Additive manufacturing (AM) expands the design space in which complex hollow structures can be fabricated, exceeding the limitations of traditional manufacturing methods. In sandwich structures, the core structure governs the impact response. Triply periodic minimal surface (TPMS) architectures show favourable impact behaviour due to their smooth curvature, the practical scalability is constrained by manufacturing resolution and cost.
This thesis investigates the introduction of fillets in conventional core geometries and the resulting impact behaviour. The primary objective is to systematically evaluate how the introduction of fillets in FDM-manufactured sandwich structures influences low-velocity impact behaviour.
A non-dimensional parameter is introduced: the fillet intensity factor, quantifying the degree of rounding within a structure. To isolate the effect of fillets, the relative density of the structure is kept constant by reducing the wall thickness, hexagon in-plane (HIP), hexagon out-of-plane (HOoP) and re-entrant out-of-plane (ReOoP) are investigated and experimentally tested. A computational impact model was constructed in parallel. The results were evaluated through cross-sectional scans and force histories of the specimens and the numerical simulations to localise stress concentrations.

The experimental and computational results showed that the introduction of fillets in the core structures does not eliminate stress concentrations, the introduced fillets relocate them. Observed in all geometries, the fracture location shifts with an increasing fillet intensity from corners and junctions towards mid-wall sections. The general impact behaviour remains more dominated by the geometry type than the introduced fillets. The computational model accurately locates stress concentrations within the structures during the initial impact phase, but limitations arise as fractures propagate. The work suggests that an optimum fillet intensity exists in which the fillets reduce the stress concentrations in junctions, while maintaining a sufficiently thick mid-wall section to prevent premature failure.
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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

This thesis investigates how silicon carbide (SiC) surface treatments influence bonding to a stiff polyurethane adhesive in tiled ceramic multi-layer armour and whether a stronger ceramic-adhesive interface improves ballistic performance. Stiff adhesives can exhibit poor tile retention during impact leading to poor multi-hit performance. Five cases were compared: solvent-cleaned control, grit-blasting, grit-blasting plus APTMS silane, vacuum air cold plasma, and nano-pulsed laser treatment. Surfaces were characterised by SEM, XPS, water contact angle, and confocal microscope roughness measurements, then evaluated using symmetric SiC-SiC double cantilever beam (DCB) tests and single-hit ballistic testing using the residual energy method (REM) and a post-failure multi-hit failure mode analysis. Plasma and laser treatments strongly increased the wettability, while grit-blasting increased roughness and silane introduced a new coupling chemistry. DCB peak loads observed showed notable increases relative to the control, with the highest values for grit-blasting plus silane, and grit-blasting. The multi-hit failure mode analysis showed the grit-blasted plus silane treatment most effectively reduced the ceramic-adhesive interface failure, whereas the laser treatment performance was limited by a weak secondary interlayer. Overall, the grit-blasting plus silane treatment seemed to offer the most substantial improvement to ballistic performance. ... Read more

Cone crack formation is the primary damage evolution in brittle ceramics subjected to ballistic impact. This work investigates the correlations between cone cracks generated by low velocity sphere impact and those formed under high velocity bullet impact. alumina, silicon carbide, and silicon nitride ceramic tiles of varying thickness were experimentally tested for three velocity regimes (≤250 m/s, 300–550 m/s, ≥600 m/s). Macrostructural cone geometry was characterized using 3D optical microscopy and high speed imaging, mesostructural topology was evaluated through incremental mean arithmetic roughness measurements along the crack propagation, and microscopic fracture modes were quantified via SEM-based areal occurrence analysis.

Primary cone angles exhibit discrete regime-dependent plateaus achieved at transition velocities rather than continuous velocity dependence. Statistically significant inverse linear relationships are identified between primary cone angle and tile thickness, and between secondary cone angle and secondary cone height. Cone nucleation depth and minor cone radius scale proportionally with projectile radius, independent of ceramic material and velocity regime, indicating projectile-controlled nucleation geometry. Surface roughness increases progressively along the crack path for all materials and projectiles, suggesting propagation induced development of topological features. In contrast, material dependent distinct fracture modes are identified with minimal effects from projectile geometry and velocity.

Comparison between sphere and ballistic impacts reveals overlapping primary cone angle regimes, similar surface roughness amplifications, and comparable fracture modes, indicating similar crack nucleation and propagation mechanics at similar projectile velocities. Differences are primarily expressed in magnitudes of cone fragmentation and specimen recoverability for postmortem characterization of bare ceramic tiles.

In general, geometry is dominant for determining cone crack morphology, and intrinsic material properties govern microstructural fracture modes. Sphere impact testing can serve as a representative predictive screening method for understanding ballistic cone crack behaviour within defined regimes. ... Read more

Composite materials offer opportunities to reduce the structural mass of naval vessels, but their integration into primary hull structures changes the stiffness distribution, load transfer and mechanisms governing collapse. This thesis investigates how selective steel-composite hybridisation of a naval hull girder region influences its quasi-static progressive collapse behaviour and ultimate bending response. The assessment focuses on intact structures subjected to monotonic longitudinal bending.

A Smith-type progressive collapse model was adapted to represent steel and composite component responses, neutral-axis migration and reduced composite participation at the steel-composite interface. An all-steel reference box and three hybrid concepts incorporating Glass-Fibre Reinforced Polymer (GFRP) were evaluated using monolithic laminate and sandwich panels, first as isolated box girders and subsequently within a reference frigate cross-section. The steel formulation was assessed against the published Nishihara MST-3 experimental benchmark. Nonlinear finite element analyses in ANSYS provided component load-shortening curves for comparison within the same sectional solver, while separate complete-box models examined wall interaction and cohesive adhesive behaviour. Sensitivity analyses assessed the influence of material properties, interface participation and modelling assumptions.

Among the investigated concepts, B3, which replaces the side webs with GFRP while retaining the steel deck, bottom flange and corner framing, provided the most favourable balance between mass reduction, bending resistance and dependence on composite participation. In the analytical isolated-box assessment, the monolithic B3 design was 11.6% lighter than the steel reference while meeting or exceeding its sagging and hogging capacities. Following embedding and recalibration within the frigate cross-section, it reduced structural mass by 52.1 kg⋅m−1 relative to the section containing steel reference boxes, maintaining sagging capacity with a 1.17% reduction in hogging capacity. Sandwich variants offered no meaningful mass advantage within the adopted longitudinal-bending model.

The finite element comparisons showed that the predicted resistance depends on how component behaviour and structural interaction are represented. Using finite element component curves produced ultimate moments 0.7–17.5% below the corresponding analytical predictions. This sectional assessment predicted lower sagging resistance for B3 than for the steel reference, while its small hogging advanta
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As part of the development of primary adhesively bonded joints, there is a desire for an element-level in-plane shear strength test of such joints. Due to its large gauge size and its theoretical ability to apply a state of uniform pure shear to the specimen, the picture frame shear test is a promising test for this application. However, use of the picture frame test as a strength test of joints was previously unexplored. Therefore, this study serves as an initial investigation and development of such a test, with the aim of developing an experimental testing procedure resulting in reliable and repeatable tests and the ability to detect damage initiation in the joint. Furthermore, accompanying finite element models correlated to the behaviour of the fixture and results of the experimental jointed tests are developed.

The mechanics and adverse effects of the fixture were first characterised. Characteristics of fixture symmetry, mechanism friction, specimen strain state, and fixture deformation were considered. These properties were studied primarily through tests with solid flat plate metal specimens, without a joint, and finite element analysis. Measurements of the experimental tests were made using test bench data, Digital Image Correlation (DIC), and strain gauges. Additionally, a detailed finite element model of the entire picture frame fixture was developed in Abaqus and correlated to the results of the characterisation tests. This model was used to further characterise the mechanics of the fixture.

Secondly, a comprehensive testing procedure was developed aimed at maximising the reliability and repeatability of tests. Minimisation of slip, minimisation of play in the fixture mechanism, and optimalisation of the measurement setup were considered. The developed testing procedure was validated through tests with jointed specimens. In these tests, single lap shear, thick adherend, and foil-defect specimen configurations were included. The results of these test are compared to those of finite element analysis.

Characterisation of the fixture behaviour demonstrates that the test does not succeed in imposing a strain state of uniform pure shear on the specimen, though this strain state is approached at the centre of the test section. It is shown that fixture deformation is the main driver behind this non-idealised fixture behaviour, brought on by the high stiffness of the metal specimens. The resulting peaks of tearing stress at the top and bottom corners greatly reduce shear strain purity in those areas and were correctly predicted by the detailed fixture finite element model.

The developed testing procedure drastically improved repeatability of the test, primarily through the inclusion of a preload cycle and the reduction of slip. The results of the consequent jointed specimen tests demonstrate limited scatter in the results and limited ability to detect the early onset of damage. Strain gauges perform better than DIC at detecting damage, though they remain limited to analysis of a single location. Additionally, analysis of numerical and experimental tests with thick adherend specimens of varying thickness revealed that the thickness of the specimen influences the failure load of the specimen, instead of solely the strength of the adhesive joint.

Given the findings of the present research, the picture frame shear test is concluded to be a fundamentally complex test for the shear strength of adhesively bonded joints. Nevertheless, reliability and repeatability were greatly improved through the development of a comprehensive test procedure. Therefore, the picture frame test remains a promising element-level shear test for larger-scale tests of bonded joints, such as those with foil-defects. ... 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

Sustainable aviation demands alternative propulsion system, such as Liquid Hydrogen (LH2). LH2 needs to be cryogenically stored in large, lightweight fuel tanks. Carbon-fibre Reinforced Polymer (CFRP) composite tanks offer promising weight-savings.

Liner-less composite tanks are prone to safety-critical issues such as leakage. Leakage is caused by the presence of microcrack networks in the laminate. It is known that these crack networks arise due to thermo-mechanical loads. Element level testing is required to build a foundational understanding of the damage and leakage phenomena. The bulge test rig is capable of such element level testing, by mimicing multi-axial loads experienced by the tank structure at cryogenic temperature.

A digital twin is developed to provide deeper insights into the behaviour of the test rig and specimen. The validity of this digital twin is quantified by a 'multi-sensor' experimental campaign. The validation dataset is obtained on a range of test configurations, both at room and cryogenic temperature. Specifically, strain data is acquired using strain gauges, fibre-optic sensors and digital image correlation. Qualitative damage assessment of post-mortem specimen is conducted using ultrasound and optical microscopy, to compare the inter and intra-laminar damage initiation predictions of the digital twin.

The experimental-numerical analysis of the work is envisioned to contribute to a more representative design of bulge test specimen.

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Composite over-wrapped pressure vessels are the state-of-the-art for hydrogen storage, with Type IV vessels currently using a thermoplastic liner and a thermoset composite. The next innovation, Type V vessels, aims to use a thermoplastic composite over-wrap that acts as both the barrier and container, requiring new materials and manufacturing methods like automated fibre placement.

To accurately characterize the new materials, the effect of their manufacturing process must be captured. While the standard split-disk test exists for wound specimens, it underpredicts their burst strength and fatigue life. Other attempts in literature have improved strength estimations but are not viable in fatigue, thus creating a challenge.

A new test method was developed, which uses a polyurethane ring to apply cyclic internal pressure. It was designed through finite-element analyses, fine-tuned through rigorous tests, and benchmarked against the standard method. It showed improved strain distribution and near-consistent performance in fatigue tests. Hence, it demonstrated potential for more accurate static and fatigue strength characterization of wound composites, with room for improvement through better alignment and even friction. ... Read more

The increasing urgency to mitigate climate change has underscored the need to transition from conventional fossil-based aviation fuels, such as kerosene, to sustainable alternatives. Hydrogen stands out due to its potential to significantly reduce greenhouse gas emissions, making it a promising energy carrier for the aviation sector. However, adopting hydrogen presents substantial challenges, with the development of specialized fuel containment systems being one of the foremost obstacles. Double-walled tanks employing vacuum insulation offer an effective solution for cryogenic hydrogen storage, but they require a robust supporting structure for the inner vessel. Various solutions have been proposed to support the inner vessel of cryogenic tanks; however, despite the diversity of designs, there remains a noticeable lack of comprehensive research focusing specifically on the structural behaviour and feasibility of these inner vessel supporting structures for commercial aircraft applications. This study proposes a novel fibre-based suspension technique for the inner vessel of a double-walled integral tank designed for liquid hydrogen storage in large commercial aircraft. A finite element model was developed to evaluate the structural interaction between the inner and outer vessels and the supporting fibres, enabling structural sizing optimization to assess the impact of added loads. A parametric study was conducted to explore the influence of fibre design parameters on structural performance, mass, displacement, and thermal behaviour. Key design guidelines were established. First, using more than two longitudinal anchoring points results in the unwanted transfer of bending loads from the outer to the inner vessel. Second, while increasing the number of circumferential fibres reduces peak loads and displacements, the associated anchoring mass is the primary limiting factor, as thermal conduction was found to be negligible. Lastly, fibre orientation should prioritize low stiffness in the contraction direction to minimize tensile forces under initial filling. This should be combined with fibres angled in the longitudinal direction to improve longitudinal stiffness and displacement control. The results confirm the structural feasibility of the suspension system, showing only a marginal structural mass increase of approximately 1.88 % compared to a baseline integral tank without internal support. These findings provide practical guidance for the implementation of fibre-based suspension systems in cryogenic tank structures, supporting the development of hydrogen storage solutions for aviation. ... Read more

Liquid hydrogen propulsion presents itself as one of the most promising technologies for a zero emission commercial aviation future. However, two of the main challenges are the low volumetric energy density and the cryogenic storage temperature of the fuel. In collaboration with Airbus and the ZEROe project, this work develops a model to predict the response of the liquid hydrogen tank when the thermal insulation is lost and very high amount of heat are transferred to the tank and its pressure is increased. The model includes the most important physics driving the response, and enables the study of the system and its improvement, maximizing the amount of fuel carried while being able to safely control the pressure rise. ... Read more

The W-type is a motion-compensated offshore crane capable of hoisting up to 2 mt of cargo, thereby supporting the maintenance processes for the rapidly growing offshore wind farms. However, the rated cargo mass is limited by the dynamic amplification factor (DAF), which is a governing parameter in the structural design of offshore cranes. During the design phase, the industry currently makes use of a one-dimensional spring-mass model to estimate the DAF. In this study, a simplified flexible multibody dynamic model was developed, and validated with experimental data, to simulate the DAF instead. It was found that the previous method underpredicts the DAF by up to 56%, because it neglects the effects of crane inertia and structural damping. Besides, the active controller of the luffing cylinders was modelled for the first time, which reduced the DAF up to 13%. Therefore, the simple spring-mass model is not deemed accurate for this application. ... 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

Double-walled vessels are recognized as an effective storage solution for liquid hydrogen (LH2) due to their superior thermal insulation. Similar to a thermos flask, the vacuum space between the inner cryogenic vessel and the outer vacuum shell prevents convective heat transfer, which would otherwise result in the rapid boil-off of the stored LH2. A critical component of this technology is the inner support structure which supports the inner vessel within the outer vacuum shell. It must be robust enough to handle operational loads and potential crash events specific to aviation applications, prevent excessive thermo-mechanical stresses caused by the contracting inner vessel, and minimize heat transfer through conduction. Despite its importance, there is a noticeable lack of comprehensive designs and research addressing this component, particularly in the context of aviation.

The primary objective of this thesis is to develop a design methodology for creating effective inner support structures for double-walled vessel technology. The research is grounded in a practical context through a collaboration with AeroDelft, a student team at TU Delft currently retrofitting a Sling 4 aircraft for hydrogen-powered electric flight. As part of the next milestone in their Project Phoenix, AeroDelft aims to store 6 kg of LH2 in a double-walled vessel to meet the mission's requirements.

To begin, functional, operational, and constraint requirements were established for the inner support structure. A baseline design for the inner vessel and outer shell was then developed. Subsequently, four distinct inner support structure concepts were introduced, each with its own design intents and key considerations. These concepts were subjected to various analyses, including modal, thermo-mechanical, crash loads, and heat leakage, to assess their viability and ability to meet essential functional requirements. To determine the most performative design(s), the concepts were evaluated and compared based on the following performance metrics: gravimetric efficiency, heat leakage, safety, and feasibility of manufacturing and assembly.

The analyses demonstrated that all four design concepts meet the essential functional requirements, providing a solid foundation for further development or potential prototyping. While the focus was on a vessel designed for a small aircraft, the methodology and design insights are applicable to larger systems. Hence, this research contributes to advancing cryogenic storage solutions for hydrogen-powered aviation.
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Structural design for unconventional aircraft can be quite challenging and could undergo many iterations until a solution is found for all load-cases such as static sizing, stability and crashworthiness. Ultimately, all of the design aspect are integrated in the earliest design changes for which Topology Optimization (TO) could provide the solution. The Flying-V is one such aircraft with challenging crashworthiness design due to its small sub-floor volume. However, a suitable formulation for aircraft crashworthiness and TO is yet to be found.
This thesis aimed at incorporating the conventional aircraft-crashworthiness characteristic of plastic energy dissipation in plastic hinges in the energy based Hybrid Cellular Automata framework. The HCA principle is to optimize on cellular level, using information of each cell’s direct neighbors. For the proposed method, the inter-cell stress and strain information is used to reconstruct the cellular bending energy throughout the non-linear dynamic crash-analysis, providing greater stability than intra-level bending energy. The bending energy formulation was based on the assumption of linearly varying stresses and strains in bending. Several formulations have been formed to manipulate this energy promoting hinge-formation.
The framework has been tested on size optimization of a small-scale slender beam impact model, representing the final design case. Results show peak force reduction and greater displacements compared to the standard HCA crashworthiness model. However, improvements can be made such as inclusion of the non-linear stress and strain variation in the bending energy formulation, to more accurately account for large plastic strain.
Before testing whether the methodology improves the crashworthiness of aircraft, verification of the voxel-mesh approach was required. An A350-like aircraft has been simplified and idealized accordingly and compared with its detailed FEM crash-assessment. Simplifications include modelling only one frame and replacing the cabin and floor with a rigid body to reduce computational cost. Results show good agreement for the required level of detail of conceptual design, although it is recommended to idealize the cabin and floor as well. Once verified, the framework has been applied to the entire domain. ... Read more

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

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

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

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

These findings highlight the potential of TBDC interleaves to enhance joint toughness, presenting new pathways to improve the safety of composite bonded structures.
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Liquid Hydrogen (LH2) appears as one of the leading solutions for sustainable aviation, with rear-fuselage tanks being one of the preferred options for fuel integration in conventional tube & wing designs. Studies have already identified some limitations of these concepts concerning the larger horizontal tailplane required. However, no studies so far considered the industry approach of sharing a single tailplane in an aircraft family for this type of aircraft.

The thesis draws some first guidelines for the design of LH2 aircraft families with rear tanks, with a special focus on the performance penalties due to tailplane commonality and its comparison to conventional designs. For this purpose, a methodology has been developed that systematically sizes the tailplane of an aircraft considering potential family members already in the preliminary design stage. The results revealed that lower performance penalty due to tailplane commonality can be expected for an LH2 family compared to kerosene powered designs. ... Read more

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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. ... 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.
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