Soft body armour is usually constructed by layering numerous layers of the same fabric. Such a construction, however, may not be the most efficient in providing the required protection due to different ballistic resistant efficiency of each layer. This research aims to optimise the construction of the panels for soft body armour by hybridisation in order to achieve the improvement of ballistic performance and reductions in weight. Twaron woven fabrics with different weave structures and Dyneema uni-directional (UD) laminates were used as components for the hybrid design of panels. Two complementary research approaches were employed in this study, namely the empirical method and the Finite Element (FE) analysis. The first part of this research systematically revealed the different ballistic characteristics of each layer in different positions of an armour panel and the way of energy absorption in the panel. The fabric layers in the front, middle and back of the panel exhibited different extent of transverse deformation and stress distribution. The energy absorption increases from front layer and reaches to the maximum value in the last perforated layer and then decreases gradually in the following back layers. Such pattern of energy absorption was not affected by either the striking velocity or the total number of layers in the panel, but the position, in the thickness, of the peak value in energy absorption was shifted more towards the back of the panel when the striking velocity increases. Such findings contribute to the understanding of different ballistic responses in different positions of an armour panel under ballistic impact. The second part of this research put forward a new hybrid design concept. According to above theoretical understandings of different ballistic characteristics in different positions of an armour panel, the fabric layers in the panel were discretely divided into three groups. In addition to the performance of different components for the panel and the influences of the laying sequence, a procedure for constructing hybrid armour panels has been established. The first group was composed of the first few layers on the striking face. The heavyweight fabrics as heat resistant layers were used in this group to resist the heat generated on the striking face. The second group contained some middle layers close to the last perforated layers. The lightweight fabric was combined in this group due to the higher energy absorption capacity. All back layers were classified into the third group. Dyneema UD laminates were placed in this group to constrain the large transverse deflection of the lightweight fabric and to minimize BFS of the panel. Two hybrid panels were designed and evaluated. In the perforation ballistic tests, the hybrid panel was more likely to stop the projectile compared to Twaron woven panels with the same areal density. In the non-perforation ballistic tests, the hybrid panel exhibited significantly lower BFS and achieved the reductions in weight. Such hybrid design makes best use of different available materials to achieve the improvement of ballistic performance and lightweight of a panel. It has a practical significance for the soft armour panel design.

This thesis focuses on the micromechanical modeling of particulate viscoelastic composite materials in the quasi-static frequency domain to approximate macroscopic damping behavior and has two main objectives. The first objective is the development of a robust frequency dependent multiscale model. For this purpose, the self-consistent (SC) mean-field micromechanical model introduced by Cherkaoui et al [J. Eng. Mater. Technol. 116, 274-278 (1994)] is extended to include frequency dependence via the viscoelastic correspondence principal. The quasi-static model is then generalized using dilute strain concentration tensor formulation and validated by comparison with complex bounds from literature, acoustic and static experimental data, and established models. The second objective is SC model implementation as a tool for the design of high loss materials. This objective is met by integrating the SC model into a Compromise Decision Support Protocol (CDSP) to explore the microstructural design space of an automobile windshield. The integrated SC-CDSP design space exploration results definitively indicate that one microstructural variable dominates structure level acoustic isolation and rigidity: negative stiffness. The work concludes with a detailed description of the fundamental mechanisms leading to negative stiffness behavior and proposes two negative stiffness inclusion designs.

<p>The use of composite materials in vehicle structures could reduce the weight and thereby the fuel consumption of vehicles.</p><p>As the road safety of the vehicles must be ensured, it is vital that the energy absorbing capability of the composite materials are similar to or better than the commonly used steel structures. The high specific bending stiffness of sandwich structures can with advantage be used in vehicles, provided that the structural behaviour during a crash situation is well understood and possible to predict. The purpose of this thesis is to identify and if possible to describe the failure initiation and progression in in-plane compression loaded sandwich panels.</p><p>An experimental study on in-plane compression loaded sandwich panels with two different material concepts was conducted. Digital speckle photography (DSP) was used to record the displacement field of one outer face-sheet surface during compression. The sandwich panels with glass fibre preimpregnated face-sheets and a polymer foam core failed due to disintegration of the face-sheets from the core, whereas the sandwich panels with sheet molding compound face-sheets and a balsa core failed in progressive end-crushing. A simple semi-empirical model was developed to describe the structural response before and after initial failure.</p><p>The postfailure behaviour of in-plane compression loaded sandwich panels was studied by considering the structural behaviour of sandwich panels with edge debonds. A parametrical finite element model was used to determine the influence of different material and geometrical properties on the buckling and postbuckling failure loads. The postbuckling failure modes studied were debond crack propagation and face-sheet failure. It could be concluded that the postbuckling failure modes were mainly determined by the ratio between the fracture toughness of the face-core interface and the bending stiffness of the face-sheets.</p>

Solid foams have been studied for years for their ability to mitigate damage from sudden impact. Small explosive attacks threaten to damage or destroy key structures in some parts of the world. A newly developed material, carbon foam, may offer the ability to mitigate the effects of such blasts. This project investigates the energy absorbing properties of carbon and polyurethane based foams in dynamic compression to illustrate their viability to protect concrete structures from the damaging effects of pressure waves from a small blast. Cellular solid mechanics fundamentals and a survey of the microscopic cellular structure of each type of foam are discussed. Experiments were performed in three strain rate regimes: low strain rate compression testing, middle strain rate impact testing, and high strain rate blast testing to reveal mechanical behavior. Experiments show a 7.62 cm (3”) thick hybrid composite layered foam sample can protect a concrete wall from a small blast.

Over the past several decades increasing focus has been paid to the impact of structures where energy, during the impact event, needs to be absorbed in a controlled manner. This has led to considerable research being carried out on energy absorbers, devices designed to dissipate energy during an impact event and hence protect the structure under consideration. Energy absorbers have found common usage in applications such as vehicles, aircraft, highway barriers and at the base of lift shafts. A type of energy absorber which has received relatively limited attention in the open literature is the tapered rectangular tube. Such a structure is essentially a tube with a rectangular cross-section in which one or more of the sides are inclined to the tube's longitudinal axis. The aim of this thesis was to analyse the impact and energy absorption response of tapered and non-tapered (straight) rectangular tubes. The energy absorption response was quantified for both axial and oblique loading, representative of the loading conditions typically encountered in impact applications. Since energy absorbers are commonly used as components in energy absorbing systems, the response of such a system was analysed which contained either straight or tapered rectangular tubes as the energy absorbing components. This system could typically be used as the front bumper system of a vehicle. Detailed finite element models, validated using experiments and existing theoretical and numerical models, were used to assess the energy absorption response and deformation modes of straight and tapered tubes under the various loading conditions. The manner in which a thin-walled tube deforms is important since it governs its energy absorption response. The results show that the energy absorption response of straight and tapered rectangular tubes can be controlled using their various geometry parameters. In particular, the wall thickness, taper angle and the number of tapered sides can be effectively used as parameters to control the amount of absorbed energy. Tapered rectangular tubes display less sensitivity to inertia effects compared with straight rectangular tubes under impact loading. This is beneficial when the higher crush loads associated with inertia effects need to be reduced. Furthermore, though the energy absorption capacity of thin-walled rectangular tubes diminishes under oblique impact loading, the capacity is more maintained for tapered rectangular tubes compared with non-tapered rectangular tubes. Overall, the results highlight the advantages of using tapered rectangular tubes for absorbing impact energy under axial and oblique loading conditions. Understanding is gained as to how the geometry parameters of such structures can be used to control the absorbed energy. The thesis uses this knowledge to develop design guidelines for the use of straight and tapered rectangular tubes in energy absorbing systems such as for crashworthiness applications. Furthermore, the results highlight the importance of analysing thin-walled energy absorbers as part of an energy absorbing system, since the response of the absorbers may be different to when they are treated on their own.

Out-of-Autoclave (OoA) processes for manufacturing aerospace-grade parts needs to be better understood to further the development and success of industries that are manufacturing reusable launch vehicles, military and commercial aircraft, and spacecraft. Overcoming the performance limitations associated with OoA, also known as low-pressure prepreg curing, methods (void count, energy absorption, etc.) will help decrease the costs associated with aerospace composite manufacturing and the negative environmental effects correlated with high-pressure composite curing methods. Experimental, theoretical, and numerical approaches are used to explore both low and high-pressure curing cycles and how the two different processes affect final cured parts. Quasi-static uniaxial compression tests on 33mm diameter tubular specimens concluded that the high-pressure curing methods (up to 90 psi) increased the likelihood of a final part with increased stiffness compared to the lower atmospheric-pressure methods (14.7 psi) on an order of 22%. After further extension and deformation past the linear elastic region, tests concluded that although the autoclaved specimens may have been higher-quality parts, the low-pressure-cured specimens performed more efficiently with respect to energy absorption. Considering the specific energy absorption (SEA) and crush force efficiency (CFE) are both on average around 6% higher for the low-pressure specimens, it is concluded that they can perform similarly to the high-pressure specimens and possibly even more efficiently depending on the loading conditions and desired purpose of the structure.

This research project has been undertaken to produce and characterize the behavior of High Strength Cementitious Mix (HSCM), which has considerably higher compressive strength compared to conventional concrete. Components of HSCM are cement, silica fume, sand, water, and high range water reducer. The material is tested for compressive strength and impact energy absorption capacity while the amount of above mentioned components are varied parametrically. The effect of these parameters are extensively studied and trends are reported. Finally, this research projects attempts to find correlations among compressive strength, compressive toughness, and impact toughness. Limitations of the experimental program are discussed and future direction for improvement and expansion of the research program is suggested.

Indiana University-Purdue University Indianapolis (IUPUI) / Lattice structures have seen increasing use in several industries including automotive, aerospace, and construction. Lattice structures are lightweight and can achieve a wide range of mechanical behaviors through their inherent cellular design. Moreover, the unit cells of lattice structures can easily be meshed and conformed to a wide variety of volumes. Compliant mechanism make suitable micro-structures for units cells in lattice structures that are designed for impact energy absorption. The flexibility of compliant mechanisms allows for energy dissipation via straining of the members and also mitigates the effects of impact direction uncertainties. Density-based topology optimization methods can be used to synthesize compliant mechanisms. To aid with this task, a proposed optimization tool, coded in MATLAB, is created. The program is built on a modular structure and allows for the easy addition of new algorithms and objective functions beyond what is developed in this study. An adjacent investigation is also performed to determine the dependencies and trends of mechanical and geometric advantages of compliant mechanisms. The implications of such are discussed. The result of this study is a compliant mechanism lattice structure for impact energy absorption. The performance of this structure is analyzed through the application of it in a football helmet. Two types of unit cell compliant mechanisms are synthesized and assembled into three liner configurations. Helmet liners are further developed through a series of ballistic impact analysis simulations to determine the best lattice structure configuration and mechanism rubber hardness. The final liner is compared with a traditional expanded polypropylene foam liner to appraise the protection capabilities of the proposed lattice structure.

Indiana University-Purdue University Indianapolis (IUPUI) / Design optimization of composite structures is of importance in the automotive, aerospace, and energy industry. The majority of optimization methods applied to laminated composites consider linear or simplified nonlinear models. Also, various techniques lack the ability to consider the composite failure criteria. Using artificial neural networks approximates the objective function to make it possible to use other techniques to solve the optimization problem. The present work describes an optimization process used to find the optimum design to meet crashworthiness requirements which includes minimizing peak crushing force and specific energy absorption for a square tube. The design variables include the number of plies, ply angle and ply thickness of the square tube. To obtain an effective approximation an artificial neural network (ANN) is used. Training data for the artificial neural network is obtained by crash analysis of a square tube for various samples using LS DYNA. The sampling plan is created using Latin Hypercube Sampling. The square tube is considered to be impacted by the rigid wall with fixed velocity and rigid body acceleration, force versus displacement curves are plotted to obtain values for crushing force, deceleration, crush length and specific energy absorbed. The optimized values for the square tube to fulfill the crashworthiness requirements are obtained using an artificial neural network combined with Multi-Objective Genetic Algorithms (MOGA). MOGA finds optimum values in the feasible design space. Optimal solutions obtained are presented by the Pareto frontier curve. The optimization is performed with accuracy considering 5% error.

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