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.

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.

Welding is a joining method for thermoplastic composites (TPCs) that offers multiple advantages over the more traditional methods of mechanical fastening and adhesive bonding. A particularly promising welding technique is ultrasonic welding, which features very short process times as a result of the high heating rates that can be achieved. This spot welding technique is hypothesized to have potential for improved damage tolerance compared to more commonly used continuous welding techniques: in a multi-spot welded joint, evolving damage will need to re-initiate in subsequent spots. The fact that damage initiation will need to occur multiple times might delay overall damage evolution through the joint compared to a continuous welded joint, where damage initiation needs to occur only once. This work is a first exploratory step into the domain of fatigue of multi-spot welded joints in TPCs. Existing research on the fatigue behavior of four-spot welded steel joints in various layouts served as the main reference throughout this research: its methodology was transferred to four-spot welded joints in TPCs. By comparing fatigue behavior across both materials, it was evaluated to what extent existing knowledge and design rules for steel could potentially be transferred to TPCs. Differences were observed in the results obtained for TPC and steel joints. Most notably, in steel joints the dominant failure mode was seen to change from spot fracture to sheet fracture at higher fatigue lives. In TPCs, joints consistently showed spot fracture across all load levels. A different interrelation between layout performances was seen in the steel and thermoplastic composite joints, assumed to be a result of localized material strengthening in the steel joints from interference of adjacent heat-affected zones. These results indicate that existing knowledge on multi-spot welded joints in steel cannot be readily transferred to TPCs, as failure modes and material mechanisms may differ. It was discovered that, when one spot failed prematurely as a result of existing damage in the joint, the remaining layout no longer seemed to have an effect on fatigue life performance. This was attributed to asymmetry in the remaining joint layout, meaning one spot would always become a preferred location for damage initiation and subsequent evolution. Therefore, subsequent damage evolution would only be restricted by a single spot up to the point where the shear strength of the joint was exceeded.

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.

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.

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.

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.

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.

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.

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.

Abstract

This master thesis investigates the effect of surface morphology of Selectively Laser Melted Ti-6Al-4V on fatigue performance. The literature study performed here iden- tifies roughness as one of three major causes for the decreased performance of the additive manufactured alloys (along side porosity and residual stresses). In order to eliminate the two other causes for poor fatigue, and isolate the surface roughness, a hot isostatic pressing (HIP) treatment was performed on every sample. Luckily, a first set of tests proved that chemical treatments, specifically electropolishing and plasma- polishing, greatly improve the surface quality. A second series tests (rotating bending fatigue tests) concluded that, although specific roughness parameter such as Ra reach lows of 0.2μ there was no statistically sound relation between fatigue performance and an improved surface roughness. Upon further investigation of the specimens, it was discovered through CT scans, that their porosity was in fact much closer to that of non HIP treated specimens. This master thesis illustrates once more that the effect of surface roughness on SLM Ti-6AL-4V alloys cannot be properly identified if the bulk material shows internal defects.

Abstract

Meta-biomaterials are porous biomaterials created by additive manufacturing techniques such as Selective Laser Melting. These materials are built up form a repeating unit cell, resulting in a porous structure that can be applied for bone implants or prosthetics. The mechanical properties of these meta-biomaterials can be tailored by variations in unit cell type and strut thickness, resulting in different porosity values. This makes it possible to create a material with mechanical properties similar to bone, which prevents negative side effects of conventional bone implants such as stress shielding. The fatigue behavior of these meta-biomaterials has been studied before, but only at a single stress ratio of R=0.1 This study investigates the fatigue behavior at different stress ratios which result in an increased mean stress. A cylindrical porous structure that is built up from a diamond unit cell is tested at stress ratios of R=0.1, R=0.3, R=0.5, R=0.7 and R=0.8. Two samples types are made of Ti-6Al-4V ELI powder, resulting in a theoretical porosity of 80% and 10%. Also an experimental DIC method is developed, to visualize the deformation behavior during the fatigue tests.

Abstract

Damage is found in resin rich bead corner radii of RTM 6 epoxy-based composite ribs due to in-service thermal-mechanical loading after an aircraft inspection. The same types of damage are obtained through pure thermal cycling of a single bead. However, thermal cycling is a time-consuming process. Therefore, a faster way of damage investigation is required. A potential way to achieve this objective is by mechanically cycling a coupon specimen with simpler geometry than the bead. To investigate potential solutions to the problem a literature review is performed. The scope of the literature review covers several topics as follows. Laminate fatigue damage modes and their impact on the laminate is researched. It is discovered that it is typical for RTM 6 epoxy-based laminates to build-up high matrix residual tensile stresses after manufacturing. Several differences between thermal and mechanical cycling are discovered. It is found that the most common way to test composites is by the use of ASTM standards. However, there is little available information about the fatigue behavior of laminates with resin rich areas. Investigation of the fatigue behavior of laminates with resin rich areas is performed by the use of FEA and physical tests. Four specimen types labeled from A to D are manufactured. Type A is a dog-bone pure RTM 6 specimen. Types B to D are all composite specimens with the same in-plane geometry and different layups and manufacturing processes. All specimens are tested statically and in fatigue. In the fatigue testing session, fractography of the damage occurring at different test conditions for different specimen types is performed. In addition, two FE models are created. The first model is of the bead. The second model is a harmonized model applicable to all composite specimen types with required layup readjustments for each specimen type. It is discovered by FEA that the maximum principal stress in the resin rich area is perpendicular to the matrix cracks in the bead and the composite specimens. It is also discovered that the maximum principal stress cycle of the matrix at the resin rich layer interface with the fabric is similar in the bead and the specimens. The similar fatigue parameters are the R-ratio and the stress amplitude. The parameter similarity suggests they could potentially drive the resin rich layer fatigue damage initiation. Moreover, a positive through-the-thickness stress gradient is discovered, which suggests the cracks are likely to initiate bellow the resin rich layer surface. This hypothesis is further supported by fractographic observations of cracks not reaching the laminate free surface. Static and fatigue tests are performed. The static test provides the UTS of all specimen types, based on which cyclic load levels are selected. In the specimen fatigue tests several results are observed. In the first place, damage similar to the bead damage is found, namely cracks and delamination. In the second place, the damage is observed to penetrate through the laminate thickness and to be dependent upon the laminate compaction. However, this penetration depth dependency on the compaction might be influenced by the second curing cycle of specimen type C, in which the damage was observed. Finally, reduction in the specimen stiffness is observed due to fatigue damage accumulation in time. Based on the results recommendations are formulated. For design purposes resin rich area formation should be avoided both inside and outside the laminate. If their formation is inevitable, at least the laminate should be kept well compacted and the resin rich area location should be kept only at the surface. For future research, two topics are identified as requiring such. First, is the laminate stiffness reduction. Second, is the influence of the second curing cycle of the well compacted specimen type C.

Abstract

A model was developed for delamination growth in bonded repair patches under constant amplitude fatigue loading. The model used the finite element method, employing the virtual crack closure technique, to determine the strain energy release rate (SERR) as a function of delamination length. Interaction effects between multiple delaminations, and the effect of delamination shape was also investigated. Fatigue cycling of coupon specimens was performed in order to find a relation between the SERR and the delamination growth rate. A power law (Paris-type) relation was established. Using this relation and the relation between SERR and delamination length, delamination growth predictions were produced. This predictions agreed well with the results of the coupon tests. A further validation by tests on more representative patch repair specimens was inconclusive due to the lack of delamination growth in the patch repair specimens.