Resonance vibration testing accumulates fatigue cycles far faster than conventional servo-hydraulic methods by driving a specimen at its own natural frequency, and the same response that applies the load also carries information about the damage accumulating within it. However, the literature offers no systematic comparison of competing vibration-based damage indicators measured simultaneously on the same metallic specimens, and no characterisation of how far ahead of failure such indicators support a useful remaining-life estimate.

This thesis addresses both gaps. Eight notched Al~6061-T6 beam specimens were driven to failure at a fixed excitation frequency and interrogated between load blocks with a scanning laser Doppler vibrometer, yielding 33 features per block. A five-layer pipeline ranked those features by sensitivity, combined them into six competing damage indices, and estimated remaining useful life by trend extrapolation and by a population Gaussian Process. Two further specimens were interrupted at the point of detection for optical
fractography.

Measurement reliability, rather than indicator choice, governed what the campaign resolved. Only two modes could be tracked stably, mode-shape correlation between two undamaged blocks of the same specimen fell as low as 0.785, and mode labels did not consistently correspond to the same physical mode. Modal parameters changed abruptly at the transition to full-amplitude loading rather than progressively, which physical inspection attributed to the clamping and load-transfer arrangements. Indicators requiring no modal identification, principally the harmonic distortion index and amplitude RMS, proved more dependable than frequency, damping, or coherence, and no damage index reached a resolved population-level prognostic horizon.

These outcomes form a traceable account of where vibration-based fatigue prognosis succeeds, where it does not, and which failures are attributable to method rather than principle. It delivers a ranked indicator comparison, the first prognostic horizon characterisation for this application on metals, quantified design requirements for resonance fatigue fixturing, and three reusable open software tools. ... Read more

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

Fatigue characterization of fiber-reinforced polymer composites is slow and costly, which motivates the search for faster screening tools. This thesis assesses whether Dynamic Mechanical Analysis (DMA), a quick measurement of viscoelastic properties, can indicate the fatigue performance of such composites, and what a DMA-based screening method would require. Two unidirectional glass/epoxy materials, denoted KS02 and KS03, made from the same fiber and resin but differing in glass transition temperature, were characterized by DMA temperature and frequency sweeps and compared against existing transverse-tension fatigue, hysteresis, and static tensile data. When the DMA and fatigue results proved difficult to reconcile, the scope was extended to Differential Scanning Calorimetry (DSC) and optical microscopy.

DMA separates the two materials clearly in their glass transition temperatures, which differ by roughly \SI{20}{\celsius}, but their fatigue lives overlap across most of the stress range, so this thermal difference does not carry through to fatigue life. The pristine DMA damping metrics, room-temperature $\tan\delta$ and loss modulus $E''$, instead track the character of damage accumulation: the more dissipative KS03 shows wider hysteresis loops, a more gradual early-life loss of stiffness, and a more textured, deflected fracture path, while the stiffer KS02 retains its stiffness until abrupt, brittle failure. DSC indicates that the two materials differ in more than cure state, most likely in resin-to-hardener mixing ratio, leaving cure state and composition confounded. The results are consistent with an interpretation in which the matrix governs the character of damage while the fiber-matrix interface governs fatigue life. DMA is therefore found to carry information about the character of fatigue damage but not, for this material pair, about fatigue life, and a calibrated screening method cannot be built from two materials whose S--N curves overlap and confound several processing variables. A calibration dataset varying one processing variable at a time, with the composition difference first confirmed by Fourier-transform infrared spectroscopy, is identified as the necessary next step. ... Read more

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

Cold spray is a material deposition process where a high-pressure gas stream accelerates small particles through a supersonic nozzle and deposits them in a coating on the substrate material. The powder is typically shipped and stored before use, so degradation could occur. This research investigates the influence of time-dependent degradation of cold spray powder materials and how this affects the sprayed coatings. Conditions of four different samples of AA6061 and AA2024 powders were evaluated with Raman spectroscopy, X-Ray diffraction, and SEM-EDS over several time intervals to monitor the development of oxides and alterations in powder surfaces. Single particle launches using a laser system were performed at Mines Paris-PSL to observe the particle-scale behavior during replicated spraying experiments. A spraying test comparing three batches of AA6061 powders using two different nozzle traverse speeds provided more information for the coating behavior of the powders.

The powder condition was shown to be altering with time, though the underlying causes other than oxidation and possible phase transformation could not be identified. There were few particles adhering during the single particle experiments, and some rebounding particles left behind pieces of oxides on the substrates. The powder degradation is expected to result in higher compressive stress in the coating caused by shot-peening effect from successive rebounds and a chance of higher oxide content. ... Read more

Human presence in cave systems stretches back thousands of years. These caves have served as vital shelter, protecting people from harsh weather, wild animals, and other threats. Beyond their practical role in survival, caves also offer invaluable cultural insights. The walls of many caves are adorned with paintings that reflect aspects of identity, hunting practices, religious beliefs, and much more, providing a window into the lives and values of early humans... ... Read more

This thesis discusses the design of a novel pan-and-tilt unit, which can be used in several fields of application, with the aim of designing a lightweight and cost-efficient product. The unit is engineered to withstand severe vibrations, high shock loads and extreme temperature fluctuations, ensuring reliable performance in a harsh maritime environment. The pan-and-tilt unit is designed to comply with several military standards and to be operational with high accuracies during several thermal and vibration loads. However, during the analysis of requirements it was discovered that achieving operational accuracy during the proposed standards for vibration (AECTP400) would result in an overly stiff and heavy design. This insight led to a reassessment of the system, allowing for a slight loosening of the vibration operationality criterion.

Evaluating the full list of requirements the main design driver was the operationality during deck vibrations, leading to a stiffness driven design instead of a strength driven design. A conceptual design study explored several configurations, including L-brackets, U-brackets and a T-bracket concept. A trade-off led to the development of a compact T-bracket design, integrating high-tech components as a ring encoder, band brake and bearing pair. Initial concepts made from aluminum showed promising weight reductions but introduced excessive stresses on the bearings made from 100Cr6 steel, ultimately leading to a shift to a full stainless steel design for improved material compatibility. Welding was chosen as the primary production technique due to its accessibility and flexibility for thin walled stainless steel, though additive manufacturing is identified as a promising alternative for future iterations, especially for enabling complex geometry.

Various design options were explored and evaluated based on mechanical robustness, weight efficiency and manufacturability. A modal analysis, thermal analysis and shock analysis proved the pan-and-tilt unit could be constructed with thin-walled stainless steel. The thermal analyses included research on the bearing stresses, asymmetric radiation and internal heat generated. The findings offer a comprehensive assessment of viable design choices, justifying the selection of the final conceptual design. This research contributes to the advancement of stabilized pan-and-tilt platforms in dynamic and extreme environments.
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Composites are widely used in structural applications due to their high strength-to-weight ratio, stiffness, and design flexibility, but their sustainability remains a limitation. To address this, flax fibre-reinforced polymer (FFRP) composites offer competitive mechanical performance while being biodegradable and less energy-intensive to produce. However, the use of FFRPs in load-bearing structural applications is constrained, in particular, by the susceptibility of flax fibres to environmental conditions such as temperature and humidity, and by a limited understanding of their delamination behaviour in the primary loading modes. Therefore, this study investigates the interlaminar fracture toughness of FFRP composites under various hygrothermal conditions in Mode I loading.

The experimental analysis was conducted under environmental conditions representative of natural weathering, including hot-wet, hot-dry, room, and cold environments. The samples were initially conditioned at the respective hygrothermal conditions and subjected to quasi-static and fatigue loading in an environmental chamber. The results demonstrate a strong dependence of fracture toughness on the applied hygrothermal conditions, indicating that FFRP composites are highly sensitive to both temperature and relative humidity. Under quasi-static loading, the fracture toughness increased with higher humidity and lower temperature, indicating enhanced crack growth resistance due to moisture and improved
fibre bridging, while a reduction in fracture toughness was observed under low-humidity conditions. Fatigue results showed distinct Paris curves, with a rightward shift observed under high-humidity and low-temperature conditions, indicating improved resistance to fatigue crack propagation, whereas Paris curves corresponding to low-humidity environments shifted leftward, reflecting decreased resistance to fatigue crack growth.

Fractographic analysis using optical microscopy and scanning electron microscopy (SEM) revealed common microstructural features such as technical fibre bridging, fibre pull-out, yarn loosening, fibre patches, scarps, and matrix cracking. The nature of fracture transitioned from ductile under high humidity and elevated temperature to brittle at low temperature, highlighting a shift in the dominant failure mechanism from interfacial debonding to matrix-dominated cracking. Surface roughness measurements, however, exhibited considerable statistical scatter across all environmental conditions, likely due to the strong influence of technical fibre bridging on the measured roughness. Consequently, the observed changes in Mode I interlaminar fracture toughness with humidity and temperature were not clearly reflected in the roughness parameters.

Overall, the findings emphasise the strong dependence of the fracture behaviour of FFRP composites on environmental exposure. Understanding these effects is critical for the reliable design and durability prediction of FFRP composites in structural applications. The results contribute to establishing a foundational understanding of the fracture mechanics of FFRPs.
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This thesis investigates the low-cycle fatigue behaviour of a deployable composite boom developed in the SpaceMast project at the German Aerospace Center (DLR). The boom, made from a single layer of plain-weave Carbon Fibre Reinforced Polymer (CFRP) with a lenticular cross-section, is intended for small satellite missions requiring compact stowage, low mass, and repeated deployment. However, repeated coiling and uncoiling can induce fatigue damage, motivating a detailed study of the governing mechanisms and life prediction methods.

The research aimed to develop and validate a methodology for assessing fatigue life through combined experimental and analytical approaches. Two key questions were addressed: (1) What mechanisms drive fatigue damage during repeated deployment? and (2) How can fatigue life be predicted for a given boom configuration? Coupon-level bending tests characterised the static and fatigue response of the CFRP material using a custom jig and Digital Image Correlation (DIC). Results revealed non-linear behaviour due to compressive softening and showed that pure bending does not accurately represent operational loading, though it provided insight into stiffness degradation and critical strain limits.

Full-scale deployment tests were then conducted to replicate realistic cycling. Fatigue damage manifested as transverse cracks on the compression side, driven by local buckling and amplified by frictional effects. Excessive coiling tension accelerated failure. The study concludes that fatigue in deployable booms is governed by local buckling and friction, and the developed framework offers a foundation for improving the reliability and design of future space-deployable composite structures. ... Read more

This thesis investigates matrix microcracking behaviour in thermoplastic composites at various temperatures, with a focus on Toray Cetex TC1225, a high-performance thermoplastic composite material reinforced with carbon fibres. The research examines the feasibility of using room temperature data to predict low-temperature performance, to minimize the need for extensive cryogenic testing. Both experimental and analytical methods are used to assess how mechanical properties and residual stresses influence microcracking. Comprehensive testing at room and low temperatures (-65°C) was conducted, with the resulting correlations analysed in detail. The findings of this study contribute to the development of more reliable cryogenic storage solutions, particularly ... Read more

The demand for more efficient aircraft and new modes of transportation lead to a departure from the traditional tail and wing configuration. In order to operate an aircraft, it needs to be certified. Most Aircraft with a capacity to carry more than 10 passengers have to fulfil a variety of bird strike related certification standards. These standards define an impact velocity to design for, critical locations and tested areas however are chosen based on experience of previous aircraft. This is not possible if the design is radically different, as it is in the case of the Flying-V. The present thesis proposes a methodology to identify and rank all possible bird strike scenarios, based on geometry and flight path. The quantification of impact scenarios may offer a cost-effective way to assess and visualize vulnerabilities, ultimately reducing certification cost and time. Thereby allowing new concepts to be certified. The methodology synthesized in this report relies on decoupled analytical bird strike load models from the late 70s to quantify bird strike intensity. An algorithm has been developed to determine impact scenarios over the area of arbitrary computer aided design (CAD) geometries. Ray tracing allows for the exclusion of areas that can not be hit. The results are not sufficient to determine critical impact locations, as the damage is significantly influenced by the structural response. However, it is possible to quickly generate probable impact scenarios over large areas based on the flight path. Despite the discrepancy between predicted damage and impact intensity, intensity maps can indicate areas of interest for investigation.
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This thesis investigates the effects of hygrothermal aging on the mode I and mode II fracture toughness of flax fiber-reinforced polymer composites (FFRP) under quasi-static (QS) and fatigue (F) loading conditions. A key motivation for this study is the potential of FFRP to replace synthetic fiber composites, as FFRP offers competitive mechanical properties while being biodegradable and less energy-intensive to produce. However, one of the main limitations of flax fibers is their susceptibility to environmental conditions such as temperature and humidity.

Delamination is a common failure mode in composites, and conducting fracture testing under mode I and mode II conditions is crucial for designing durable components. Double Cantilever Beam (DCB) and End-Loaded Split (ELS) specimens were manufactured for mode I and mode II tests, respectively. Subsequently, hygrothermal aging was simulated by subjecting the specimens to one or two cycles of humidification and drying at elevated temperatures within a climate chamber. Quasi-static testing was performed on unaged, 1-cycle aged, and 2-cycle aged specimens, while fatigue testing was conducted exclusively on unaged and 1-cycle aged specimens.

Testing resulted in significant plastic deformation of the specimens, this was attributed to their insufficient stiffness. This invalidated the assumption of Linear Elastic Fracture Mechanics (LEFM). To better capture these effects, the analysis was conducted using the J-integral, based on non-linear fracture mechanics. While the J-integral cannot account for all observed effects, it provides for a more realistic approximation for comparative evaluation of fracture toughness between aging states.

The results reveal that in mode I QS testing, the initiation fracture toughness on average improved by 19% after one aging cycle, with no further increase observed after a second cycle, while mode II QS fracture toughness was insensitive to aging. In mode I fatigue testing, a reduction in delamination growth resistance was observed after one aging cycle. Mode II fatigue testing exhibited substantial variability within aging states, making it challenging to determine the influence of aging, although a reduction in variability was noted after aging. The increase in QS initiation fracture toughness is likely due to the plasticization of fibers and matrix.

These results indicate that aging does not have a straightforward effect on fracture toughness, as its impact varies between modes and regions of crack growth. These findings provide valuable insights for the design of FFRP and other biofiber composites, contributing to the development of more sustainable materials.
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This thesis investigates the design and analysis of cryogenic liquid hydrogen storage tanks in the context of aircraft retrofit. Hydrogen propulsion offers significant potential for achieving zero emissions in aviation, but its adoption introduces challenges related to safe and efficient storage.
The research focuses on comparing single-wall and double-wall tank architectures, assessing their ability to meet stringent operational, thermal, and structural performance requirements. In the preliminary assessment of a tank design’s viability, two key performance requirements are cruise time and dormancy time. A minimum cruise time of 20 minutes ensures the tank can support basic flight operations, while a dormancy time of 1 day ensures no hydrogen loss occurs if the aircraft remains stationary for an extended period, accounting for potential delays.
The methodology to calculate cruise time involves determining the maximum time the aircraft can remain in the cruise phase based on the inner tank dimensions, fill ratio, and mission profile. The dormancy time is the time required for the tank pressure to reach the venting pressure, at which point hydrogen must be released, and is calculated by implementing a thermodynamic model that simulates the tank's dynamic behavior over time, accounting for heat inflow from the external environment.
The evaluation of the single-wall tank reveals its simplicity and potential cost-effectiveness, but also exposes considerable limitations in terms of thermal insulation for the specific retrofit case study. This design approach is a viable option for larger-scale applications, where a lower surface area-to-volume ratio reduces heat transfer and, consequently, hydrogen boil-off. However, the compact dimensions of the tanks required for aircraft retrofitting present a significant challenge due to the inherently higher surface area-to-volume ratio, which leads to increased thermal losses and prevents the single-wall architecture from meeting the performance requirements imposed by this specific case study.
In contrast, the double-wall tank, equipped with a vacuum layer and multi-layer insulation (MLI), offers improved thermal performance. The heat transfer from the external environment is significantly reduced, allowing to preserve the cryogenic temperature of the hydrogen fuel. However, the added complexity introduces new challenges, particularly regarding the design of the inner vessel support system which must maintain the inner vessel’s position while accommodating thermal displacements and managing structural loads. Assessing the heat leakage budget for the support structure is the final critical step, as it determines the maximum allowable heat inflow through the support system, ensuring the tank meets its dormancy time requirements while allowing for design optimization.
The thesis develops a design methodology for the inner vessel support system, balancing the need for flexibility (to accommodate thermal contraction experienced during the first filling of the tank) with sufficient stiffness (to withstand operational loads, including emergency landing conditions). This approach involves selecting suitable materials and geometries that meet thermal requirements, while accurately determining the support structure’s stiffness properties. Different loading scenarios, such as normal operations and emergency landing conditions, are evaluated to analyze stresses and displacements in both the tank and support system. Adjustments to the design are made if stress or displacement exceed safe limits. The analysis reveals that optimizing the support structure is critical for the double-wall tank’s overall feasibility. While the double-wall design is technically viable and meets the thermal performance requirements, its success depends on further refinement of the support system to minimize heat leakage and ensure structural integrity.
The results of this study suggest that, although single-wall tanks are not suitable for this application, double-wall tanks offer a promising solution for retrofitting aircraft with cryogenic liquid hydrogen storage. Nonetheless, significant challenges remain, particularly in designing efficient support structures that can handle the operational demands without compromising thermal performance. Future work should focus on optimizing the support system design, exploring flexible materials, and considering additional factors such as sloshing loads to further improve tank reliability and performance.
In general, this thesis contributes to the development of a robust methodology for the preliminary design of cryogenic hydrogen storage tanks, providing a foundation for further advancements in hydrogen-powered aviation. ... Read more

Aerospace manufacturers increasingly rely on composite materials for the most advanced aircraft, due to their superior performance and tailorability. As the adoption of such structures grows, so does the occurrence of various kind of damages throughout their service life. Therefore, there is the necessity to develop robust, reliable and repeatable procedures to fully restore the structural integrity of composite components. An additional challenge is posed by composite structures that include functional inserts: the functionality of such inserts needs to be re-instated on top of the integrity of the overall structure, while ensuring a seamless repair finish. This thesis considered two damage scenarios in a CFRP fuselage access panel whose edges are wrapped with a functional material, and defined the most suited repair procedure to tackle them.
The first scenario is represented by damages located in the CFRP structural part of the fuselage access panel. The scarf repair method was identified as the optimal one for this instance, research effort was therefore directed towards its improvement. Indeed, such technique does not currently allow to achieve a fully flush surface, as a mismatch between the repaired area and the undamaged one remains noticeable. Eliminating such unevenness is crucial for stealth and eventually aerodynamic reasons, and therefore needs to be investigated. Two repair configurations were implemented: the first one consists of re-milling the surface once the repair is completed, while the second one relies on a thinner repair patch that — once properly aligned — allows for a flush surface. The first option allowed for an improvement of over 80\%, reducing the surface unevenness from more than 1/2 of a millimeter to less than a 1/10, while ensuring a smooth, continuous surface finish. At the same time, it proved capable of meeting all the mechanical requirements, performing closely to a reference repair configuration in several tests. On the other hand, the second option only partially improved the surface finish, but fell short of the fatigue life requirement by a large margin and also showed a significantly poorer mechanical performance compared to the reference repair and the other configuration.
The second scenario is represented by small, cosmetic damages located in the functional edge of the fuselage access panel. It was identified that such damages are best addressed with repair procedures based on a filler compound. Guidelines to define such repair compounds were defined. Then, two repair procedures based on the use of these compounds were thought out: the first one is similar to conventional filling repair processes, while the second one relies on a bespoke tool to inject the repair compound. The former was implemented and produced promising results: it allowed to precisely restore the original profile and achieve a seamless surface finish. Damages affecting both the structural and functional parts were also briefly addressed, paving the way for future developments. ... Read more

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

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

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

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

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