Aircraft designed for naval aircraft carriers experience great airframe stress during landing due to the high vertical velocities that they must maintain as a consequence of the extremely short runway and shallow landing angle of attack. This creates a need for structural rigidity to counteract the forces that land-based aircraft never experience. This is not ideal if it otherwise limits the performance and flying capabilities of the aircraft that are otherwise necessary for the environments they might find themselves in. As such, a new approach to protecting the aircraft from the immense loads they experience during landing could be to add flexibility to the airframe and landing gear, promoting deflection instead of failure. This thesis aims to investigate this idea, starting with an elementary set of tests, looking into material flexibility, and then moving on to adding this concept to progressively more advanced structural systems. Using balls of varying material, preliminary drop tests indicated that material flexibility could assist the dissipation of landing energies, showing that the coefficient of restitution increases with the stiffness. Drop tests involving mass-spring-damper systems as well as cantilever plates and transverse beams also indicated that the strain energy a body can absorb from a set load case can be increased if its flexibility also grows. This finding led to the important conclusion and finding that a flexible body can transfer and store at least 10 times its initial contribution of energy to a system in the form of strain energy. Through these tests, it was shown that flexible structures can be a beneficial design feature in combatting and dissipating vertical landing energies. / Master of Science / Historically, airplanes landing on naval aircraft carriers are subject to high impact loads when they land because the plane is traveling at a high velocity downward and has a short runway to stop on. This impact on the runway is so severe that it requires the structure of the airplane to be reinforced, which in turn makes the plane heavier and less capable in flight. This reinforcement also implies that the plane is quite stiff in all of its components. One solution to this issue is to reverse the design logic historically taken, and impose flexible structures into the main body of the plane, which can bend and absorb some of the vertical energy that the plane possesses. This theory was investigated using a series of drop tests, starting with ball drop tests of varying materials. These tests showed that the material of a ball can affect the energy that it absorbs and how much is kept by the ball after it collides with the ground. Next, more complex structures were tested, using shock absorbers, metal plates, and metal beams. These components were combined to form drop systems, which were dropped to measure the bending in the plates and beams, as well as the shock absorbers. The conclusion made from these tests is that a more flexible structure can absorb a higher percentage of energy compared to its initial contribution, than its stiffer and heavier counterpart. This important conclusion shows that the application of flexible structures could be a vital step in improving the design of airplane wing and body structures to promote the longevity of the structure of the aircraft.

Predicting the performance of complex structures exposed to harsh thermal environments is a crucial issue in many of today's aerospace and space designs. To predict the thermal stresses a structure might be exposed to, the thermal properties of the independent materials used in the design of the structure need to be known. Therefore, a noninvasive estimation procedure involving Genetic Algorithms was developed to determine the various thermal properties needed to adequately model the Outer Wing Subcomponent (OWS), a structure located at the trailing edge of the High Speed Civil Transport's (HSCT) wing tip. Due to the nature of the nonlinear least-squares estimation method used in this study, both theoretical and experimental temperature histories were required. Several one-dimensional and two-dimensional finite element models of the OWS were developed to compute the transient theoretical temperature histories. The experimental data were obtained from optimized experiments that were run at various surrounding temperature settings to investigate the temperature dependence of the estimated properties. An experimental optimization was performed to provide the most accurate estimates and reduce the confidence intervals. The simultaneous estimation of eight thermal properties, including the volumetric heat capacities and out-of-plane thermal conductivities of the facesheets, the honeycomb, the skins, and the torque tubes, was successfully completed with the one-dimensional model and the results used to evaluate the remaining in-plane thermal conductivities of the facesheets, the honeycomb, the skins, and the torque tubes with the two-dimensional model. Although experimental optimization did not eliminate all correlation between the parameters, the minimization procedure based on the Genetic Algorithm performed extremely well, despite the high degree of correlation and low sensitivity of many of the parameters. / Master of Science

The ongoing drive for lighter and more efficient structural components by the commercial engineering industry has resulted in the rapid adoption of the finite element method (FE) for design analysis. Satisfied with the success of finite elements in reducing prototyping costs and overall production times, the industry has begun to look at other areas where the finite element method can save time, and in particular, improve designs. First, the mathematical methods of optimisation, on which the methods of structural design improvement are based, are presented. This includes the methods of: topology, influence functions, basis vectors, geometric splines and direct sensitivity methods. Each method is demonstrated with the solution of a sample structural improvement problem for various objectives (frequency, stress and weight reduction, for example). The practical application of the individual methods has been tested by solving three structural engineering problems sourced from the automotive engineering industry: the redesign of two different front suspension control arms, and the cost-reduction of an automatic brake tubing system. All three problems were solved successfully, resulting in improved designs. Each method has been evaluated with respect the practical application, popularity of the method and also any problems using the method. The solutions presented in each section were all solved using the FE design improvement software ReSHAPE from Advea Engineering Pty. Ltd.

<p>Damaged components and a lack of spare components are issues which are currently affecting military aircraft capability. Laser Cladding is an additive manufacturing technique which shows promise in repairing damaged aviation components. However, there are considerable certification requirements for critical components which stand to gain the most benefits from laser clad repair methodologies. These requirements involve establishing crack growth rate data for the laser clad material to gain confidence in the reliability of the repair's performance on in-service aircraft. This research seeks to understand the fatigue behavior of Ti-6Al-4V that has undergone a simulated laser clad repair, with unrepaired specimens also tested to allow for comparison. </p>

10ABSTRACTEnergetic materials are widely used as rocket propellants and explosives in the field of aerospace and defense. Understanding the nature of impact in polymer-bonded explosives is crucial safety and transportation of energetic materials. The formation of hotpots in energetic materials leads to unexpected initiations, posing a safety hazard. An attempt was made to study the mechanical behavior of energetic materials under different shapes of impactors. In particular, the likelihood of hotspot formations was discussed in spherical and Spherical Impactors(SI). Spherical and planar-shaped impactors were modeled with a cohesive finite element frame work to simulate the behavior of granular energetic materials with cyclo-tetramethylene-tetranitramine(HMX) embedded in a hydroxyl-polybutadiene binder. Temperature distribution and stresses induced around crystals on expanding stress profile of SI and uniform pressure profile from a SI are compared to determine the possibility of detonation.<div><br></div><div>In this work, the dependence of sample morphology on induced stresses in the microstructure is highlighted by using three different microstructures. A digitized polymer-bonded-explosive microstructure was analyzed for possible initiations with different impact velocities. The effect of the shape of grains and volume fractions on the likeliness of hotspot formation were studied using rounded and sharp-edged idealized crystals. Impactor behavior on samples was compared based on force chains, temperature profiles, and stress distributions</div>

Aerospace structures have stringent pointing and shape control requirements during long-term exposure to a hostile environment with no scheduled maintenance. This makes them excellent candidates for a smart structures approach as current passive techniques prove insufficient. This study investigates the feasibility of providing autonomous dimensional control to aerospace structures by embedding shape memory alloy elements inside composite structures. Increasing volume fractions of nitinol wire were embedded in cross-ply graphite/ epoxy composite panels. The potential of this approach was evaluated by measuring the change in longitudinal strain with increasing temperature and volume fraction. Reduction of thermal expansion is demonstrated and related to embedded volume fraction. Classical lamination theory is used to formulate a two-dimensional model which included the adaptive properties of the embedded nitinol. The model was used to predict the increased modulus and reduction of thermal strain in the modified plates which was verified by the experimental data. / Master of Science

<div>The service life of aircraft structure, primarily composed of aluminum alloys, is markedly lower when galvanic corrosion is present due to early crack initiation at localized pitting, with the likelihood of cracking being higher at pits spanning several microns. To understand the joint effect that the mechanical and chemical behavior of AA7050-T7451 have on the evolution of corrosion prior and until its transition to cracking, the microstructure, constituent particles, mechanical strains, and the corrosion morphology were experimentally characterized using high-resolution methods and the mechanical stresses are computationally modeled at the micrometer level using a FFT-based crystal plasticity framework. </div><div><br></div><div>The material was corroded under both mechanically loaded and unloaded conditions under different corrosion intervals to properly capture the evolution of corrosion before, during, and after particle fallout. For the events prior to cracking, statistical cross-correlations between the mechanical state of the material and the corrosion morphology were performed to understand the mechanisms driving corrosion at its various stages. For the cracking event and its subsequent growth, the joint analysis of strains and stresses obtained from 3D crystal plasticity models were used to calculate Fatigue Indicator Parameters (FIPs) that can quantitatively give an insight of the major mechanisms driving crack initiation and growth in pre-corroded materials. The development of micromechanical models that account for both the environmental degradation and the microstructure in the material allowed to accurately predict the location of crack initiation arising from pits, which has been a longstanding problem in the field of corrosion. It is concluded that both corrosion growth and its transition to cracking are multivariable events, where corrosion growth is jointly driven by the local chemistry and the micromechanics, and crack initiation is driven by the coupled interaction between the corrosion geometry and the micromechanics.</div><div><br></div>

Platelets made of slit and chopped unidirectional, carbon-fiber prepreg are becoming a popular option for use as a high performance molding compound because of their high fiber volume fraction and increased ability to flow compared to continuous fiber systems. As this molding compound is newly introduced to industry, increasing amounts of research have gone into understanding how platelets flow during molding and how components perform mechanically based on the final orientation state of platelets. This work investigates the performance of prepreg platelet molding compound (PPMC) as a viable alternative to continuous fiber systems for use with geometrically complex structural members on vehicles subjected to collisions. In doing so, the crash performance, energy absorption, and failure morphology of crush tubes made with PPMC are investigated and quantified. Then, a simulation methodology is developed to obtain manufacturing-informed performance models to predict the effect of platelet orientation state on mechanical behavior of PPMC components. This methodology uses a building block approach where each block in modeling is verified against closed-form solution (when available) and validated against experimental results. Once confidence is developed in a modeling block, the complexity of the simulation is increased until a component with full platelet orientation distribution is captured. The result is PPMC component models that are capable of predicting mechanical performance in orientation regimes that are not investigated experimentally.

<p>Aeronautical products additively manufactured by Selective Laser Melting (SLM), are known to have fatigue properties which are negatively impacted by porosity defects, microstructural features and residual stresses. Little research is available studying these phenomena with respect to the short fatigue crack growth (FCG) inconsistency problem, the large focus being on the long FCG. This thesis seeks to add useful knowledge to the understanding of the mechanisms for short crack growth variability in SLM manufactured Ti-6Al-4V, with the two variables for the process conditions and build directions investigated. An in-situ FCG investigation using x-ray synchrotron computed micro-tomography (μXSCT) was used to visually observe and quantify the short crack path evolution. Crack growth, deflections and porosity interactions were noted and discussed in relation to microstructure, build layer thickness and build layer orientation. A novel use of in-situ energy dispersive x-ray diffraction (EDD) was able to show the lattice strains evolving as a propagating crack moved through a small region of interest. The results presented show the ability to reliably obtain all six elastic strain tensor components, and interpret useful knowledge from a small region of interest. </p> <p> </p> <p>There are conflicting views in literature with respect to the damage tolerance behavior of as built SLM manufactured Ti-6Al-4V. In the 2018 review by Agius et al., the more prominent studies were considered with Leuders et al. showing the highest long FCG rates for cracks parallel to the build layer and Cain et al. showing cracks propagating through successive build layers as highest [1]–[3]. Cain et al. and Vilaro et al. report significant anisotropy in long FCG for different build orientations whereas Edwards and Ramulu present similar FCG behavior for three different build directions [2]–[5]. Kruth et al. concluded that for optimized build parameters without any (detectable) pores, the building direction does not play a significant role in the fracture toughness results [6]. All of the mentioned literature reported martensitic microstructures and the presence of prior grain structures for as built SLM Ti-6Al-4V.</p> <p> </p> <p>No studies to the authors knowledge have considered the short FCG of SLM manufactured Ti‑6Al‑4V and its implications to the conflicting damage tolerance behaviors reported in literature [1]. In this work small cross-sectional area (1.5 x 1.5 ) samples in two different build conditions of as built SLM Ti-6Al‑4V are studied. The short FCG rate of three different build directions was considered with cracks parallel to the build layers shown to be the most damaging. The microstructure and build layer are shown to be the likely dominant factors in the short FCG rate of as built Ti-6Al-4V. In terms of porosity, little impact to the propagating short crack was seen although there is local elastoplastic behavior around these defects which could cause toughening in the non-optimized build parameter samples tested. The fracture surfaces were examined using a Scanning Electron Microscope (SEM) with the results showing significant differences in the behavior of the two build conditions. From the microindentation hardness testing undertaken, the smooth fracture surface of the optimized sample correlated with a higher Vickers Hardness (VH) result and therefore higher strength. The non-optimized samples had a ‘rough’ fracture surface, a lower VH result and therefore strength. Furthering the knowledge of short FCG in SLM manufactured Ti-6Al-4V will have positive implications to accurately life and therefore certify additive manufactured aeronautical products.</p>

Micro-mechanical Raman spectroscopy is an excellent tool for direct stress measurements in the structure. The presence of mechanical stress changes the Raman frequency of each Raman modes compared to the Raman frequencies in absence of stress. This difference in Raman frequency is linearly related to stress induced and can be calibrated to stress by uniaxial or biaxial tension/compression experiments. This relationship is not generally linear for non-linear behavior of the materials which limits its use to experimentally study flow stress and plastic deformation behavior of the material. In this work strontium titanate ceramic particles dispersed inside epoxy resin matrix were used to measure stress in epoxy resin matrix with non-linear material behavior around it. The stress concentration factor between stress induced inside ceramic particles and epoxy resin matrix was obtained by non-linear constitutive finite element model. The results of finite element model were used for training a machine learning model to predict stress in epoxy resin matrix based on stress inside ceramic particles. By measuring stress inside ceramic particles using micro-mechanical Raman spectroscopy, the stress inside epoxy matrix was obtained by pre-determined stress concentration factor.