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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

To enable weight reduction of skin stiffened structures in aerospace applications they can be loaded in the post-buckling regime during service. However, to do this safely the fatigue delamination growth behaviour of post-buckled skin stiffened structures must be well understood. To this end, skin-stiffener separation of single stringer compression specimens with an initial delamination loaded in post-buckling has been investigated using the virtual crack closure technique. Both quasi-static and fatigue delamination propagation were studied using Abaqus FEA 2017. Two recently introduced fatigue delamination growth methods, the Constant Amplitude Fatigue method and the Simplified Fatigue method, were investigated. The methods were verified at the simple specimen level using double cantilevered beam and mixed mode bending specimens before they were used to predict delamination behaviour in single stringer compression specimens For the single stringer compression specimen a clear correlation has been observed between the local buckling of the delaminated stiffener flange and sudden delamination extensions, both under quasi-static and under fatigue loading.

Research to analyse and/or develop novel materials and techniques is necessary to address competition and safeguard craftmanship in shipbuilding. In this master thesis, two new approaches to reduce the production and maintenance costs are studied: {metal foam core, metal face sheet} sandwich materials and corner adstir fillet stationary shoulder friction stir welding (FSW). The current project is a preparatory analysis for the research in which the two components will be combined. It is anticipated that their combination may generate more added value due to mutual reinforcement.
The study of metal foam based sandwich materials showed that looking at the material level, the {aluminium foam core, steel face sheet} sandwich material is theoretically promising with respect to the conventional steel solid plate in specific situations and applications. When the sandwich material is implemented as part of the stiffened panel, the expected benefit could not be realised. This confirms the search for a more selective use, such in naval vessels when one needs a high resistance to impacts and blasting, and when one wants to conserve the limited internal space.
Compared with the currently existing arc welding technique of double sided T-joints, the corner adstir fillet stationary shoulder FSW scores better on hardness and fatigue resistance of the weld. Since the properties of the weld material correspond to those of the parent material, the weld can no longer be seen as the weakest link of the structure. The fatigue experiment occasionally revealed a new failure type at the undercut, being a sharp corner created by the modified FSW shoulder. This second failure type did not impact on the overall expected failure resistance; however, the limited occurrence does not allow a final statistical interpretation.
Based upon our analyses, there is not yet a place for widespread use of {aluminium foam core, steel face sheet} sandwich materials, but there is an added value for the corner adstir fillet stationary shoulder FSW.

Mines and improvised explosive devices are the cause of many mortalities of vehicle occupants. Both experimental and numerical research in this field aims to improve the safety of military vehicles. TNO has advanced Finite Element (FE) models to simulate such events. The numerical research is done to better understand mine blasts. Another aim is to reduce experimental costs for Defence Material Organization (DMO). For full vehicle simulations the empirical Westine model is the basis of the in-house TNO mine blast model. The model consists of a triangular pressure pulse consistent with the Westine impulse which is calibrated using the a test rig developed by TNO and DMO. Experiments are compared with numerical results using the TNO mine blast model and the jump height, representative for total impulse transferred, shows scattered results for different vehicle tests. A possible cause can be accuracy of the current mine blast model. This research will study a new technique to validate mine blast models. When this new technique results in improvements in validation of mine blast models it can be used to study and improve the current TNO model. The aim of the new validation method is to calculate the interaction pressure of an plate excited by a mine blast loading. The obtained pressure can be compared with existing mine blast models for validation of the mine blast model. The methodology is based on solving the inverse problem which requires transient Digital Image Correlation (DIC) measurements of the deforming plate for the duration of the mine blast loading. Such measurements are done with the state of the art test setup from TNO and DMO. Several methods to solve the inverse problem will be studied. The proposed best way to solve the problem at hand is by solving the corresponding optimization problem using an iterative gradient of descent algorithm. The main difficulty in this approach will be the calculation of the gradient of the objective function. To do this the corresponding transient adjoint problem has to be solved. First an algorithm to solve the inverse problem is implemented for a linear Kirchhoff Love plate model to study the behaviour of the inverse problem for a relatively simple test case. For this algorithm the transient adjoint problem is derived. The forward and adjoint problem are numerically solved. The implementation is verified using the method of manufactured solutions and a convergence study. The algorithm is verified using three different bench mark test pressures. It was found that without any exception the displacement of the algorithm converged with great accuracy to the applied displacement. The corresponding pressure converged good for smooth pressure distributions. Non smooth pressure distributions representative for mine blast interaction pressure did not converge. This shows the non-uniqueness of the solution of the inverse problem. It was realized after these tests that the forward problem acts as a low pass filter for time and spatial oscillations of the pressure distribution. This implies that the inverse problem in non-unique and that noise in the displacement data will be amplified in the obtained pressure from the inverse solution. The total force and radial position as function of time, obtained after integration over the spatial domain, for a localized load are accurately captured back. These parameters could be useful to validate a mine blast model such that research was continued for a continuum model. A non-linear elastic material model is proposed to model the deforming plate assuming monotonically increas- ing strain. This model is used and calibrated against the Johnson-Cook plasticity model. One plate simulation excited by the TNO mine blast model verifies that the non-linear elastic and Johnson-Cook material model result in almost the same displacement. The adjoint problem for a general continuum with elastic material model is derived. The same optimization algorithm is implemented for the continuum model using the calibrated non-linear elastic material model. The forward and adjoint problem are solved using the author’s transient FEM implementation which is verified using commercial software. From the benchmark tests it was verified that the displacement converged quite well however less compared to the linear problem studied earlier. The corresponding pressures behaved similar. It was concluded that the total force and centroid position of a localized load are not always in agreement for the benchmark and solved pressure. This research shows that the limitations of the inverse problem shed light on the limitations in the validation methods employed by many researchers in the field.

A new certification approach for bonded primary Fiber Metal Laminate (FML) structures is investigated: using bolts as Disbond Arrest Features (DAF)s to contain the growth of bond line damages so that they can be found and repaired by inspection before becoming critical. By fatigue testing with coupon specimens and model analysis, it has been demonstrated that reducing the Mode I Strain Energy Release Rate (SERR) is the main driver for arrest. The peak stress associated with a disbond front can initiate adherend fatigue cracks during slow growth. The effect of adherend fatigue cracks on the arrest of disbond growth could not be determined and must be investigated in future work. In the process, a novel quasi-analytical disbond growth model has been developed and validated. An algorithm is developed and verified that utilizes the strain field measured by Digital Image Correlation (DIC) to locate the disbonded region.

The high specific strength and stiffness of composite materials have resulted in a widespread introduction of composite structures in the commercial aviation industry. Although composite materials offer the possibility to design lightweight structures, they also suffer from complex failure modes of which damage progression is badly understood. This thesis research is about Structural Health Monitoring (SHM) for composite aircraft structures. By permanently attaching a sensor network to a structure, maintenance activities can be improved by reducing costs and by optimizing maintenance scheduling. SHM could furthermore be used to increase the understanding of composite behavior. This research focuses on investigating an image reconstruction algorithm which can be used to visualize impact damage on a composite plate using piezoelectric (PZT) sensors. The relationship between the reconstructed images and damage growth is analyzed using different excitation frequencies and different formulations of a damage index while also assessing the influence of sensor failure on the algorithm. Moreover, an experimental campaign is performed during which composite plates are subjected to Quasi-Static Indentation (QSI) testing in order to create impact damage and monitored to obtain data.

This master thesis research investigates the possibility to repair carbon fiber thermoplastic aircraft
structures using induction welding. Carbon fiber is conductive and heats up when placed inside an
alternating magnetic field. A generator, coil, and pressure frame are needed to perform induction
welding. The support plates needed to pressurize the carbon fiber parts need to be non-conductive,
non-magnetic, temperature resistant, stiff at high temperatures, and thermally insulating.
The material used is five harness satin weave carbon fiber PPS supplied in pre-consolidated plates
and unconsolidated semi-preg. Three different joint geometries are used in this investigation: a conventional
scarf, a continuous scarf, and a stepped lap joint. The stepped lap joint and the conventional
scarf are milled using a CNC machine. The continuous scarf is produced in a press using specialized
tooling and a press program prescribed by TenCate.
Of each type two specimens are produced: an induction welded specimen and a press joined specimen.
The specimens are tested in tension to determine the tensile strength and stiffness. This gives
the performance of the induction welded joints with respect to the press joined specimens. Additionally,
both of the welded specimens are compared to the pristine specimens.
By measuring the temperature along the weld-line of multiple test specimens, an induction welding
program is obtained for each joint type. Achieving a consistent temperature along the weld-line is
challenging due to the thermal conductivity of carbon fibers and other effects inside the laminate. Similar
to the continuous scarfed specimens, the press joined specimens are created using specialized tooling.
Before testing the specimens in tension, they are scanned using a C-scan. The press bonded
specimens show no flaws, whereas the induction welded specimens do not return the signal to the
transducer. Micrography of the specimens shows similar results as the C-scan. The press joined
specimens show little to no flaws and the induction welded specimens show a significant amount of
voids and small unjoined sections near the tips for the continuous scarf and stepped lap specimen.
The tensile tests show that all induction welded joints perform less than the press joined specimens
with a percentage of about 73-78%. The conventional scarf and the continuous scarf result in a similar
failure strength recovery of about 44% of the pristine failure strength for the press joined specimens.
Both the conventional scarf and the continuous scarf show similar behavior in welding and the tensile
testing, but the continuous scarf is more difficult to produce. Therefore the conventional scarf joint is
preferred over the continuous scarf joint. The stepped lap joint has the best performance, recovering
about 59% of the pristine stiffness. The stiffness of all tested specimens is between 92%-99% of the
pristine stiffness.
Inspection using digital image correlation and finite element modeling indicates the failure initiates
inside the PPS matrix in between the parts at the location of the first or last step. Finite element modeling
also suggests that a smaller angle for the scarf joint or a longer overlap for the stepped lap joint would
increase failure strength.

Damage is found in resin rich bead corner radii of RTM 6 epoxy-based composite ribs due to in-service thermal-mechanical loading after an aircraft inspection. The same types of damage are obtained through pure thermal cycling of a single bead. However, thermal cycling is a time-consuming process. Therefore, a faster way of damage investigation is required. A potential way to achieve this objective is by mechanically cycling a coupon specimen with simpler geometry than the bead.
To investigate potential solutions to the problem a literature review is performed. The scope of the literature review covers several topics as follows. Laminate fatigue damage modes and their impact on the laminate is researched. It is discovered that it is typical for RTM 6 epoxy-based laminates to build-up high matrix residual tensile stresses after manufacturing. Several differences between thermal and mechanical cycling are discovered. It is found that the most common way to test composites is by the use of ASTM standards. However, there is little available information about the fatigue behavior of laminates with resin rich areas.
Investigation of the fatigue behavior of laminates with resin rich areas is performed by the use of FEA and physical tests. Four specimen types labeled from A to D are manufactured. Type A is a dog-bone pure RTM 6 specimen. Types B to D are all composite specimens with the same in-plane geometry and different layups and manufacturing processes. All specimens are tested statically and in fatigue. In the fatigue testing session, fractography of the damage occurring at different test conditions for different specimen types is performed. In addition, two FE models are created. The first model is of the bead. The second model is a harmonized model applicable to all composite specimen types with required layup readjustments for each specimen