B. A. Cheeseman

Ballistic impact into fabric and compliant composite laminates

The development of tough, high-strength, high-modulus fibers has led to the use of fabrics and compliant composite laminates for a number of impact-related applications, such as turbine blade containment, fuselage protection and body armor. Numerous studies have been conducted to identify material properties and system mechanisms that are important to the performance of these ballistic textiles. The current paper presents a review of the factors that influence ballistic performance; specifically, the material properties of the yarn, fabric structure, projectile geometry and velocity, far field boundary conditions, multiple plies and friction. Each physical mechanism is described in detail, and original references are cited to allow further investigation.

The effect of degree of saturation of sand on detonation phenomena associated with shallow-buried and ground-laid mines

A new materials model for sand has been developed in order to include the effects of the degree of saturation and the deformation rate on the constitutive response of this material. The model is an extension of the original compaction materials model for sand in which these effects were neglected. The new materials model for sand is next used, within a non-linear-dynamics transient computational analysis, to study various phenomena associated with the explosion of shallow-buried and ground-laid mines. The computational results are compared with the corresponding experimental results obtained through the use of an instrumented horizontal mine-impulse pendulum, pressure transducers buried in sand and a post-detonation metrological study of the sand craters. The results obtained suggest that the modified compaction model for sand captures the essential features of the dynamic behavior of sand and accounts reasonably well for a variety of the experimental findings related to the detonation of shallow-buried or ground-laid mines.

Material Modeling and Ballistic-Resistance Analysis of Armor-Grade Composites Reinforced with High-Performance Fibers

A new ballistic material model for 0°/90° cross-plied oriented ultra-high molecular weight (UHMW) polyethylene fiber-based armor-grade composite laminates has been constructed using open-literature data for the fiber and polymeric-matrix material properties and the general experimental/field-test observations regarding the deformation and failure modes in these types of materials. The present model is an extension of our recently developed unit cell-based ballistic material model for the same class of composites (M. Grujicic, G. Arakere, T. He,W.C. Bell, B. A. Cheeseman, C.-F. Yen, and B. Scott, A Ballistic Material Model for Cross-Plied Unidirectional Ultra-High Molecular-Weight Polyethylene Fiber-reinforced Armor-Grade Composites, Mater. Sci. Eng, A 2008, 498(1-2), p 231-241) which was found to be physically sound, but computationally not very efficient. The present model is constructed in such a way that it can be readily integrated into commercial finite element programs like ANSYS/Autodyn (ANSYS/Autodyn version 11.0, User Documentation, Century Dynamics Inc., a subsidiary of ANSYS Inc., 2007), as a User Material Subroutine. To validate the model, a series of transient nonlinear dynamics computational analyses of the transverse impact of armor-grade composite laminates with two types of bullets/projectiles is carried out and the computational results compared with their experimental counterparts. Relatively good agreement is found between the experiment and the computational analysis relative to: (a) the success of the armor panels of different areal densities in defeating the bullets at different initial bullet velocities; (b) postmortem spatial distribution of the damage modes and the extents within the panels; (c) the temporal evolution of the armor-panel back-face bulge; and (d) The existence of three distinct armor-penetration stages (i.e., an initial filament shearing/cutting dominated stage, an intermediate stage characterized by pronounced filament/matrix debonding/decohesion, and a final stage associated with the extensive filaments extension and armor-panel back-face bulging).

Fully coupled thermomechanical finite element analysis of material evolution during friction-stir welding of AA5083

Abstract Interactions between the rotating and advancing pin-shaped tool (terminated at one end with a circular—cylindrical shoulder) with the clamped welding plates and the associated material and heat transport during a friction-stir welding (FSW) process are studied computationally using a fully coupled thermomechanical finite element analysis. To surmount potential numerical problems associated with extensive mesh distortions/entanglement, an arbitrary Lagrangian—Eulerian (ALE) formulation was used, which enabled adaptive remeshing (to ensure the continuing presence of a high-quality mesh) while allowing full tracking of the material-free surfaces. To demonstrate the utility of the present computational approach, the analysis is applied to the case of FSW of AA5083 (a solid—solution strengthened and strain-hardened/stabilized Al—Mg wrought alloy). To account for the competition between plastic deformation-controlled strengthening and dynamic recrystallization-induced softening phenomena during the FSW process, the original Johnson—Cook strain and strain-rate hardening and temperature-softening material strength model is modified using the available recrystallization kinetics experimental data. Lastly, the computational results obtained in the present work are compared with their experimental counterparts available in the open literature. This comparison revealed that general trends regarding spatial distribution and temporal evolutions of various material-state quantities and their dependence on the FSW process parameters are reasonably well predicted by the present computational approach.

Ballistic Performance of Alumina/S‐2 Glass‐reinforced Polymer‐matrix Composite Hybrid Lightweight Armor Against Armor Piercing (ap) and Non‐AP Projectiles

The ability of light‐weight all fiber‐reinforced polymer‐matrix composite armor and hybrid composite‐based armor hard‐faced with ceramic tiles to withstand the impact of a non‐Armor‐ Piercing (non‐AP) and AP projectiles is investigated using a transient non‐linear dynamics computational analysis. The results obtained confirm experimental findings that the all‐composite armor, while being able to successfully defeat non‐AP threats, provides very little protection against AP projectiles. In the case of the hybrid armor, it is found that, at a fixed overall areal density of the armor, there is an optimal ratio of the ceramic‐to‐composite areal densities which is associated with a maximum ballistic armor performance against AP threats. The results obtained are rationalized using an analysis based on the shock/blast wave reflection and transmission behavior at the hard‐face/air, hard‐face/backing and backing/air interfaces, projectiles’ wear and erosion and the intrinsic properties of the constituent materials of the armor and the projectiles.

Computational Analysis of Material Flow During Friction Stir Welding of AA5059 Aluminum Alloys

Workpiece material flow and stirring/mixing during the friction stir welding (FSW) process are investigated computationally. Within the numerical model of the FSW process, the FSW tool is treated as a Lagrangian component while the workpiece material is treated as an Eulerian component. The employed coupled Eulerian/Lagrangian computational analysis of the welding process was of a two-way thermo-mechanical character (i.e., frictional-sliding/plastic-work dissipation is taken to act as a heat source in the thermal-energy balance equation) while temperature is allowed to affect mechanical aspects of the model through temperature-dependent material properties. The workpiece material (AA5059, solid-solution strengthened and strain-hardened aluminum alloy) is represented using a modified version of the classical Johnson-Cook model (within which the strain-hardening term is augmented to take into account for the effect of dynamic recrystallization) while the FSW tool material (AISI H13 tool steel) is modeled as an isotropic linear-elastic material. Within the analysis, the effects of some of the FSW key process parameters are investigated (e.g., weld pitch, tool tilt-angle, and the tool pin-size). The results pertaining to the material flow during FSW are compared with their experimental counterparts. It is found that, for the most part, experimentally observed material-flow characteristics are reproduced within the current FSW-process model.

Filament-Level Modeling of Aramid-Based High-Performance Structural Materials

Molecular statics and molecular dynamics are employed to study the effects of various microstructural and topological defects (e.g., chain ends, axial chain misalignment, inorganic solvent impurities, and sheet stacking faults) on the strength, ductility, and stiffness of p-phenylene terephthalamide (PPTA) fibers/filaments. These fibers can be considered as prototypes for advanced high strength/high-stiffness fibers like Kevlar®, Twaron®, New Star®, etc. While modeling these fibers, it was taken into account that they are essentially crystalline materials consisting of stacks of sheets, with each sheet containing an array of nearly parallel hydrogen-bonded molecules/chains. The inter-sheet bonding, on the other hand, was considered as mainly being of van der Waals or p-electron character. The effects of various deviations of the PPTA fiber structure from that of the perfectly crystalline structure (i.e., microstructural/topological defects) on the material’s mechanical properties are then considered. The results obtained show that while the presence of these defects decreases all the mechanical properties of PPTA fibers, specific properties display an increased level of sensitivity to the presence of certain defects. For example, longitudinal tensile properties are found to be most sensitive to the presence of chain ends, in-sheet transverse properties to the presence of chain misalignments, while cross-sheet transverse properties are found to be most affected by the presence of sheet stacking faults.

Process Modeling of Ti-6Al-4V Linear Friction Welding (LFW)

A fully coupled thermomechanical finite-element analysis of the linear friction welding (LFW) process is combined with the basic physical metallurgy of Ti-6Al-4V to predict microstructure and mechanical properties within the LFW joints (as a function of the LFW process parameters). A close examination of the experimental results reported in the open literature revealed that the weld region consists of a thermomechanically affected zone (TMAZ) and a heat-affected zone (HAZ) and that the material mechanical properties are somewhat more inferior in the HAZ. Taking this observation into account, a model for microstructure-evolution during LFW was developed and parameterized for the Ti-6Al-4V material residing in the HAZ. Specifically, this model addresses the problem of temporal evolution of the prior β-phase grain size (the dominant microstructural parameter in the HAZ) during the LFW process. This model is next combined with the well-established property versus microstructure correlations in Ti-6Al-4V to predict the overall structural performance of the LFW joint. The results obtained are found to be in reasonably good agreement with their experimental counterparts suggesting that the present computational approach may be used to guide the selection of the LFW process parameters to optimize the structural performance of the LFW joints.

Modifications in the AA5083 Johnson-Cook Material Model for Use in Friction Stir Welding Computational Analyses

Johnson-Cook strength material model is frequently used in finite-element analyses of various manufacturing processes involving plastic deformation of metallic materials. The main attraction to this model arises from its mathematical simplicity and its ability to capture the first-order metal-working effects (e.g., those associated with the influence of plastic deformation, rate of deformation, and the attendant temperature). However, this model displays serious shortcomings when used in the engineering analyses of various hot-working processes (i.e., those utilizing temperatures higher than the material recrystallization temperature). These shortcomings are related to the fact that microstructural changes involving: (i) irreversible decrease in the dislocation density due to the operation of annealing/recrystallization processes; (ii) increase in grain-size due to high-temperature exposure; and (iii) dynamic-recrystallization-induced grain refinement are not accounted for by the model. In this study, an attempt is made to combine the basic physical-metallurgy principles with the associated kinetics relations to properly modify the Johnson-Cook material model, so that the model can be used in the analyses of metal hot-working and joining processes. The model is next used to help establish relationships between process parameters, material microstructure and properties in friction stir welding welds of AA5083 (a non-age-hardenable, solid-solution strengthened, strain-hardened/stabilized Al-Mg-Mn alloy).

An Improved Mechanical Material Model for Ballistic Soda-Lime Glass

In our recent work (Grujicic et al., Int. J. Impact Eng., 2008), various open-literature experimental findings pertaining to the ballistic behavior of soda-lime glass were used to construct a simple, physically based, high strain rate, high-pressure, large-strain mechanical model for this material. The model was structured in such a way that it is suitable for direct incorporation into standard commercial transient non-linear dynamics finite element-based software packages like ANSYS/Autodyn (Century Dynamics Inc., 2007) or ABAQUS/Explicit (Dessault Systems, 2007). To validate the material model, a set of finite element analyses of the edge-on-impact tests was conducted and the results compared with their experimental counterparts obtained in the recent work of Strassburger et al. (Proceedings of the 23rd International Symposium on Ballistics, Spain, April 2007; Proceedings of the 22nd International Symposium on Ballistics, November 2005, Vancouver, Canada). In general, a good agreement was found between the computational and the experimental results relative to: (a) the front shapes and the propagation velocities of the longitudinal and transverse waves generated in the target during impact and (b) the front shapes and propagation velocities of a coherent-damage zone (a zone surrounding the projectile/target contact surface which contains numerous micron and submicron-size cracks). However, substantial computational analysis/experiment disagreements were found relative to the formation of crack centers, i.e. relative to the presence and distribution of isolated millimeter-size cracks nucleated ahead of the advancing coherent-damage zone front. In the present work, it was shown that these disagreements can be substantially reduced if the glass model (Grujicic et al., Int. J. Impact Eng., 2008) is advanced to include a simple macrocracking algorithm based on the linear elastic fracture mechanics.