The effect of plain-weaving on the mechanical properties of warp and weft p-phenylene terephthalamide (PPTA) fibers/yarns

Coarse-grained molecular statics/dynamics methods are first used to investigate degradation in the PPTA fiber/yarn tensile strength, as a result of the prior compressive or tensile loading. PPTA fibers/yarns experience this type of loading in the course of a plain-weaving process, the process which is used in the fabrication of ballistic fabric and flexible armor. The more common all-atom molecular simulations were not used to assess strength degradation for two reasons: (a) the size of the associated computational domain rendering reasonable run-times would be too small and (b) modeling of the mechanical response of multi-fibril PPTA fibers could not be carried out (again due to the limited size of the computational domain). However, all-atom simulations were used to (a) define the coarse-grained particles (referred to as “beads”) and (b) parameterize various components of the bead/bead force-field functions. In the second portion of the work, a simplified finite-element analysis of the plain-weaving process is carried out in order to assess the extent of tensile-strength degradation in warp and weft yarns during the weaving process. In this analysis, a new material model is used for the PPTA fibers/yarns. Specifically, PPTA is considered to be a linearly elastic, transversely isotropic material with degradable longitudinal-tensile strength and the longitudinal Young’s modulus. Equations governing damage and strength/stiffness degradation in this material model are derived and parameterized using the coarse-grained simulation results. Lastly, the finite-element results are compared with their experimental counterparts, yielding a decent agreement.     

Computational Investigation of Hardness Evolution During Friction-Stir Welding of AA5083 and AA2139 Aluminum Alloys

A fully coupled thermo-mechanical finite-element analysis of the friction-stir welding (FSW) process developed in our previous work is combined with the basic physical metallurgy of two wrought aluminum alloys to predict/assess their FSW behaviors. The two alloys selected are AA5083 (a solid-solution strengthened and strain-hardened/stabilized Al-Mg-Mn alloy) and AA2139 (a precipitation hardened quaternary Al-Cu-Mg-Ag alloy). Both of these alloys are currently being used in military-vehicle hull structural and armor systems. In the case of non-age-hardenable AA5083, the dominant microstructure-evolution processes taking place during FSW are extensive plastic deformation and dynamic re-crystallization of highly deformed material subjected to elevated temperatures approaching the melting temperature. In the case of AA2139, in addition to plastic deformation and dynamic recrystallization, precipitates coarsening, over-aging, dissolution, and re-precipitation had to be also considered. Limited data available in the open literature pertaining to the kinetics of the aforementioned microstructure-evolution processes are used to predict variation in the material hardness throughout the various FSW zones of the two alloys. The computed results are found to be in reasonably good agreement with their experimental counterparts.     

Transformation toughening in the γ-TiAl–β-Ti–Vsystem: Part II A molecular dynamics study

Molecular dynamics simulations of the evolution of materials in a region surrounding a crack tip were carried out for the case of a crack in a γ-TiAl phase impinging at a right angle onto the interface between a γ-TiAl phase and a metastable Ti–15V (at %) phase. The corresponding linear anisotropic solutions for the singular stress and displacement fields were used to both generate the crack in the original crystal and to prescribe the boundary conditions applied to the computational crystal during the molecular dynamics simulation runs. The atomic interactions were accounted for using appropriated embedded atom method (EAM) type interatomic potentials. The crack-tip behaviour for the two-phase γ–β material was ultimately compared with the one in the corresponding single-phase material, i.e. to the one in pure γ and the one in pure β crystals. The simulation results showed that under the same applied level of external stress, the crack tip became blunt and the crack stopped propagating in the γ-TiAl–β-Ti–15V bicrystal and in the single β-phase crystal while the crack extended by brittle cleavage in the single-phase γ crystal. The blunting process was found to be controlled by the martensitic transformation that took place in the β-phase ahead of the crack tip. Depending on the local stress conditions the crystal structure of martensite was found to be either hexagonal close packed (h.c.p.), body centred orthorhombic (b.c.o.) and/or face centred orthorhombic (f.c.o.). Finally the implications of crack tip martensitic transformation on the toughness of the materials are analysed in quantitative terms using the concept of Eshelby's conservation integral, i.e. the energy release rate. This revised version was published online in November 2006 with corrections to the Cover Date.     

Coarsening resistance of M2C carbides in secondary hardening steels: Part I. Theoretical model for multicomponent coarsening kinetics

The development of very high-strength levels in many alloy steels is achieved by a secondary hardening reaction. In high Co-Ni steels containing the strong carbide-forming elements Mo, Cr, and W, secondary hardening is accomplished by the precipitation of fine-scale M₂C alloy carbides. Coarsening resistance of the M₂C precipitates depends on the alloy content of these elements, and there should be an addition to the alloy of these carbide-forming elements which optimizes the M₂C coarsening resistance. Current Lifshitz-Slyozov-Wagner (LSW) theory^[2,3] cannot properly be used to describe, the coarsening behavior of multicomponent carbides, which involves concentrations and diffusivities of two or more solutes and nonspherical carbide morphologies. A model is introduced for the coarsening resistance of multicomponent carbides. This model treats the coarsening of shape-preserving particle and is applicable to rodlike particles.     

Crystal plasticity analysis of earing in deep-drawn OFHC copper cups

The theory of thermally-activated slip is used to derive a crystal-plasticity materials constitutive model for deformation of OFHC copper single crystals. The mechanical response of the polycrystalline material is next determined from the single-crystalline materials constitutive relations using the classical Taylor approximation for apportionment of the deformation gradient between grains. Simulations of the deep drawing of cylindrical cups from as-rolled OFHC-copper blanks are next carried out using an explicit finite element formulation. The results obtained show that the crystallographic texture in as-rolled sheets, which can be accounted for through the use of crystal-plasticity, gives rise to rim-earing in fully-drawn cups. It is further shown that the extent of rim-earing can be greatly reduced by properly modifying the shape of the blank. A procedure is next proposed for optimization of the blank shape.     

Electrochemical and AFM study of nickel nucleation mechanisms on vitreous carbon from ammonium sulfate solutions

Reaction and nucleation mechanisms of nickel in ammoniacal solutions have been investigated as a function of nickel concentration, solution pH, deposition potential, temperature and conditioning potential. Electrochemical mechanisms of nickel reduction were found to be pH dependent, while their kinetics was concentration dependent. A surface film formed by anodic oxidation passivates nickel clusters preventing their further oxidation. Nickel nucleation on vitreous carbon, which proceeds according to the progressive nucleation model, shows a large degree of inhibition at both pH 6 and pH 9. Cluster sizes were larger when electrodeposition was carried out from solutions with higher nickel concentrations. The clusters were also larger at more negative deposition potentials and at higher solution pH. Cluster population density increased with the increasing solution temperature. Different activation energies for the nickel-aquo and nickel-ammino complexes calculated from Arrhenius diagram indicate that electroreduction of nickel-ammino complex is energetically more demanding. All electrochemical results were further verified by the atomic force microscopy investigations.     

Ballistic-Failure Mechanisms in Gas Metal Arc Welds of Mil A46100 Armor-Grade Steel: A Computational Investigation

In our recent work, a multi-physics computational model for the conventional gas metal arc welding (GMAW) joining process was introduced. The model is of a modular type and comprises five modules, each designed to handle a specific aspect of the GMAW process, i.e.: (i) electro-dynamics of the welding-gun; (ii) radiation-/convection-controlled heat transfer from the electric-arc to the workpiece and mass transfer from the filler-metal consumable electrode to the weld; (iii) prediction of the temporal evolution and the spatial distribution of thermal and mechanical fields within the weld region during the GMAW joining process; (iv) the resulting temporal evolution and spatial distribution of the material microstructure throughout the weld region; and (v) spatial distribution of the as-welded material mechanical properties. In the present work, the GMAW process model 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. The model is upgraded through the introduction of the sixth module in the present work in recognition of the fact that in thick steel GMAW weldments, the overall ballistic performance of the armor may become controlled by the (often inferior) ballistic limits of its weld (fusion and heat-affected) zones. To demonstrate the utility of the upgraded GMAW process model, it is next applied to the case of butt-welding of a prototypical high-hardness armor-grade martensitic steel, MIL A46100. The model predictions concerning the spatial distribution of the material microstructure and ballistic-limit-controlling mechanical properties within the MIL A46100 butt-weld are found to be consistent with prior observations and general expectations.     

Material-Model-Based Determination of the Shock-Hugoniot Relations in Nanosegregated Polyurea

Previous experimental investigations reported in the open literature have indicated that applying polyurea external coatings and/or internal linings can substantially improve ballistic penetration resistance and blast survivability of buildings, vehicles, and laboratory/field test-plates, as well as the blast-mitigation capacity of combat helmets. The protective role of polyurea coatings/linings has been linked to polyurea microstructure, which consists of discrete hard-domains distributed randomly within a compliant/soft matrix. When this protective role is investigated computationally, the availability of reliable, high-fidelity constitutive models for polyurea is vitally important. In the present work, a comprehensive overview and a critical assessment of a polyurea material constitutive model, recently proposed by Shim and Mohr (Int J Plast 27:868-886, 2011), are carried out. The review revealed that this model can accurately account for the experimentally measured uniaxial-stress versus strain data obtained under monotonic and multistep compressive loading/unloading conditions, as well as under stress relaxation conditions. On the other hand, by combining analytical and finite-element procedures with the material model in order to define the basic shock-Hugoniot relations for this material, it was found that the computed shock-Hugoniot relations differ significantly from their experimental counterparts. Potential reasons for the disagreement between the computed and experimental shock-Hugoniot relations are identified.     

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.