The thesis extends Lennart Krieg's modified linear viscoelastic fatigue life prediction framework to matrix-dominated angle-ply 45 glass-fiber/epoxy composite laminates. To account for matrix nonlinearity, Schapery's nonlinear viscoelastic constitutive theory was integrated into Krieg's fatigue life prediction equation via an augmentation parameter derived from creep-recovery tests. Applying a saturated capped augmentation parameter significantly improved life predictions, reducing root-mean-square errors across stress ratios (e.g. from 0.656 to 0.316 decades for R=0.3). Independent evaluations using dynamic mechanical analysis and stress-sweep testing yielded well-aligned fatigue limit estimates (69 - 77 MPa). Overall, this work provides a successful proof-of-concept for physics-based fatigue prediction in matrix-dominated composites ... Read more

The drive toward sustainable manufacturing has increased interest in repurposing metal scrap as a feedstock for powder production. Previous work shows scrap-derived powders can produce parts with mechanical properties comparable to virgin feedstock, but has focused largely on gas atomization (GA). Alternative atomization routes, such as ultrasonic atomization (UA), remain underexplored.

This thesis characterizes wire-derived UA SS316L as a feedstock for laser powder bed fusion (LPBF), comparing directly with a commercial GA powder. A UA process window was first established through Archimedes density and image analysis of cubes. The densest UA parameter set was then applied to both feedstocks to fabricate cube, tensile, and hardness specimens under two scan strategies (67° and contour), and BCC lattices under 67°. Cubes were characterized for porosity, microstructure, crystallographic texture (EBSD), and phase composition (XRD). Mechanical performance was evaluated through tensile testing, Vickers hardness, and lattice compression. Powder reuse was assessed over five successive builds.

Both feedstocks achieved relative densities above 99%. Across both scan strategies, UA specimens showed slightly lower yield and ultimate tensile strengths than GA with slightly higher ductility, though the differences were modest. Feedstock type had limited influence on grain morphology, whereas scan strategy strongly affected crystallographic texture. Tensile and hardness values for all specimens fell within the range typically reported for LPBF SS316L. UA-derived BCC lattices exhibited compressive response comparable to GA counterparts. After five reuse cycles, UA powder exhibited only limited degradation in feedstock and part properties.

Overall, wire-derived UA SS316L is shown to be a viable LPBF feedstock for dense bulk components and BCC lattice structures. These findings provide a useful foundation for future work on ultrasonic atomization as a scrap-based route for metal AM feedstocks. ... Read more

This work presents the analysis of on-edge impact loading of carbon fibre reinforced polymer(CFRP) material. On-egdge impact are rarely researched and they often consume a lot of time and resources to be tested. This work presents a possible easier set-up to analyze the response of the CFRP using a conventional droptower. First, quasi-static tests are performed to gain initial mechanical understandings of the material, including the energy absorbed by the specimen. Then efforts are dedicated to impact load the specimen at same energy using the droptower. Then the impact energies are increased to further investigate the effects of the strain-rate loading for on-edge loading. The damages are compared through the means of microscopy and ultrasonic C-Scan to assess the visible damages and the extent of the damages. While it was possible to perform reliable energy analysis and the damages were compared in a detailed manner per strain-rate used, the force and impulse data of the specimen were not able to evaluated to full extent. The nature of the droptower, measuring the force experienced by the impactor and the vibration caused due to the impact hinders the data significantly. For future, it is recommended to present a set-up which can measure what the specimen experiences directly, rather than relying on the information from the droptower to gain information about the specimen.
... Read more

Ultra-High Temperature Ceramic (UHTC) additives have the potential to improve the oxidation resistance of C/C-SiC composites used in thermal protection systems and other extreme-temperature aerospace applications. This study investigates the applicability of established Ceramic Matrix Composite (CMC) manufacturing techniques, including slurry infiltration and alloy infiltration, for incorporating zirconium diboride additives into Reactive Melt Infiltration (RMI)-produced C/C-SiC composites. The objective was to identify manufacturing routes capable of achieving a homogeneous distribution of UHTC particles and to evaluate their influence on composite microstructure and high-temperature oxidation behaviour. Composite plates were characterised by scanning electron microscopy throughout the production process and following two high-temperature oxidation tests - a furnace test and an Oxyacetylene torch test. The results indicate that slurry-based processing routes provided most uniform particle distribution and offer favourable prospects for industrial upscaling through tape-winding prepreg manufacturing. However, process modifications are required to reduce residual porosity, which exceeded 20% at pyrolysed stages (compared to 7-14% in non-slurry techniques), in turn resulting in 4-5% remaining porosity at silicon infiltration, as opposed to 1-2%. In all specimens, ZrB₂ agglomeration was observed within Si-rich and SiC-rich regions, suggesting the occurrence of an alloying or reaction process during manufacture, independent of overall porosity levels. During high temperature testing, all plates showed formation of SiO₂ protective layer, however the slurry plate had the lowest weight loss and showed ZrO₂ formation closely matching that of what has been seen in literature for regimes over 2500°C. Slurry-based techniques were identified as the overall most promising approach for incorporating UHTC additives into RMI-manufactured C/C-SiC composites due to their superior particle distribution, easier handleability and compatibility with scalable manufacturing routes. Further work is required to determine the effect of ZrB₂ agglomeration on the mechanical performance and long-term oxidation resistance of the material. ... Read more

Stiffened composite panels are a lightweight, high stiffness-to-weight structural concept widely used in modern airframes, but the designs returned by structural optimization are rarely manufacturable. Free-size optimization produces a continuous, per-angle thickness field with jagged boundaries and sub-scale features that no Automated Fiber Placement (AFP) machine can lay down, and closing that gap is normally left to manual reinterpretation of the optimized result. This thesis therefore develops an integrated optimization and design-for-manufacturing framework that, within a single automated workflow, takes a stiffened composite panel from an arbitrary initial stiffener layout on a flat skin to an optimized stiffener layout on a blended, manufacturable variable-thickness laminate, and quantifies the structural cost of making that design manufacturable.

The stiffener layout is optimized explicitly using the Moving Morphable Components method in a Python-OptiStruct loop, after which the skin is sized by free-size optimization based on the converged layout, which returns a homogenized skin definition, a continuous total thickness and fiber-angle percentages per element, but no lay-up. This thickness field is made manufacturable in three stages: clustering into discrete thickness zones, a stacking sequence table parametrization that builds a guide laminate from the free-size lamination parameters and drops plies in an importance-driven order subject to the standard laminate and ply-drop guidelines, and a morphological filter that regularizes each fiber angle along its own direction at the AFP minimum cut length. A secant-based compensation step then returns the filtered design to its target material budget.

The framework is applied to an aircraft flat pressure bulkhead under uniform out-of-plane pressure, in-plane compression and asymmetric pressure, with the stiffener layout optimizer first verified against a reference configuration from the literature. Compared at equal volume, the final manufacturable designs improve on a uniform panel baseline by 15%, 18% and 7%, corresponding to structural knockdowns of 22%, 8% and 18% against the free-size theoretical optimum. The delivered designs therefore recover between roughly a third and three quarters of the performance benefit available from the optimization. The cost of manufacturability is not a fixed penalty: relative to the total available performance gain from optimization, it is smallest for the membrane-dominated compression case, whose stiffness depends on how much material is present rather than exactly where, and largest where the optimum relies on a fragmented, bending-critical reinforcement network that the filter is forced to remove.

Sensitivity studies over requested volume fraction, minimum cut length and ply thickness confirm this trend, and a guideline relaxation study shows that removing laminate constraints does not reliably improve the delivered design. This traces to the structure of the framework itself: the guide laminate and ply-drop order are selected on structurally motivated but approximate criteria that are never re-evaluated against the compliance of the design they produce. Together with a compliance-based objective and a four-angle suite, this bounds how closely the manufacturable design can approach the theoretical optimum, and indicates where the framework can most readily be improved.

Overall, this work shows that structural optimization and AFP manufacturing constraints can be combined in a single automated workflow, and quantifies the resulting structural cost for stiffened composite panels under a variety of load cases. This allows the cost of manufacturability to be predicted early in the design process and weighed when choosing parameters such as minimum cut length and ply thickness, rather than being discovered in later design stages

... Read more

This thesis investigates whether the picture-frame test can be used as an element-level validation method for conduction-welded thermoplastic composite joints. Pristine and defect-containing panels were tested under free-buckling and anti-buckling conditions, with their response assessed using force–displacement data, Digital Image Correlation, strain gauges, visual inspection, and ultrasonic C-scans. The experiments were compared with finite element models using Cohesive Zone Modelling and the Virtual Crack Closure Technique, based on independently obtained interface fracture properties. The results show that the picture-frame test can generate interface failure at panel scale, while also revealing the influence of buckling, weld geometry, manufacturing variability, and competing laminate damage on the structural response. The numerical analyses reproduce selected experimentally observed deformation and damage mechanisms and provide useful insight into the transfer of coupon-derived fracture properties to a larger structural scale. The picture-frame test can therefore be used as an element-level validation method, but its reliability is conditional rather than universal. ... Read more

This thesis investigates the application and extent of validity of a novel physics-based energy-balance approach with the Sliding Box analogy. The scope focuses on predictive modeling of fatigue crack growth in aluminum alloys at low temperatures. The motivation for this research arises from the increasing demand for reliable fatigue-life prediction methods in the context of hydrogen-powered aviation, where liquid-hydrogen storage tanks and adjacent structures are exposed to cyclic mechanical and thermal loading in cold environments. Traditional empirical approaches, such as Paris’ law, are limited by their reliance on extensive experimental calibration, especially when material properties and environmental factors vary significantly.

The research aims to expand and validate the Energy Balance with the Sliding Box analogy (EBSB) framework by comparing its predictions to a comprehensive experimental dataset covering both room-temperature and low-temperature conditions down to -30°C. The study considers a diversified selection of aluminum alloys with quasi-static property differences that emulate changes expected at low temperature. The investigated materials are 7075-T6, 2024-T3, 6061-T6, and an artificially over-aged 2024-A300. Additionally, thicknesses from 1 to 6.5 mm and two stress ratios, R=0.1 and R=0.5, are varied to evaluate their influence on the modeled fatigue crack-growth response. The methodology combines quasi-static tensile testing to determine case-specific elastic--plastic properties with constant-amplitude fatigue crack-growth experiments on standard geometry specimens. Crack growth is measured via periodic high-contrast image capture. Furthermore, Digital Image Correlation (DIC) is employed to evaluate plastic-zone development from periodic displacement-field measurements around the advancing crack tip.

The results demonstrate that the EBSB model—particularly in its direct sliding-box form—captures the general trends and order of magnitude of fatigue crack growth in aluminum alloys well. However, predictive accuracy is limited by the current formulation of the plastic energy dissipation term and by the treatment of stress-state transitions (plane stress/plane strain). The quantification of plastic-volume growth within the energy balance was varied in an attempt to capture increased energy absorption at low temperature. The model tends to overestimate crack-growth rates, especially for pronounced variations in yield strength and strain hardening relative to the 7075-T6 baseline. This effect is not mitigated by adopting larger plastic-zone approximations. For lower thicknesses of 7075-T6, however, using a larger plastic-zone estimate reduces model error, consistent with better agreement with the plane-stress assumption.

Overall, the EBSB model is sensitive to material-specific strain-energy absorption characteristics, particularly yield strength and the degree of strain hardening. Experimental observations confirm the expected increase in yield strength and work hardening at low temperature, but also reveal an unexpected decrease in ductility for the low-temperature 7075-T6 tests, highlighting the complex coupling between material behavior and environment. Improvements to the EBSB framework are proposed, including a parametrically defined plastic strain energy density derived from quasi-static stress--strain data and the introduction of a damping term to account for crack-shielding effects. Recommendations for future work include expanded cryogenic testing, improved DIC resolution, and the development of more robust analytical and numerical treatments to better capture strain hardening, stress-state transitions, and plastic-zone evolution. ... Read more

This research investigates the influence of Ultra-High Temperature Ceramic (UHTC) additives on the microstructure and oxidation behavior of Ceramic Matrix Composites (CMCs) synthesized via Reactive Melt Infiltration (RMI). The study focuses on three key parameters of the UHTC additives: particle size, additive volume, and chemical composition. Comprehensive material characterization was conducted throughout the RMI process to study the relationships between these parameters and the resulting microstructural quality. The synthesized samples were subjected to furnace oxidation tests at 1000°C and 1200°C over varying exposure durations. The experimental results demonstrated the oxidation response of ZrB2- and ZrC-based UHTCMC materials produced by RMI, providing insights into optimizing additive selection and processing conditions for improved (ultra)-high temperature application. ... Read more

The increased use of adhesively bonded joints in aerospace structures has given rise to ample research opportunities on the topic of ensuring structural integrity. One such method of verifying bond strength is through the use of non-destructive testing methods. One particular defect type, kissing bonds or zero-volume bonds, is notoriously difficult to detect using traditional techniques such as ultrasonic C-Scan. This thesis explores the feasibility of using Acoustic Emission (AE) monitoring to detect defects and characterize failure modes for adhesively bonded composite joints. Numerous Double Cantilever Beam (DCB) specimens containing various defect types were manufactured and tested under static Mode I loading conditions. Defect types included were pristine specimens, inclusion defects and artificial kissing bonds replicated through contamination of the adherend surface using release agent. Mechanical testing revealed that bondline defects significantly reduce the effective fracture toughness Gef f of defective specimens and induced alternative failure modes instead of complete cohesive failure. Most notably, specimens contaminated with release agent showed adhesion failure consistent with kissing bond type defects, while remaining undetectable when using conventional ultrasonic C-Scan inspection. AE monitoring allowed for identification of signal characteristics corresponding to cohesive failure, delamination and adhesion failure caused by kissing bonds and other defect types. Clustering of acoustic signals revealed characteristic frequency bands for certain defect types and failure modes. In particular, a cluster of signals of 130 [kHz] and 170 [kHz] peak frequency respectively could be correlated to delamination and cohesive failure. Wavelet transforms of the measured signals further revealed the broad frequency spectrum present in all specimen types. However, inconsistencies in the characteristic failure modes attributed to certain frequency bands make this mode of identification insufficient in its current state to detect failure modes with complete certainty. ... Read more

Fused filament fabrication (FFF) of thermotropic liquid crystal polymers (LCPs) presents an attractive route toward lightweight components with excellent in-plane mechanical properties, enabled by molecular alignment during extrusion. However, the layer-by-layer nature of FFF leads to weak interlayer adhesion, which severely compromises mechanical performance in the build direction (Z-axis). This anisotropy limits the structural reliability of printed LCP parts under demanding conditions. Inspired by through-thickness reinforcement methods used in laminated fiber-reinforced composites, this thesis investigates the potential of z-pinning to enhance interlayer strength and damage tolerance in FFF-printed LCPs.

This work focuses on adapting the z-pinning concept, previously applied in PLA systems, to the challenges of anisotropic, shear-aligning LCPs. A z-pinning methodology is developed around the commercial filament Vectra® A950, utilizing a bottom-up insertion process enabled by custom G-code routines. Vertical pins are extruded into pre-formed voids during the print, allowing control over pin shape, height, and placement. The approach leverages standard FFF hardware, requiring only a narrow-tip nozzle and careful synchronization of extrusion timing to ensure consistent pin deposition.

Mechanical testing was conducted on both pinned and unpinned tensile specimens printed in the Z-direction. The results demonstrate that z-pinning significantly enhances performance when properly implemented. The best configuration, tall pins arranged in an ABA staggering pattern, achieved a 40% increase in peak load and a tenfold increase in energy absorption before reaching peak load. Fracture analysis revealed more distributed fracture patterns, with signs of crack deflection and arrest, indicating a transition from brittle delamination to
more progressive failure modes.

These findings validate the feasibility of z-pinning for improving the mechanical properties of 3D-printed LCP components. However, the benefits are highly sensitive to process execution, as poor pin deposition may negate reinforcement or introduce stress concentrators. The study underscores the importance of concurrent design and manufacturing development, showing that even in single-material systems, structural performance can be engineered through localized deposition strategies. This opens a path toward more robust, anisotropy-mitigated 3D-printed parts using high-performance polymers. ... Read more

This report investigates the damage behavior of cold spray repairs subjected to low velocity impact (LVI) loading, with a focus on their potential application for aircraft cowling inlet lip repairs. Cold spray technology is increasingly considered as a promising alternative to traditional thermal spray and Laser- clad technologies due to its ability to significantly reduce repair costs, decrease residual tensile stresses in parts and decrease turn-around time during maintenance and overhaul. Unlike conventional thermal spraying methods, cold spray operates at temperatures below the melting point of the feedstock powder, allowing particles to remain in the solid state throughout the deposition process. As a result, it avoids the formation of heat-affected zones, preserves material properties, and induces favorable compressive residual stresses. These advantages make cold spray particularly suited for both structural and non- structural aerospace repairs. However, insufficient research exists to verify the quality of cold spray repairs on aircraft components to obtain the technology certified.
The primary objective of this research was to identify and characterize the dominant damage mecha- nisms that occur in cold spray coatings and repairs under LVI conditions. Six types of cold sprayed aluminum alloy coupons were evaluated, including both flat coatings and double curved blend-out re- pairs and cold spray depositions of AA1XXX and AA2024. The depositions were applied using either a robotic system or a manual gun. All specimens were subjected to controlled impact loading via drop weight and/or quasi static indentation tests. The results were analyzed using high-speed camera imaging, optical microscopy, and scanning electron microscopy of both surface and cross-sectional features.
Key output parameters such as dent depth, dent width, absorbed energy, and crack formation were analyzed and compared across different coupon types and impact energies. Results show that:
• Deposition geometry plays a critical role in how the material deforms under impact.
• The ductility of the deposition and substrate materials (e.g., AA1XXX vs. AA2024) influences both energy absorption and crack resistance for varying impact location.
• Coarse-gritsurfacepreparationenhancesbondingandincreasesresistancetodelaminationcom- pared to finer surface treatments.
• Barely visible impact damage does not apply to cold spray depositions as a dent is always visible. However, the substrate (non-impact side) showed different crack thresholds and damage mech- anisms compared to the cold spray deposition. The substrate cracked more severely compared to the cold spray deposition.
• Manual gun-sprayed depositions exhibit non-uniform properties, leading to variable crack thresh- olds across the repair area.
To compare performance independently of thickness, the impact data was normalized by the deposi- tion thickness. This revealed that thin coatings (like AA1XXX on top of AA2024) show lower energy absorption per millimeter compared to blend-out repairs.
In conclusion, the study highlights the critical influence of deposition material, geometry, thickness, and surface treatment on the performance of cold spray repairs under impact. It demonstrates key differences between robotic and manual spray systems, particularly in terms of uniformity and damage treshodls. The results show that cold spray depositions are able to withstand low velocity impact and can thus be used to repair dents in cowling inlet lips. The insights contribute to the ongoing research to integrate manual cold spray gun systems into aerospace repairs. ... Read more

Sandwich structures are widely used in lightweight engineering applications due to their high stiffness-to-weight ratio and good energy absorption capabilities. However, their behavior under highly localized and dynamic loads remains complex to model accurately. This thesis investigates the transient response of circular sandwich panels subjected to high-dynamic pressure waves. An analytical model based on classical plate theory and modal superposition was developed to predict the displacement evolution of the panel. An analytical method to predict delamination onset through an energy-based criterion was developed as well. The results were validated through comparison with experimental data. The comparison shows good agreement in terms of the overall de- flection shape and magnitude. The proposed model captures the essential physics with minimal computational cost and offers a foundation for design optimization of dynamic load-resistant sandwich components. Future developments may focus on incorporating progressive damage models, as well as higher-order plate theories to enhance the predictive capability of the present model. ... Read more

Finite Element Methods (FEMs) are widely employed for damage simulation in composites, with the Cohesive Zone Model (CZM) being particularly favored for its robustness and scalability. Despite the CZM's mesh-dependent nature, the Discontinuous Galerkin Cohesive Zone Model (DG/CZM) formulation allows for relatively arbitrary crack propagation thanks to highly refined meshes and its strong suitability for parallel implementation, which is crucial for fracture simulation in composites.

However, a challenge with the CZM in modeling damage in composites lies in determining which damage mechanism occurs at a given numerical interface. To date, fracture simulation in composite laminates has mainly relied on methods designed for isotropic materials, which come with notable modeling constraints such as limiting the arbitrariness of damage evolution and requiring experimental data on fracture propagation.

In this thesis, a novel approach is proposed to ensure arbitrary damage development solely based on the structural stress state using the DG/CZM approach. ... Read more

Ice accumulation on aircraft surfaces is a prevalent issue, and icephobic coating strategies can be implemented to assist in the removal of or ice on an aircraft. However little is known regarding the effects of novel icephobic coatings and surfaces on ice adhesion strength, which is difficult to measure and quantify, and is typically reported in literature with a high standard deviation and scatter. In this thesis, a reliable set-up for testing ice adhesion strength is successfully designed, constructed, and validated. Using this set-up, it is possible to examine the relationships between various surface parameters and ice adhesion strength, and to analyse the scatter in order correlate the results both qualitatively and quantitatively. Additionally, the influence of material and topology on the adhesion strength and failure mechanisms of ice is investigated. ... 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.
... Read more

To facilitate the transition to liquid hydrogen (LH2) as an alternative aircraft fuel, the stor age tanks need to be closely monitored to allow for safe operation. This research explores the feasibility of using an acoustic emission-based structural health monitoring method to detect and localise simulated damage on a metal double-walled vacuum insulated tank. The complicated inner and outer tank geometry makes it a novel application of an established method.
To be able to perform experiments, a metal double-walled vacuum insulated tank with a fixed and flexible support was cut in half to make the inner tank accessible. Pencil lead break tests were performed on the inner and outer tank and recorded by lead zirconate titanate (PZT) sensors to assess localisation accuracy. It was found that a pencil lead break (PLB) performed on the inner tank could be detected by a sensor placed on the outer tank. Subsequently, an accuracy of 80% was achieved in making the distinction whether the PLB was located on the inner or outer tank. For the PLBs placed on the inner tank a 1D localisation accuracy of 17 [mm] and 21 [mm] was achievable for the flexible and fixed support respectively. For the outer tank a 1D localisation accuracy of 46 [mm] was achievable, while for 2D localisation this was 41 [mm].
It is concluded that with sensors placed solely on the outer tank, both the inner and outer tank can be monitored. Additionally, simulated damage on the inner and outer tank could be localised with an accuracy below 10% of the tank’s dimension. Therefore, based on the feasibility study performed, acoustic emission-based structural health monitoring is deemed a viable method to enhance the reliability and safety of metal double-walled vacuum insulated LH2 tanks. ... Read more

Fibre Metal Laminates (FMLs) have been widely utilised in aircraft structures for their specific strength, durability and damage tolerance. Most aircraft structures, due to their size, require joining, and FMLs allow for a special type of joint called the splice, a unique but a lesser known joint.

The bulk of research into the splice joint has been conducted in the early 2000s for the A380 program, where a specific type of splice has been implemented, the overlap splice. While it is a proven design, it was expected that other splice designs could be as good if not better than the overlap splice. Further, the pre-existing design guidelines on the splices were not suited for detailed design that would target specific mechanical strengths of the joint, like durability and
damage tolerance. Therefore, this research focused on design iterations of the two splice types, the butt splice and the overlap splice, that would provide insights into splice design guidelines against fatigue and demonstrate which splice configuration is a superior joint. An extension
of this research focuses on identification of location and lifetime to damage initiation within the splice using analytical numerical methods, which was previously done on uninterrupted FMLs, but not on spliced FMLs.

Research comprises of a multi-disciplinary approach, combining Finite Element Method (FEM), numerical predictive modelling and experimental investigations that then also doubled as means of validation of the models built. Design iterations were based on the influence that the parameters, labeled a through e, within the splice joint have on the overall stress field within the joint and how that affects the joint’s fatigue life to damage initiation, Ni, a fatigue
performance indicator of the study. It has been discovered that changing the tolerances within the splices provides little influence on the stress field within the splice, which resulted in several iterations of the splice design approaching smaller and more lightweight joint alterations. The designs showed excellent durability characteristics when compared to the Limit of Validity (LOV) cycles set by the aviation authorities, such as EASA and FAA. The Finite Element (FE) model was able to accurately depict the stress field within the splice, validated by the experimental data through strain fields captured using Digital Image Correlation (DIC) technique, and consequently accurately pointed to the location of damage initiation, which in all cases was the outer-most overlap on the flush side of the joint. Along with the adapted predictive numerical model it was possible to predict the damage initiation lives in the spliced specimens with a blunt notch. The predictions in the samples without the notch resulted in far lesser agreement with experiments. This was likely caused by to the limitations of the model, which only took into account quasi-static loading without damage and is highly dependent on the reference data used.

It was concluded that splices can be designed smaller and lighter than previously done due to the tighter tolerances allowed within the splice. Overlaps of 5 mm in both the unnotched butt splice and the unnotched overlap splice, however, resulted in an alternative damage progression mode, specifically a complete delamination of the external overlap rather than metal fatigue cracking. This is attributed to the rising average shear stress in the adhesive. Regardless, the smallest and lightest iterations of the butt splice and the overlap splice with
5 mm overlaps and gaps between aluminium interruptions showed excellent durability and the design iterations were found to affect the damage initiation life very little, considering that fatigue damage initiation is a subject to scatter. It is hard to draw a concrete conclusion on the damage tolerance of the updated designs due to alternative damage modes and lack of research thereof. While both splice types were deemed to be successful in their role of a joining structure, the butt splice was concluded to be superior to the overlap splice when it came to fatigue performance, consistently developing visible damage later than the overlap splice. This gap in fatigue performance is expected to expand if thicker layers or larger number of layers are to be considered. This is because of the secondary bending which occurred in the joint, more so for the overlap splice than the butt splice judging from FEA results and experiments.

It is recommended that a more extensive FE model is developed using the model built in this research as a foundation. The current model could be expanded in several ways, such as simulations of damage progression, individual modelling of the fibre layers, fibre failure models, and a curing simulation. This will improve the validity of the methodology, specifically concerning the unnotched splice specimens, and improve the predictions of not only initiation lives, but also failure lives.
... Read more

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

The concept of lightweight design is driving future aircraft to exploit all the available strength of materials and further reduce the weight of an aircraft leading to lower fuel consumption and more sustainable aviation. Current designs introduce rivets and bolts to join the structure and simultaneously create holes in the pristine material, which is not wanted due to the local stress concentration. If the design wants to exploit all the strength of the material, stress concentration should be avoided using suitable joining technologies that don't require holes in the structure. This is possible with the adhesive bonding technology. However, many factors, such as the effects of manufacturing and impact-induced damage are still not fully understood. Thus, the damage tolerance of the design cannot be guaranteed. This results in conservatory safety factors being prescribed in the design process, which possibly reduces the exploitation of all available material strength.

The occurrence of barely visible impact damage (BVID) in aircraft composite structures, especially in adhesive joints, is a serious issue that can jeopardise an aircraft's structural safety during operation. However, virtual testing and numerous numerical approaches allow high-fidelity simulation of damage initiation and propagation in adhesives and composite adherends to predict the residual strength of the bonding accurately. Although the development is fast, accurate impact simulation is still computationally very expensive and thus not applicable in industrial cases where behaviour needs to be analysed to assess the structures' design, maintenance and repair.

This thesis investigated two topics that allow robust and accurate simulation methods to assess the effect of manufacturing and impact damage on the residual strength of bonded joints. The first topic is related to the development of the composite material damage model, where the multiscale material model is created by the use of a representative volume element (RVE). The second topic of simulation methodology is the development of the quasi-static simulation approach for the damage tolerance assessment of bonded joints. Modelling approaches to simulate residual strength are investigated through the finite element modelling of three groups (i) pristine, (ii) artificially damaged during manufacturing, and (iii) impacted bonded joints. In the numerical models, an approach based on the observation of the fracture surface of single-lap joints in different geometry and layup configuration is proposed in which the damage assessment focuses on the bondline area up to the first 0° ply. For the modelling of the damage in joints, two modelling techniques were studied, first removing elements in the damaged area and second detachment of the elements. Utilising those techniques, simplified approaches to model damage resulting from the impact were studied, with the modelling of impact damage as a hole, where all through-thickness elements are deleted and as delamination, where interlaminar cohesive zone elements are deleted.

The developed multiscale material model allowed for an accurate representation of failure modes occurring in the composite material by the characterisation of elastic, damage and plasticity parameters of the fibre and matrix constituents in the homogenisation and inverse characterisation process. The results showed that the deletion of the elements can be used to represent defects and damage of different kinds in the composite bonded joints, both in the adhesive and adherend. The comparison of different representations of the impact damage in the single lap joint configuration revealed that the best prediction in terms of the ultimate load, failure mode and size of the model is obtained with the simplified representation as a single delamination positioned before the first 0° ply in the layup and in the studied adherend configuration that was between the first (45°) and second (0°) ply in the layup. The study ends with the conclusion that in different geometries of single lap joints and different damage types studied, a numerical analysis should focus on the region of the overlap edge and in the thickness direction from the bondline up to the first 0° ply in the lay-up. The results and numerical method developed during this thesis create a base for the further investigation of a variety of impact cases on bonded joints. ... Read more

Efficient maintenance scheduling is a critical objective for aircraft carriers and Maintenance Repair and Overhaul Organizations (MROs) to minimize aircraft downtime. While predictive maintenance models have improved, accurately identifying materials, especially spare parts, for specific maintenance events remains challenging. This paper combines the challenges of identifying spare parts by MROs and aftermarket distributor demand models by developing a robust prediction model (SPSO-CM) to forecast subsequent customer-specific purchases of maintenance planning document (MPD) related spare parts, considering technical documentation and previous sales records. The proposed architecture employs a gradient-boosting algorithm with numerous data mining improvement techniques to predict the likelihood of a subsequent spare part purchase from a customer. A k-means clustering algorithm is used to group spare parts with similar characteristics, as certain specific spare part properties significantly influence demand prediction models. A unique feature selector and nested group K-fold TimeSeriesplit cross-validation method were developed and incorporated into the Bayesian search space to optimize hyperparameters and improve performance. Two test cases were simulated, and the results demonstrated that SPSO-CM is more effective in forecasting proprietary parts and frequently purchased spare parts than those with extremely lumpy demand patterns. Two potential applications for an aftermarket distributor are discussed, one from a customer-level perspective and another at a larger supply-chain level, highlighting its promising capabilities. ... Read more