Blast-wave impact-mitigation capability of polyurea when used as helmet suspension-pad material

Traumatic brain injury (TBI) is generally considered as a signature injury of the current military conflicts, with costly and life-altering long-term effects. Hence, there is an urgent need to combat this problem by both gaining a better understanding of the mechanisms responsible for the blast-induced TBI and by designing/developing more effective head protection systems. In the present work, the blast-wave impact-mitigation ability of polyurea when used as a helmet suspension-pad material is investigated computationally. Towards that end, a combined Eulerian/Lagrangian fluid/solid transient non-linear dynamics computational analysis is carried out at two levels of blast peak overpressure: (a) one level corresponding to the unprotected-lung- injury-threshold; and (b) the other level associated with the corresponding 50% lethal dose (LD₅₀), i.e. with a 50% probability for lung-injury induced death. To assess the blast-wave impact-mitigation ability of polyurea, the temporal evolution of the axial stress and the particle (axial) velocity at different locations within the intra-cranial cavity are analyzed. The results are compared with their counterparts obtained in the case of a conventional foam suspension-pad material. This comparison showed that, the use of polyurea suspension pads is associated with a substantially greater reduction in the peak loading experienced by the brain relative to that observed in the case of the conventional foam. The observed differences in the blast-wave mitigation capability of the conventional foam and polyurea are next rationalized in terms of the differences in their microstructure and in their mechanical response when subjected to blast loading.     

Computer Modeling of the Evolution of Dendrite Microstructure in Binary Alloys During Non-isothermal Solidification

Two-dimensional simulations of the evolution of dendrite microstructure during isothermal and non-isothermal solidifications of a Ni-0.41Cu binary alloy are carried out using the phase-field method. The governing evolution equation for the phase field variable, the solute mole fraction and the temperature are formulated and numerically solved using an explicit finite difference scheme. To make the computations tractable, parallel computing is employed. The results obtained show that under lower cooling rates, the solidification process is controlled by partitioning of the solute between the solid and the liquid at the solid/liquid interface. At high cooling rates, on the other hand, solute trapping takes place and solidification is controlled by the heat extraction rate. An increase in the cooling rate is also found to have a pronounced effect on the dendrite microstructure causing it to change from poorly developed dendrites consisting of only primary stalks, via fully developed dendrites containing secondary and tertiary arms to the diamond-shaped grains with cellular surfaces. These findings are in excellent agreement with experimental observations.     

Tensile testing and TEM analysis of nitrogen-strengthened Fe-Ni-Cr single crystals

A conventional abnormal grain growth annealing heat treatment has been used to produce low (0.009 wt%) and high (0.12 wt%) nitrogen single crystalline Fe-40Ni-15Cr wt% base alloys. Constant strain-rate tensile tests were carried out in the temperature range between 77 and 298 K at two different strain-rates. The results obtained were analysed using the standard procedure for the thermally activated glide of dislocations, and the possible rate-controlling mechanisms for athermal and thermal nitrogen-induced strengthening have been discussed. A conventional two-beam bright-field transmission electron microscope was next used to determine the character of the dislocations and their dependence on the amount of nitrogen in the alloy. It was found that the dislocations became predominantly screw in character as the nitrogen content in the alloy was increased. These findings have been discussed in the light of the existing models for the nitrogen strengthening of the Fe-Ni-Cr austenite.     

Concurrent Computational and Dimensional Analyses of Design of Vehicle Floor-Plates for Landmine-Blast Survivability

Development of military vehicles capable of surviving landmine blast is seldom done using full-scale prototype testing because of the associated prohibitively-high cost, the destructive nature of testing, and the requirements for major large-scale experimental-test facilities and a large crew of engineers committed to the task. Instead, tests of small-scale models are generally employed and the model-based results are scaled up to the full-size vehicle. In these scale-up efforts, various dimensional analyses are used whose establishment and validation requires major experimental testing efforts and different-scale models. In the present work, an approach is proposed within which concurrent and interactive applications of the computational analyses (of landmine detonation and the interaction of detonation products and soil ejecta with the vehicle hull-floor) and the corresponding dimensional analysis are utilized. It is argued that this approach can guide the design of military-vehicle hull-floors which provide the required level of protection to the vehicle occupants under landmine blast attack without introducing unnecessarily high weight to the vehicle. To validate this approach, a combined Eulerian/Lagrangian formulation for landmine detonation and the interaction of detonation products and soil ejecta with the vehicle hull-floor (developed in our previous work) has been utilized along with the experimental results pertaining to small-scale model and full-scale vehicle testing.     

Multi-length scale modeling of chemical vapor deposition of titanium nitride coatings

Chemical Vapor Deposition (CVD) of TiN coatings has been analyzed at three different length scales: (a) At chemical reactor length scale, by solving the appropriate reactive-gas, fluid-dynamics, heat-transfer boundary value problem; (b) At the atomic scale, by applying a kinetic Monte Carlo method to model the deposition process in a stochastic manner and (c) At the coating-grain scale, by employing an improved van der Drift-type model to simulate the evolution of surface morphology, grain size distribution, evolution of the morphological and crystallographic texture, etc. in polycrystalline TiN coatings. It has shown that by combining the three modeling schemes, one can establish a direct link between the processes parameters and the microstructure (and thus the properties) of as CVD-grown TiN coatings. This, in turn, enables optimization of both the coating deposition process, and the microstructure and properties of CVD-grown coatings.     

Computational Modeling of Microstructural-Evolution in AISI 1005 Steel During Gas Metal Arc Butt Welding

A fully coupled (two-way), transient, thermal-mechanical finite-element procedure is developed to model conventional gas metal arc welding (GMAW) butt-joining process. Two-way thermal-mechanical coupling is achieved by making the mechanical material model of the workpiece and the weld temperature-dependent and by allowing the potential work of plastic deformation resulting from large thermal gradients to be dissipated in the form of heat. To account for the heat losses from the weld into the surroundings, heat transfer effects associated with natural convection and radiation to the environment and thermal-heat conduction to the adjacent workpiece material are considered. The procedure is next combined with the basic physical-metallurgy concepts and principles and applied to a prototypical (plain) low-carbon steel (AISI 1005) to predict the distribution of various crystalline phases within the as-welded material microstructure in different fusion zone and heat-affected zone locations, under given GMAW-process parameters. The results obtained are compared with available open-literature experimental data to provide validation/verification for the proposed GMAW modeling effort.     

Development of a Robust and Cost-Effective Friction Stir Welding Process for Use in Advanced Military Vehicles

To respond to the advent of more lethal threats, recently designed aluminum-armor-based military-vehicle systems have resorted to an increasing use of higher strength aluminum alloys (with superior ballistic resistance against armor piercing (AP) threats and with high vehicle-light weighing potential). Unfortunately, these alloys are not very amenable to conventional fusion-based welding technologies and in-order to obtain high-quality welds, solid-state joining technologies such as Friction stir welding (FSW) have to be employed. However, since FSW is a relatively new and fairly complex joining technology, its introduction into advanced military vehicle structures is not straight forward and entails a comprehensive multi-step approach. One such (three-step) approach is developed in the present work. Within the first step, experimental and computational techniques are utilized to determine the optimal tool design and the optimal FSW process parameters which result in maximal productivity of the joining process and the highest quality of the weld. Within the second step, techniques are developed for the identification and qualification of the optimal weld joint designs in different sections of a prototypical military vehicle structure. In the third step, problems associated with the fabrication of a sub-scale military vehicle test structure and the blast survivability of the structure are assessed. The results obtained and the lessons learned are used to judge the potential of the current approach in shortening the development time and in enhancing reliability and blast survivability of military vehicle structures.     

Multi-Length Scale Modeling of High-Pressure-Induced Phase Transformations in Soda-Lime Glass

Molecular-level modeling and simulations are employed to study room-temperature micro-structural and mechanical response of soda-lime glass when subjected to high (i.e., several giga-Pascal) uniaxial-strain stresses/pressure. The results obtained revealed the occurrence of an irreversible phase-transformation at ca. 4 GPa which was associated with a (permanent) 3-7% volume reduction. Close examination of molecular-level topology revealed that the pressure-induced phase transformation in question is associated with an increase in the average coordination number of the silicon atoms, and the creation of two- to fourfold (smaller, high packing-density) Si-O rings. The associated loading and unloading axial-stress versus specific-volume isotherms were next converted into the corresponding loading Hugoniot and unloading isentrope axial-stress versus specific-volume relations. These were subsequently used to analyze the role of the pressure-induced phase-transformation/irreversible-densification in mitigating the effects of blast and ballistic impact loading onto a prototypical glass plate used in monolithic and laminated transparent armor applications. The results of this part of the study revealed that pressure-induced phase-transformation can provide several beneficial effects such as lowering of the loading/unloading stress-rates and stresses, shock/release-wave dispersion, and energy absorption associated with the study of phase-transformation.     

Derivation,Parameterization and Validation of a Sandy-Clay Material Model for Use in Landmine Detonation Computational Analyses

A set of large-strain/high-deformation-rate/high-pressure material models for sand-based soils with different saturation levels and clay and gravel contents was recently proposed and validated in our study, and the same has been extended in this study to include clay-based soils of different saturation levels and sand contents. The model includes an equation of state which reveals the material response under hydrostatic pressure, a strength model which captures material elastic-plastic response under shear, and a failure model which defines the laws and conditions for the initiation and evolution of damage and ultimate failure of the material under negative pressure and/or shear. The model was first parameterized using various open-literature experimental results and property correlation analyses and, then, validated by comparing the computational results obtained in an ANSYS/Autodyn-based transient non-linear dynamics analysis of detonation of a landmine buried in sandy-clay with their experimental counterparts.     

Modeling of AA5083 Material-Microstructure Evolution During Butt Friction-Stir Welding

A concise yet a fairly comprehensive overview of the friction stir welding (FSW) process is provided. This is followed by a computational investigation in which FSW behavior of a prototypical solution-strengthened and strain-hardened aluminum alloy, AA5083-H131, is modeled using a fully coupled thermo-mechanical finite-element procedure developed in our prior study. Particular attention is given to proper modeling of the welding work-piece material behavior during the FSW process. Specifically, competition and interactions between plastic-deformation and dynamic-recrystallization processes are considered to properly account for the material-microstructure evolution in the weld nugget zone. The results showed that with proper modeling of the material behavior under high-temperature/severe-plastic-deformation conditions, significantly improved agreement can be attained between the computed and measured post-FSW residual-stress and material-strength distribution results.