factorial Design of Experiments (DoE) with two settings (high and low) for the following processing parameters: heated length, nip-point temperature and compaction force. The tape width was measured for all the specimens to investigate the influence of the different processing parameters and some post-processing analyses were carried out to understand the tape width deformation mechanism. This included width measurement in the heating phase of the process, surface roughness analysis, tape cross-section profile inspection and fiber-resin content analysis.

From the post-processing analyses and investigations, it was found that the tape width deforms in the heating as well as the consolidation phase of the process. Additionally, the cross-section images show that the conformable roller led the tape edge profile to have a gradual decrease in thickness with a clear slope and the tape edges show a clear indication of spreading of the fiber-resin mixture due to the presence of both fibers and resin. Moreover, the surface roughness data show an indication of the role of temperature distribution because the as-received tape surface roughness was achieved for the higher temperature and longer heated length settings which are assumed to promote better heat distribution in the material.

The influence of the processing parameters on the tape width deformation did not show clear trends for all specimen configurations. However, the exceptions pointed towards the role of temperature distribution in the tape that led to the overshadowing of the effect of other processing parameters. Considering this, it was observed that the change in heated length did not have a significant effect on the tape width except for one configuration i.e., 300 N, 370 °C, wherein a clear increase with no overlap in data was seen. For the effect of temperature and compaction force, it was found that they have an influence on the tape width for temperatures lower than the melting temperature of the polymer resin (Tm). Additionally, the fiber straining effect on the tape width deformation with compaction force was suggested for the higher temperature specimens.

Laser-assisted fiber placement (LAFP) is an automated manufacturing technique with promising potential to produce larger aerospace structural components using thermoplastic composites. However, during the rapid heating phase, deconsolidation of composite tapes can occur, resulting in negative effects on bond forming ability. This study aimed to investigate the effect of LAFP process parameters on the rapid laser heating deconsolidation forms of CF/PEEK tape samples by developing an experimental setup. Six different combinations of placement speed and consolidation pressure were tested, and the specimens were characterized for roughness, degree of effective intimate contact (DEIC), void content, and thickness. The results showed that an increase in placement speed and pressure can yield a positive effect on the resolvement of deconsolidation. The nip-point temperature was found to have a significant influence, with expected higher temperatures resulting in better re-compaction of the fiber bed and flow of the matrix material. The developed experimental setup provides a foundation for further investigation into the in-situ consolidation of LAFP for meeting aerospace standards.

Robot manipulators are significantly more accurate than their human counterparts and enhance the repeatability of various tasks. However, manufactures still provide reduced information regarding the robot controller functions, typically affecting robots' predictability. Additionally, industrial examples show that system transparency could be improved, helping users to understand the process and apply corrective actions. One step in the direction of improved predictability is the identification of the Limiter block associated with the KUKA robot controllers that limit the acceleration and jerk obtained during motions. This work aims to provide dynamically feasible trajectories with reduced performance deterioration. Specifically, the Limiter's behaviour was analyzed and later predicted in a iterative manner, aiming for the maximum velocity, acceleration and jerk that can be achieved, starting from the current robot state and given specific robot information. Using the predicted bounds, the trajectory generator will determine the maximum achievable point that fulfills all the constraints, running an optimization problem in an iterative fashion. Nonetheless, the practical experiments represent a significant component allowing data collection and results validation.

Fibre-reinforced laminated composites are constructed layer-by-layer, allowing their material properties to be easily tailored relative to their isotropic counterparts (metallic alloys). Moreover, a composite structure with variable stiffness (VS) can be made by spatially tailoring stiffness across a laminate. This permits better load distribution within a structure, efficiently using a given material, and allow for lighter construction. Typically, the design of VS structures follows a bi-level procedure: first, the structure’s stiffness distribution is optimised using lamination parameters (LPs), and second, the stacking sequences (SS) of differently oriented fibres are designed to achieve the desired stiffness distribution. However, the designs envisioned using LPs are only sometimes exactly matched by the designed SS. The shortcomings faced during this conversion are called the ’Inverse Problem’. Upon reviewing the existing techniques in literature to handle the inverse problem, it was understood that converting LPs into SS is challenging without incurring substantial computational costs or imposing significant restrictions on allowable fibre orientations in the design. Such restrictions tend to under-utilise the directional properties of the fibres. Thus, there was a need to explore and develop better stiffness design methods for laminated composites by bridging the gap between LPs and SS in a computationally efficient way.

In response to this challenge, a hierarchical design framework was proposed to handle the Inverse Problem. Diverging from conventional methods that directly design the SS and try to match a given set of LPs, this framework divides the problem into two distinct stages. Initially, the focus is to use the In-Plane LPs and determine the number of layers in each orientation within a laminate, also called the Fibre Angle Distribution (FAD). Subsequently, the FAD serves as an interim solution, and the SS can be designed by using the Out-of-Plane LPs along with it. This problem partitioning enhances computational efficiency and offers potential benefits in solving the Inverse Problem more effectively. Given the time constraints inherent to a master thesis, the primary undertaking of this study was to efficiently design the FAD while accommodating a wide range of possible fibre orientations. To address this, a novel method using the Fast-Fourier Transform (FFT) was developed to facilitate FAD design with ply angle multiples of 15° (or [∆15° ] = [0, ±15, ±30, ±45, ±60, ±75, 90]). Moreover, empirical guidelines for SS design, such as the Symmetry and Balancing rule, were incorporated into the design step.

The primary contribution of this report is introducing an FFT-based method to design FADs, a novel addition to the existing body of research. Upon extensive testing, it was shown that this implementation could convert LPs into multiple unique FAD solutions in less than 0.3 seconds on a regular office laptop, outperforming other methods in literature by at least ten times. Furthermore, the implementation was also used to demonstrate the benefits of designing laminated composites using [∆15°] over the conventional [∆45°] orientations (or [0, ±45, 90]). In light of the positive results, it is pointed out that the In-Plane LPs (and consequently the In-Plane stiffness) are known to be sensitive only to the FAD and not their SS. As such, the readers of this thesis are presented with a very computationally efficient approach for designing laminated composites for In-Plane Stiffness...

The use of Grid-stiffened structures has been increasing in the space industry over the last decade. Thanks to it is high specific mechanical properties and excellent damage tolerance characteristics, researchers also started looking into the potential of using them for aeronautical applications. However, not much has been done to investigate the reduction in strength after barely visible impact damage and visible impact damage, and how much energy is needed to cause such damage, which is an important requirement for aircraft certification. In this project, a grid-stiffened replica of an existing reference cargo drone is created, and the design is tested to check its compliance with the requirements. This involved modelling of the design, manufacturing of test coupons, impact and mechanical tests, and model validation. The results showed a possible weight reduction, with no major reduction in strength observed with impacting using the cut-off energy level set by the manufacturer.

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.

The state-of-the-art design approach of composite structures in Formula One (F1) rely on expensive full-scale crash tests and simple models that cannot describe the underlying physical phenomena during crushing. Although the models can describe the crush forces and resulting accelerations, their predictive capabilities are poor, and several iterations are usually required to arrive at a satisfactory design, causing the teams money and resources. Reductions in costs could be possible by reducing the number of full-scale experiments and increasing the amount of analysis.

This thesis aims to evaluate the capabilities of a commercially available material model at two distinct modelling scales by conducting physical and virtual experiments on two different specimen geometries; tubular coupons and a new open-section channel specimen. Firstly, the new open-section specimen with a geometry representing the typical F1 crash structures was designed, manufactured and tested. The newly proposed specimen showed controlled failure modes, and displayed typical characteristics of composite structures under crushing loading. Secondly, the two specimen geometries were numerically analyzed on the macro-scale (laminate-level) and meso-scale (ply-level) using a commercially available material model implemented in the Abaqus finite element software. Mesh-dependency of the macro-scale approach was discussed and numerical aspects required to perform crush simulations on the meso-scale were identified and discussed. Lastly, calibration of the numerical models was performed to understand the effects of model inputs on the output responses, to evaluate the validity of the input parameters, and to assess two different calibration approaches; the first method is based on macro-scale modeling, while the second method is based on surrogate-based inverse parameter identification at the meso-scale.

The numerical models could not reproduce the experimental crush responses of either geometry when using experimentally characterized material data. Following the second calibration approach, the qualitative and quantitative crush responses of the tubular specimen could be accurately described, but the channel specimen showed no improvements in their response. The macro-scale modelling approach with the current material model was judged to be impractical for large-scale analysis, and the meso-scale approach showed need for further developments.

Carbon fibre reinforced polymer (CFRP) composites are well-known for their specific strength and stiffness properties. For the past two decades, various industries have increasingly used these materials to improve performance and reduce emissions. This increased use has caused a problematic growth in both production and end-of-life CFRP waste. Recent developments in recycling processes allow recycled carbon fibres (rCF) to have undamaged and clean surfaces with intact sizing, retaining the mechanical properties of non-recycled virgin carbon fibres (vCF) at a considerable cost reduction.
Unfortunately, CF recyclate from these recycling procedures is highly diffuse making them useless for structural application. For making rCF material viable to be used in structural applications, further processing of the recyclate is necessary. To manufacture high-performance rCF materials which feature homogenously distributed, orientated and aligned fibres, considerable effort has to be spent in controlling the microstructure during processing.
The presented thesis has focused on introducing an initial characterisation method that allows for quantitative analysis and comparison between rCF microstructures. Characterisation is performed by analysing elliptical CF footprints in perpendicular cross-section micrographs. Fibres in the micrograph are identified and segmented through a semi-supervised machine learning tool. The elliptical fibre shape is measured by the method of ellipses (MoE) in FIJI ImageJ. This MoE extracts the position and elliptical shape of every fibre footprint in the micrograph. Analysis of the microstructure is done in python to quantify the microstructural morphology by fibre diameter, fibre orientation distribution (FOD), global-local fibre volumes (Vf), fibre density distribution (FDD) and Fibre-to-Fibre (FtF) spacing.
It was found that the method is accurate and reliable in identifying and measuring fibres present in the cross-section micrograph. Similarly, calculated values for Vf, FDD and FtF spacing can be used for characterising the quality of studied materials.
Unfortunately, despite being frequently used in literature to characterise highly aligned and orientated CF materials, all fibre orientation measurements were flawed by the inherent susceptibility of the MoE to calculate fibre orientation with large biases. This orientation bias was caused by the error sensitivity of the arccosines function used for calculating the out-of-plane (OoP) angle. Minute elliptical shape differences in near perpendicular fibres with almost circular footprints resulted in large non-linear errors.
Recommended is to study FODs in samples that are sectioned at a 45 or 60° angle, resulting in large elliptical fibre footprints which are less error sensitive when analysing with the MoE. The induced section angle can easily be transformed back to yield an unbiased FOD measurement.

The aerospace industry increasingly uses fibre-reinforced plastics that are manufactured using automated fibre placement (AFP). A common problem in AFP is the occurrence of gaps and overlaps that affect the structural perfromance of laminates. Tow width fluctuation is an important cause of gaps and overlaps. Two developments in manufacturing are combined to create a new mitigation method for gaps and overlaps. One development is fibre steering, which harnesses the flexibility of AFP to vary the fibre orientation within the ply. Another development is smart manufacturing which includes responding real-time to changes. The combination is local fibre steering which compensates for tow width fluctuation by placing tows adjacent to each other. In three steps it is assesed how effective local fibre steering is in increasing the structural performance of composite laminates by reducing gaps and overlaps through compensating for tow width fluctuation in automated fibre placement. Firstly, it is determined to what extent gaps and overlaps are reduced by constructing a conventional and a steered laminate. The tow width is modelled using a cubic spline. The steered laminate uses numerical approximations of the equations for parallel parametric curves. Two problems occurs that are: the accumulation of curvature leading to self-intersections and cusps, and large angle deviations. The curvature accumulation is tackled by using smoothing splines and the fibre angle deviation can be limited by taking a weighted average. Secondly, the tow width model is validated by measuring the tow width of two different materials. Thirdly, the effect of the reduction in gaps and overlaps and the fibre angle deviation on the structural performance is determined by using a finite element method based on the defect layer method that calculated the buckling load and the effective stiffness. Results show that gaps and overlaps cannot be completely eliminated but local fibre steering is able to increase the effective stiffness.

The versatility of automated composite technologies granted the possibility of manufacturing laminates with a continuous variation of fiber orientation, also known as Variable Stiffness Laminates (VSL). Despite offering enough dexterity to be adapted for composite deposition, industrial manipulators are nontrivial to integrate into other applications than those originally envisioned, especially when complex curvilinear cartesian paths are desired. Further improvements in layup control and performance are necessary to induce an accurate tracking of fiber tow courses. The goals of this thesis were to a) reduce variability of layup control, b) eliminate experimental iterative steps associated with programming tow courses and c) enforce a constant laydown speed that aids consolidation quality. To achieve such objectives, a framework entitled LayLa (Laying Laminates) was developed as an offline programming tool that automatically computes the series of continuous robot motions to perform a tow course at a desired laydown speed. Experiments with LayLa were conducted for Automated Fiber Placement (AFP) using a six degrees-of-freedom KUKA manipulator, without actual deposition. From a VSL design algorithm, two fiber tow courses with a reduced turning radius were selected to be performed at a laydown speed of 0.1m/s. The overall results revealed reasonably accurate trajectory tracking of both joint and operating space variables, with errors at the end-effector position around 0.05mm and at the laydown speed of 3.2mm/s, corroborating the use of LayLa to create external commands for composite deposition.

In this thesis a design optimisation framework was developed, with the aim of reducing the cord usage in TANIQ’s Large Bore Hose product family. To decrease the finite element analysis time, an axisymmetric finite element model was developed using ABAQUS’s rebar method, where the directional cord properties are smeared in hoop direction. The two-dimensional rebar method showed an increased stability and an analysis time reduction of up to 70 % compared to the baseline method. A four-dimensional surrogate model was used in combination with genetic algorithm optimisation to identify the cord angle configuration resulting in the lowest cord usage. The proposed design resulted in a cord usage reduction of 14.9 % compared to a real-life reference design.

Variable Stiffness Laminates are created by spatially varying the fiber orientation resulting in designs that makes use of curved fibers rather than Uni-directional fibers to tailor the properties to design requirement. The advent of automated manufacturing methods such as Automated Fiber Placement and Tailored Fiber Placement has made variable stiffness laminate more realistic and attractive for use in aerospace and automotive sector. Importance of light weight designs in automotive sector has received new interest due to the stringent emissions rules and entry of designs based on alternative energy. Within this context, this thesis intends to create variable stiffness design for an automotive part to investigate the possible improvements in structural responses compared to a design based on conventional laminates. The design is done based on 2D Finite element analysis coupled with a convex optimization that helps to generate steered fiber designs optimized for strength, stiffness and buckling. One of the important load case studied here is the Inertia Relief. Inertia Relief method is a computationally efficient way of analyzing rigid bodies without doing a dynamic analysis. The method was implemented in the finite element framework and the responses from the analysis were taken for optimization.
Final optimization of the structure showed that significant improvement in the objective responses can be achieved by using variable stiffness laminates over a conventional laminate design (UD). The result also implies that reduction in weight can be achieved if a variable thickness optimization is to be done on the model.

Striving to improve structural efficiency the aerospace industry shows increasing interest in variable-stiffness composite laminates. Advanced fiber placement is a hybrid manufacturing technique that offers the flexibility of both filament winding and automated tape laying. With the development of this novel system curved tows can be placed and a spatially variable-stiffness laminate can be designed with continuous changing stiffness from point to point.

The increased design freedom to tailor a structure by in-plane stiffness variation leads to a challenging design optimization problem. A multi-step framework is developed by the aerospace structures and materials department to optimize variable-stiffness laminates. Variable-stiffness laminate design allows for sophisticated designs. Based on this premise it is investigated how such design could improve the structural performance of an engine thrust frame, a structural application that transfers the thrust loads from the rocket engine to the rest of the launch system. The engine thrust frame is subject to cryogenic thermal loads, something not incorporated in the available optimization framework. The goal of this work is to add thermal loads to the laminate analysis routine and to adjust the optimization routine to incorporate thermal influences.

With the thermomechanical optimization framework in place the engine thrust frame is modeled. Conceptual design optimization of the engine thrust frame under thermomechanical loads is performed to increase buckling resistance. A mismatch in the coefficient of thermal expansion is used by the optimal variable-stiffness design. The stiffened areas contract less than the inter-stiffener bay regions. Consequently a stabilizing tensile stress is induced in the prone to buckling bay regions, whereas compressive stresses are distributed to the stiffened areas. Based on the stabilizing thermal stresses and load distribution significant gains in performance are found.