The FireFly is a multi-role VTOL firefighting aircraft. It was designed with a water tank capacity of 10,000 L and a dash speed of over 400 kph. The aircraft is capable of refilling the water tank with help of a snorkel device connected to the tank whilst flying in hover.

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.

This research delves into aircraft crashworthiness, focusing on the innovative Flying-V configuration, aiming to improve safety in unconventional designs. Challenges arise due to the Flying-V's unique V-shaped fuselage, complicating traditional crashworthiness assessments. To address this, the study proposes modelling approaches, particularly for the central part of the fuselage lacking detailed structural information.
Various modelling approaches are explored, building on a finite element model of the Flying-V developed in previous work. Drop tests validate optimal section designs, emphasizing a minimum vertical impact velocity. Spatial variations in Dynamic Response Index (DRI) and Severity Index (SEV) prompt nuanced studies on impact scenarios and potential passenger side loads.
As the analysis progresses, extending the computational domain becomes crucial for reliability. Insights into weight distribution imbalances and challenges with corrective measures emerge from analyses of extended fuselage sections. Spatial fluctuations in DRIs and SEVs underscore the need for a balanced approach between computational efficiency and result realism.
A newly introduced modelling technique leveraging moments of inertia is implemented, yielding realistic results for straightforward scenarios and reducing simulation time significantly. Further analysis explores intricate landing scenarios, highlighting differences between full and reduced models, particularly at elevated pitch angles.
Recognizing the limitations of simplified methodologies, a submodelling technique is proposed for extreme crash scenarios, effectively capturing engine section dynamics with reduced computational time.
While reduced modelling techniques show promise, the study underscores the need for a comprehensive finite element method representation of the Flying-V, recommending successive simulations with a coarse overall mesh followed by submodelling for detailed assessment of critical regions.

Optimizing the routes of firefighting aircraft can reduce the time it takes to contain wildfires and make sure they remain within control. In this paper, a novel formulation of the Vehicle Routing Problem (VRP) is developed to improve aerial firefighting operations by optimizing aircraft routes. The formulation is a capacitated split delivery multi-trip VRP with time windows and hierarchical objectives. The primary objective is to minimize the time of carrying out all requested drops, and the secondary objective is to minimize the total flight time. Two types of aircraft are used: Scoopers and tankers. The main difference is that scoopers can refill their water tank from a water body. By easily adjusting the capacity and speed of the aircraft, most firefighting aircraft can be modelled using these two types, including helicopters. The program allows the user to input the number and types of aircraft available, the locations of airfield, fires, and nearest water body, intensity of each fire, and more. Several random cases and case studies were solved within the expert-recommended time limit of 5 minutes, yielding reasonable optimized routes. The problem is scalable and sizes ranging from one to 80 drops were tested and solved within 22 minutes. Furthermore, given a certain fire situation, the model can be simulated with various aircraft combinations to gain insights into fleet optimization. In one case study, it was demonstrated that replacing a scooper with a tanker can result in halving the total operation time. Strategic fleet planning is also demonstrated in a case study with the use of a Monte Carlo simulation, in order to compare the performance of different fleet options for a given setting. Therefore, the model is not only applicable in live situations, but can also be used as a supportive tool in planning for upcoming fire seasons, or reviewing and learning from past fires

The growing demand for renewable energy has led to significant developments in wind turbine technology. The ever increasing size of turbine blades and their exposure to a variety of environmental factors can affect their annual energy production and service life. Erosion caused by rainfall and hailstones is identified as two of the most detrimental types of environmental factors to the life of a turbine blade. Hailstone impact in particular is expected to affect the aerodynamic profile of the leading edge as well as cause significant damage to the composite substrate.

The aim of this research study is to investigate the effect of varying hailstone sizes on the damage mode in leading edge polyurethane coated composites subjected to hail impact. The coated glass fibre composite samples were experimentally tested using an impact gas cannon. The impact parameters were determined based on real-life scenarios of blade tip speeds and hailstone sizes. Simulated hail ice (SHI) were manufactured using de-ionized water to form monolithic ice spheres. SHI of 15 mm and 20 mm diameter were used in the research for conducting the hail impact experiments. The coated composite samples were evaluated using non-contact profilometry (optical microscopy) and non-destructive testing (ultrasonic c-scan). Observations revealed that the polyurethane coatings remain largely intact throughout the hail impacts and no visible sign of damage or delamination between the coating and substrate was noticed during damage analysis. The damage mode of matrix cracks in the substrate for the impact parameters used, remained the same for both hailstone sizes. Further, it was seen over the experiments that there exists a failure threshold energy (FTE) for each hailstone size and sample thickness, below which no surface/sub-surface damage is visible. It is hypothesized based on observations in literature that a smaller hailstone will have a lower FTE compared to a larger hailstone and will be more lethal, owing to the concentrated area of contact. Future research to develop further awareness of damage evolution in the coated composites is recommended and discussed.

One of the emerging welding techniques for thermoplastic composites is ultrasonic welding. Ultrasonic spot welding has the potential to be composite counterpart of the widely used, highly automatized spot welding technique used to join metals. State of the art research on ultrasonic welding is focusing on the welding process itself in search of the process parameters that allow consistent manufacturing of high quality welds. To complement this effort, the research of this thesis looks ahead at assessing the durability of welded products, which is the next step after assuring a good weld was created. This research aims to support the study of damage progression in ultrasonically welded components subjected to repeated loading, by providing a method of comparison between the observed surface deformation of joined components under lab experiment conditions and expected deformations predicted for the experiment with numerical modelling techniques.

This thesis proposes the interpretation of surface strain fields, measured through digital image correlation, and the interpretation of the increase in compliance of a welded joint as fatigue damage progresses, recorded by the test machine as force-displacement relationships, as indirect methods of measuring the state of damage accumulated within the ultrasonic weld spots. The interpretation of measured surface strain fields and of recorded load-displacement data is done by comparison to a collection of finite element simulations corresponding to ultrasonic weld spots containing various degrees of damage. This research aims to present a method of strain field interpretation that is generalizable to the analysis of lap shear specimens containing multiple ultrasonic weld spots.

Pistons for reciprocating compressors for industrial applications are often made of specialised materials. These prove to have problems with manufacture due to the high quality and short production times needed in combination with a low production volume per design. Additive manufacturing, specifically selective laser melting, could solve the production problems, provided that the material retains the needed mechanical properties. Ti6Al4V is the most appropriate material for this application. The most important mechanical properties for the application are the fatigue limit and the stress intensity factor, which are not well established properties for printed materials. For this reason fatigue limit and stress intensity factor tests were performed for both stress relieved and hot-isostatic pressed test pieces on longitudinal and transverse directions. Strength, toughness and fatigue limit is higher in hot-isostatic pressed test pieces of Ti6Al4V, and these are proven to be appropriate for application in compressor pistons. However the fatigue limit of stress relieved Ti6Al4V is lower, anisotropic, and has more scatter, and as such is insufficient for the application, which is due to deleteriously oriented microstructure and the presence of porosities. This can be solved by changing the printing parameters – laser power, cooling rate or heat treatment – although the exact combination of parameters for optimised values is not known and will be part-specific.

The purpose of this research study is to analyse the effect of the design of mechanical fastener on their effectiveness as Disbond Arrest Features (DAFs). This was done by means of a sensitivity study using a 3D model using the Virtual Crack Closure Technique (VCCT) to model the disbond growth of a single lap shear specimen comparing the Strain Energy Release Rate (SERR). Different fastener material stiffnesses, shaft radii, head radii and head geometries were compared. The sensitivity study showed the Mode I SERR was completely independent of the fastener design and the Mode II was suppressed more strongly for fasteners with a lower flexural flexibility. Fasteners with a countersunk head showed less reduction of the Mode II SERR, this was caused by the head rotating as a result of the fastener adherend interaction, reducing the effectiveness of the load transfer.

To expand the use of additive manufacturing in aerospace towards more critical applications, it is required to design parts in a damage tolerant context. Therefore, the damage tolerance of additive manufactured multiple load path structures is assessed by analysing the fatigue life and damage propagation of components with increasing redundancy. An experimental approach is chosen, whereby specimens with 1, 9 and 81 parallel struts are tested. A decreased fatigue life is found for the specimens with more but thinner struts. This decrease is attributed to manufacturing related effects that occur upon producing smaller elements. The failure of the multiple load path structures showed a step-wise pattern. Due to this, the decreased variation in fatigue life and decreased sensitivity to initial damage, multiple load path structures are more damage tolerant. However, in design a balanced decision should be made upon applying these structures, due to the decreased fatigue life.

This research focuses on the possible causes of variation in the tensile strength and stiffness of witness specimens that are used in the manufacturing of aerospace grade selective laser sintered parts. Using specimens and data obtained from Materialise of the PA2241FR samples, the crystallinity and porosity of the samples was determined. The effect of the flame-retardant additive was also investigated. A relationship was found between the degree of crystallinity and the tensile strength, and between the porosity and the tensile strength. Unfortunately, no relationship could be found between the crystallinity and stiffness. The effect of the FR-additive could not be tested using NMR and as a result no data on the influence was available. When analysing the other factors, they seemed to indicate the significance of an unknown parameter on the mechanical properties and especially on the stiffness. A variation of the halogenated flame-retardant additive seems to fit this effect.

To create a more sustainable future for aviation, new, lighter-weight structures and materials will need to be engineered. It will also be critical that damage tolerance and safety are not compromised in the process. Lattice materials represents one avenue of exploration; however, two key challenges arise: limited experimental work has been conducted to date regarding tensile mechanical response and lattice materials are generally considered to be less tough than traditional aerospace materials. Advancements in additive manufacturing in recent years creates the opportunity to rapidly produce high-quality complex geometries, allowing for both challenges to be more easily investigated. To address the issue of toughness and damage tolerance, nature is a source of inspiration, as all of nature’s toughest materials derive this characteristic from creating structural hierarchy using intrinsically weak
building blocks.

Two sets of lattice structures were fabricated using stereolithographic (SLA) 3D printing and tested under quasi-static tensile loading. Two sets of lattices were fabricated: lattices with uniform strut thickness, or relative density, and mixed-relative density lattices which create structural hierarchy. Using a novel method to track lattice deformation during loading, lattice stiffness-displacement response has been correlated with beam elongation and rotation behavior and the deformation of individual cells. The stiffness-displacement response of uniform lattices can be classified by relative density as either an elastomeric, elastoplastic, or hybrid response. In hierarchical lattices, cell deformations occurring in different relative density regions are directly correlated to features of the stiffness-displacement response.

Aspects of the mechanical response of hierarchical lattices, particularly fracture toughness and fracture pattern, are heavily influenced by the exact configuration of structural hierarchy, spurring a discussion of what characteristics are most important in the pursuit of increased lattice damage tolerance. While none of the lattices represent an optimal solution, each displayed characteristics which, if combined to
form a hybrid structure, could substantially improve lattice damage tolerance.

Traditional design methods are generally unsuitable for optimally designing organic shapes made possible by additive manufacturing. In this study, a simple Genetic Algorithm (GA) optimisation routine was developed for a relevant engineering design problem – the optimisation of thickness distribution for a crenelated fuselage skin panel. The basis for this optimisation is the damage tolerance behaviour of the panel in the presence of a fatigue crack. The results demonstrated that crossover and mutation are inherently more similar than expected, thus questioning whether it is not more important to design a set of search heuristics through better understanding of the fitness space, rather than the application of a flawed, nature-inspired standard crossover and random mutation. Through these insights, this research contributed to ongoing research in understanding GAs, which, if better understood, could assist engineers in finding improved designs of additively manufactured components.

Selective laser melting (SLM) is an additive manufacturing technique, which is currently on the rise of being used for manufacturing bone implants. Spinal cage, dental and hip implants can for example be manufactured using SLM. Ti6Al4V lattice structures, categorised as metamaterials, can be printed by SLM with mechanical properties close to bone tissue. Due to the lattice structure the stiffness of the Ti6Al4V is decreased, by which stress shielding can be reduced. The lattice structures enhance bone ingrowth which in turn improves the implant’s integration into bone tissue. In light of this potential, this research is focused on biomechanical properties of additively manufactured Ti6Al4V metamaterials.

The current research is aimed at improving the fatigue resistance and wettability of diamond lattice structured Ti6Al4V by applying different microstructural designs and surface engineering through hot isostatic pressing (HIP), sand blasting (SB) and chemical etching (CE). Furthermore a comparison is made between the two SLM processes in terms of continuous and pulsed laser scanning. In order to verify the developed herein post treatment procedures, the tests were also upscaled to actual spinal cage implants. Furthermore, surface modifications affect its wettability which can be linked to cell adhesion and ultimately healing time of the implant. Hence Sessile drop tests were performed to assess the wettability and compare the effect of the various surface modifications.

For both SLM methods it was found that HIP reduces porosity of Ti6Al4V metamaterials, which reduces crack initiation sites and it also serves as a heat treatment increasing the b-phase fraction and thus increasing ductility
and fatigue resistance. SB and CE were found to reduce surface indiscrepancies, which decrease the effect of stress concentration and fatigue initiation sites. Finally SB induces compressive residual surface stresses which means the surface is work hardened, increasing the overall mechanical properties.

For continuous SLM samples an increase in yield strength from 89 MPa up to 115 MPa was found by applying HIP treatment. It should be noted, however, that static mechanical properties were not affected by SB and CE treatments. Fatigue resistance, both low cycle (LCF) and high cycle fatigue (HCF), was significantly improved by a combination of HIP, SB and CE. The observed trend was similar for both pulsed and continuous SLM samples. It is worth noting that SLM samples manufactured with pulsing laser were found in general to be inferior to the
continuous laser SLM, both in terms of static and dynamic properties. The difference is likely attributed to the nature of the laser scanning process, where for pulsing laser method each bead interconnection serves as stress concentration, while for continues laser it is rather the strut interconnections that act as weakest points. Furthermore, for the continuous SLM a preferred grain growth direction was observed which indicates anisotropy. This was not observed for pulsed SLM samples.

For the wettability results it was observed that SB decreases and CE increases the contact angle. A decrease in contact angle means the surface has become more hydrophilic, hence the in this study developed SB modification could be considered as more favourable for osseointergration.

The upscaled spinal cage implants post treatment procedure showed a decrease in yield strength and an increase in fatigue resistance for the HIP+SB+CE as compared to as-processed implants. The rather limited post treatment
improvement on implants was linked to the post process treatment method, which should be modified to account for the complex geometry of these structures.

Complex structural shapes can be produced with additive manufacturing. The geometrical complexity that can be achieved translates into an increase in possible designs. Designing these structures with traditional methods is difficult. A design process with computational optimization will enable engineers to use the geometrical freedom offered by these manufacturing methods. This study explores how topology optimization can be used to design structures that are fatigue tolerant. Two optimization algorithms for fatigue tolerance were developed in this thesis. One algorithm minimizes the stress intensity factor, whereas the other one maximizes the fatigue crack growth life. Both algorithms use a resource constraint to limit the total amount of material, an enriched finite element method to analyze the crack growth performance and the method of moving asymptotes to incrementally improve the design. Example problems showed that the algorithm dramatically improves the fatigue resistance.

Recent findings have highlighted the potential of a 3D-printable high-strength Liquid Crystal Polymer, whose anisotropy can be fostered for topology optimization intents. The mesostructure of a 3D-printed liquid crystal polymer is studied: the observation of interlayer features under the form of regular notches or spiraling patterns swirls is reported on optical microscopy of cross-sections. A formation mechanism is proposed: interlayer features may be formed as a result of an offset in placement of material. Another question is raised by the observation of these crenelated shapes: by providing mechanical interlocking between layers, they are expected to enhance interlaminar shear strength of a part. Short-beam shear tests indicate that when interlayer features are tall with respect to the layer height, and oriented perpendicular to the shear loading direction, the interlaminar shear strength of the 3D-printed part is enhanced by up to 112%. Microscopic evidence further indicates the crack-arrest ability of these features.

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

Additive manufacturing allows material structuring, supporting the fabrication of multiple-level structures or metamaterials. Through the lens of classical stress reduction, nature’s cellular solid structures feature stress-homogenizing nodal topologies. Avian long bones are an example. Research into the mechanics of open cell cellular solids seems focused on the effectiveness of unit cell architecture and neglects the detailed behavior of constituent nodes. Several specimen series were printed on the nodal- and cellular solid-levels of analysis, all with varying nodal topologies. A discussion of force-displacement and digital image correlation experimental data is had; the cellular solid deflection rigidity seems highly sensitive to nodal topology under quasi-static compression. It is thought that bioinspired profiles successfully homogenize stress and improve load transfer, mitigating nodal softening: peak stresses and the propagation of nodal torsion into adjoining strut deflection decreased. This sensitivity is relevant for lightweight strain energy absorption and stiffness provision, and demands further research.