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.

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>

<p dir="ltr">The leading edges of helicopter rotor and wind turbine blades, largely made of fiber-reinforced polymer composites, are susceptible to impact from small particles of a variety of materials at velocities up to the speed of sound. To ensure structural integrity of these blades over their life spans, it is essential to fundamentally understand how these impacts damage, and subsequently erode, the blade material and structure. However, experimental investigations of single particles at impact velocities simulating realistic impact conditions are scarce. This study is designed to observe the local crater formation process from a single particle impact at the upper limit of rotor tip velocities to identify the key erodent parameters behind the damage mechanisms in a polymer composite. It is also aimed to further examine damage mechanisms in a plate sample to determine the extent of material loss and global deformation of the impact surface. In experiments presented, a glass fiber-reinforced epoxy composite was subjected to single impacts from spherical particles at velocities up to Mach 1. A specifically designed light gas gun was used to launch particles at relevant velocities. Target configurations were selected to examine the highly localized crater formation process and to measure the global deformation of a target plate subjected to high-velocity impact by spherical particles 1.59 mm in diameter. The crater formation processes from impacts by particles of five different materials were evaluated by high-speed <i>in-situ</i> imaging as well as post-mortem inspection with a scanning electron microscope. The global deformation of a target plate was examined <i>in-situ</i> using stereo DIC and post-mortem using optical profilometry. The composite in this study was found to exhibit brittle fracture behavior during the crater formation process at the local level, with ductile behavior observed at the global scale in a plate sample. The formation of the crater in the target material was found to follow a two-stage process from a single impact event. The first stage creates a debris cloud at the perimeter of the impact site, with a very consistent outer diameter across all five projectile materials. In the second stage, material at the center of the crater is removed from the target as the loading wave reflects off the free boundary after the projectile has separated from the target. The second stage behavior depended on two distinct projectile groups: those that experienced plastic deformation on impact with the target material and those that did not undergo any apparent plastic deformation. A GFRP composite with a small yarn, plain weave reinforcement structure was found to exhibit anisotropic flexural wave propagation near the impact site. This degree of anisotropy is reduced as the wave propagates further from the impact site. These results add new insights into a comprehensive fundamental understanding of impact damage and erosion mechanisms in fiber-reinforced polymer composite materials and structures.</p>

<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.