J. S. Snipes

Molecular-level computational investigation of shock-wave mitigation capability of polyurea

Various static and (equilibrium and non-equilibrium) dynamic molecular-level computational methods and tools are utilized in order to investigate the basic shock-wave physics and shock-wave material interactions in polyurea (a nano-phase segregated elastomeric co-polymer). The main goal of this investigation was to establish relationships between the nano-segregated polyurea microstructure (consisting of rod-shaped, discrete, so-called “hard domains” embedded into a highly compliant, so-called soft matrix) and the experimentally established superior capability of this material to disperse and attenuate resident shock waves (e.g., those generated as a result of blast-wave impact). By analyzing molecular-level interactions of the shock waves with polyurea, an attempt was made to identify and quantify main phenomena and viscous/inelastic deformation and microstructure-altering processes taking place at the shock front, which are most likely responsible for the superior shock-mitigation behavior of polyurea. Direct molecular-level simulations of shock-wave generation and propagation in the “strong-shock” regime are utilized in order to construct the appropriate shock-Hugoniot relations (relations which are used in the construction of the associated continuum-level material models). Extension of these relations into the “weak-shock” regime of interest from the traumatic brain injury prevention point of view is also discussed.

Multi-Length Scale-Enriched Continuum-Level Material Model for Kevlar®-Fiber-Reinforced Polymer-Matrix Composites

Fiber-reinforced polymer matrix composite materials display quite complex deformation and failure behavior under ballistic/blast impact loading conditions. This complexity is generally attributed to a number of factors such as (a) hierarchical/multi-length scale architecture of the material microstructure; (b) nonlinear, rate-dependent and often pressure-sensitive mechanical response; and (c) the interplay of various intrinsic phenomena and processes such as fiber twisting, interfiber friction/sliding, etc. Material models currently employed in the computational engineering analyses of ballistic/blast impact protective structures made of this type of material do not generally include many of the aforementioned aspects of the material dynamic behavior. Consequently, discrepancies are often observed between computational predictions and their experimental counterparts. To address this problem, the results of an extensive set of molecular-level computational analyses regarding the role of various microstructural/morphological defects on the Kevlar® fiber mechanical properties are used to upgrade one of the existing continuum-level material models for fiber-reinforced composites. The results obtained show that the response of the material is significantly affected as a result of the incorporation of microstructural effects both under quasi-static simple mechanical testing condition and under dynamic ballistic-impact conditions.

Coarse-grained Molecular-level Analysis of Polyurea Properties and Shock-mitigation Potential

Several experimental investigations reported in the open literature clearly established that polyurea (PU), an elastic copolymer, has an unusually high ability to attenuate and disperse shock waves. This behavior of PU is normally attributed to its unique nanometer-scale two-phase microstructure consisting of (high glass-transition temperature, Tg) hydrogen-bonded discrete, hard domains dispersed within a (low Tg) contiguous soft matrix. However, details regarding the mechanism(s) responsible for the superior shock-wave mitigation capacity of PU are still elusive. In the present study, molecular-level computational methods and tools are used to help us identify and characterize these mechanism(s). Because the shock-wave front structure and propagation involve coordinated motion of a large number of atoms and nano-second to micro-second characteristic times, these phenomena cannot be readily analyzed using all-atom molecular-level modeling and simulation techniques. To overcome this problem, all-atom PU microstructure is coarse-grained by introducing larger particles (beads), which account for the collective degrees of freedom of the constituent atoms, the associated force-field functions determined and parameterized using all-atom computational results, and the resulting coarse-grained model analyzed using conventional molecular-level computational methods and tools. The results thus obtained revealed that a combination of different deformation mechanisms (primarily shock-induced ordering and crystallization of hard domains and coordinated shuffle-like lateral motion of the soft-matrix segments) is most likely responsible for the superior ability of PU to attenuate/disperse shock waves.

Modeling of ballistic-failure mechanisms in gas metal arc welds of mil a46100 armor-grade steel

Purpose – The purpose of this paper is to discuss the recently developed multi-physics computational model for the conventional Gas Metal Arc Welding (GMAW) joining process that has been upgraded with respect to its predictive capabilities regarding the spatial distribution of the mechanical properties controlling the ballistic limit (i.e. penetration resistance) of the weld. Design/methodology/approach – The original model consists of five modules, each dedicated to handling a specific aspect of the GMAW process, i.e.: electro-dynamics of the welding-gun; radiation-/convection-controlled heat transfer from the electric arc to the workpiece and mass transfer from the filler-metal consumable electrode to the weld; prediction of the temporal evolution and the spatial distribution of thermal and mechanical fields within the weld region during the GMAW joining process; the resulting temporal evolution and spatial distribution of the material microstructure throughout the weld region; and spatial distribution ...

Meso-scale Computational Investigation of Shock-Wave Attenuation by Trailing Release Wave in Different Grades of Polyurea

Over the past several years, considerable research efforts have been made toward investigating polyurea, a segmented thermoplastic elastomer, and particularly its shock-mitigation capacity, i.e., an ability to attenuate and disperse shock-waves. These research efforts have clearly established that the shock-mitigation capacity of polyurea is closely related to its chemistry, processing route, and the resulting microstructure. Polyurea typically possesses a nano-segregated microstructure consisting of (high glass transition temperature, Tg) hydrogen-bonded discrete hard domains and a (low Tg) contiguous soft matrix. While the effect of polyurea microstructure on its shock-mitigation capacity is well-established, it is not presently clear what microstructure-dependent phenomena and processes control its shock-mitigation capacity. To help identify these phenomena and processes, meso-scale simulations of the formation of nano-segregated microstructure and its interaction with a leading shock-wave and a trailing release-wave is analyzed in the present work. The results obtained revealed that shock-induced hard-domain densification makes an important contribution to the superior shock-mitigation capacity of polyurea, and that the extent of densification is a sensitive function of the polyurea soft-segment molecular weight. In particular, the ability of release-waves to capture and neutralize shock-waves has been found to depend strongly on the extent of shock-induced hard-domain densification and, thus, on the polyurea soft-segment molecular weight.

Linear Friction Welding Process Model for Carpenter Custom 465 Precipitation-Hardened Martensitic Stainless Steel

An Arbitrary Lagrangian-Eulerian finite-element analysis is combined with thermo-mechanical material constitutive models for Carpenter Custom 465 precipitation-hardened martensitic stainless steel to develop a linear friction welding (LFW) process model for this material. The main effort was directed toward developing reliable material constitutive models for Carpenter Custom 465 and toward improving functional relations and parameterization of the workpiece/workpiece contact-interaction models. The LFW process model is then used to predict thermo-mechanical response of Carpenter Custom 465 during LFW. Specifically, temporal evolutions and spatial distribution of temperature within, and expulsion of the workpiece material from, the weld region are examined as a function of the basic LFW process parameters, i.e., (a) contact-pressure history, (b) reciprocation frequency, and (c) reciprocation amplitude. Examination of the results obtained clearly revealed the presence of three zones within the weld, i.e., (a) Contact-interface region, (b) Thermo-mechanically affected zone, and (c) heat-affected zone. While there are no publicly available reports related to Carpenter Custom 465 LFW behavior, to allow an experiment/computation comparison, these findings are consistent with the results of our ongoing companion experimental investigation.

Molecular-Level Study of the Effect of Prior Axial Compression/Torsion on the Axial-Tensile Strength of PPTA Fibers

A comprehensive all-atom molecular-level computational investigation is carried out in order to identify and quantify: (i) the effect of prior longitudinal-compressive or axial-torsional loading on the longitudinal-tensile behavior of p-phenylene terephthalamide (PPTA) fibrils/fibers; and (ii) the role various microstructural/topological defects play in affecting this behavior. Experimental and computational results available in the relevant open literature were utilized to construct various defects within the molecular-level model and to assign the concentration to these defects consistent with the values generally encountered under “prototypical” PPTA-polymer synthesis and fiber fabrication conditions. When quantifying the effect of the prior longitudinal-compressive/axial-torsional loading on the longitudinal-tensile behavior of PPTA fibrils, the stochastic nature of the size/potency of these defects was taken into account. The results obtained revealed that: (a) due to the stochastic nature of the defect type, concentration/number density and size/potency, the PPTA fibril/fiber longitudinal-tensile strength is a statistical quantity possessing a characteristic probability density function; (b) application of the prior axial compression or axial torsion to the PPTA imperfect single-crystalline fibrils degrades their longitudinal-tensile strength and only slightly modifies the associated probability density function; and (c) introduction of the fibril/fiber interfaces into the computational analyses showed that prior axial torsion can induce major changes in the material microstructure, causing significant reductions in the PPTA-fiber longitudinal-tensile strength and appreciable changes in the associated probability density function.

Multiphysics Modeling and Simulations of Mil A46100 Armor-Grade Martensitic Steel Gas Metal Arc Welding Process

A multiphysics computational model has been developed for the conventional Gas Metal Arc Welding (GMAW) joining process and used to analyze butt-welding of MIL A46100, a prototypical high-hardness armor martensitic steel. The model consists of five distinct modules, each covering a specific aspect of the GMAW process, i.e., (a) dynamics of welding-gun behavior; (b) heat transfer from the electric arc and mass transfer from the electrode to the weld; (c) development of thermal and mechanical fields during the GMAW process; (d) the associated evolution and spatial distribution of the material microstructure throughout the weld region; and (e) the final spatial distribution of the as-welded material properties. To make the newly developed GMAW process model applicable to MIL A46100, the basic physical-metallurgy concepts and principles for this material have to be investigated and properly accounted for/modeled. The newly developed GMAW process model enables establishment of the relationship between the GMAW process parameters (e.g., open circuit voltage, welding current, electrode diameter, electrode-tip/weld distance, filler-metal feed speed, and gun travel speed), workpiece material chemistry, and the spatial distribution of as-welded material microstructure and properties. The predictions of the present GMAW model pertaining to the spatial distribution of the material microstructure and properties within the MIL A46100 weld region are found to be consistent with general expectations and prior observations.

Computer-aided engineering analysis of tooth-bending fatigue-based failure in horizontal-axis wind-turbine gearboxes

Purpose – Wind energy is one of the most promising and the fastest growing alternative-energy production technologies, which have been developed in response to stricter environmental regulations, the depletion of fossil-fuel reserves, and the worlds ever-growing energy needs. This form of alternative energy is projected to provide 20 percent of the US energy needs by 2030. For economic reasons, wind turbines (articulated structures that convert wind energy into electrical energy) are expected to operate, with only regular maintenance, for at least 20 years. However, some key wind turbine components (especially the gearbox) tend to wear down, malfunction and fail in a significantly shorter time, often three to five years after installation, causing an increase in the wind-energy cost and in the cost of ownership of the wind turbine. Clearly, to overcome this problem, a significant increase in long-term gearbox reliability needs to be achieved. Design/methodology/approach – While purely empirical efforts a...

Molecular-Level Computational Investigation of Mechanical Transverse Behavior of p-Phenylene Terephthalamide (PPTA) Fibers

Purpose – A series of all-atom molecular-level computational analyses is carried out in order to investigate mechanical transverse (and longitudinal) elastic stiffness and strength of p-phenylene terephthalamide (PPTA) fibrils/fibers and the effect various microstructural/topological defects have on this behavior. The paper aims to discuss these issues. Design/methodology/approach – To construct various defects within the molecular-level model, the relevant open-literature experimental and computational results were utilized, while the concentration of defects was set to the values generally encountered under “prototypical” polymer synthesis and fiber fabrication conditions. Findings – The results obtained revealed: a stochastic character of the PPTA fibril/fiber strength properties; a high level of sensitivity of the PPTA fibril/fiber mechanical properties to the presence, number density, clustering and potency of defects; and a reasonably good agreement between the predicted and the measured mechanical ...