Showing papers by "Rashid K. Abu Al-Rub published in 2008"

Constitutive Modeling and Simulation of Perforation of Targets by Projectiles

Abstract: DOI: 10.2514/1.26011 The effective use of existing finite element codes in the direct simulation of hypervelocity impacts by projectiles is limited by the dependence of the size of localized failure regions on the mesh size and alignment. This gives rise to a nonphysical description of the penetration and perforation processes. A micromechanical constitutive model that couples the anisotropic thermoviscodamage mechanism with the thermohypoelastoviscoplastic deformation is presented here as a remedy to this situation. Explicit and implicit microstructural length-scale measures, which preserve the well-posed nature of the differential equations, are introduced through the use of the viscosity and gradient localization limiters. Simple and robust numerical algorithms for the integration of the constitutive equations are also presented. The proposed unified integration algorithms are extensions of the classical rateindependent return-mapping algorithms to the rate-dependent problems. A simple and direct computational algorithmisalsousedforimplementingthegradient-dependentequations.Thisalgorithmcanbeimplementedinthe existing finite element codes without numerous modifications, compared with the current numerical approaches for integratinggradient-dependent models.Modelcapabilitiesarepreliminarilyillustrated forthedynamiclocalization of inelastic flow in adiabatic shear bands and the perforation of a 12-mm-thick Weldox 460E steel plate by a deformable blunt projectile at various impact speeds.

Thermodynamic framework for coupling of elasto-viscoplasticity and nonlocal anisotropic damage for microelectronics solder alloys

Abstract: The microstructure of soldered materials is known to have a strong influence on damage initiation and propagation and being localised. Moreover, it is well-established in the literature that the final failure of a solder joint is preceded by inhomogeneities in the deformation of the joint at relatively early stages, and that predicting the non-uniform micro-damage distribution during thermo-mechanical loading allows one to ultimately predict the failure location and time and then in turn improve the performance and reliability of microelectronic solder alloys. This study develops a general consistent and systematic framework for the analysis of microelectronic solder alloys that assesses a strong coupling between rate-dependent plasticity and rate-dependent damage within the framework of thermodynamic laws and nonlocal gradient-dependent theory. The model presented in this paper can be considered as a feasible thermodynamic approach for microelectronic solder alloys that enables one to derive various coupled thermo-viscoplasticity- viscodamage theories by introducing simplifying assumptions.

Multi-scale Constitutive and Computational Modeling of Hypervelocity Impact Damage of Steel Targets

Abstract: Understanding the constraints and limitations of various potential hull structure materials and armor is paramount in design considerations of future combat vehicles. Developing and applying theoretical and computational models that guide the development of design criteria and fabrication processes of high ballistic-resistant material are essential. The ultimate objective is to develop and field a contingency armor that is thin and lightweight, but with a very high level of overpressure protection system that provides low penetration depths. Therefore, performing multiscale computational modeling and simulation of the ballistic response of fighting vehicles made of high performance materials under hypervelocity projectile impact is invaluable. However, as soon as material failure dominates a deformation process, the material increasingly displays strain softening (localization) and the finite element computations are considerably affected by the mesh size and alignment and gives non-physical descriptions of the damaged regions and failure of solids. Therefore, the effective use of existing Finite Element Codes in the direct simulation of hypervelocity impacts by projectiles is limited by the dependence of the size of localized failure regions on the mesh size and alignment. As a remedy to this situation, a micromechanical constitutive model that couples the anisotropic thermo-viscodamage mechanism with the thermohypoelasto-viscoplastic deformation is formulated in [1–3] based on the non-local laws of the thermodynamics. Explicit and implicit microstructural length scale measures, which preserve the well-posed nature of the differential equations, are introduced through the use of the viscosity and gradient localization limiters. The enhanced nonlocal gradient-dependent theory formulates a constitutive framework on the continuum level that is used to bridge the gap between the micromechanical theories and the classical (local) continuum theories. They are successful in explaining the size effects encountered at the micron scale and in preserving the well-posedeness of the initial boundary value problem governing the solution of material instability triggering strain localization. Moreover, viscosity (rate dependency) allows the spatial difference operator in the governing equations to retain its hyperbolicity and the initial boundary value problem is well-posed. Model capabilities are preliminarily illustrated for the dynamic localization of inelastic flow in adiabatic shear bands and the perforation of Weldox 460E steel plates with by a deformable blunt projectile at various high impact speeds. Comparisons with the experimental results in [4] are performed. The micro-damage model used to predict material behavior under dynamic loading conditions was earlier presented in [1–3]. Thus, only the main equations will be given in the following. The model is based on the nonlocal gradient-dependent theory. It includes the von Mises yield criterion, the non-associated flow rules, isotropic and anisotropic strain hardening, strain rate hardening, softening due to adiabatic heating and anisotropic damage evolution, and finally a

Surface/Interfacial Effects on Size Dependent Strength of Micro/Nano Systems

Abstract: The emerging areas of microand nano-technologies exhibit important strength differences that result from continuous modification of the material microstructural characteristics with changing size, with smaller being stronger. There are many experimental observations which indicate that, under certain specific conditions, the size of micro/nano-systems significantly affect their strength such that a length scale is required for predicting such size effects when using the classical theories of continuum mechanics. For example, experimental works have shown increase in strength by decreasing: (a) the particle size in nano-composites; (b) the diameter of nano-wires in torsion and uniaxial compression; (c) the thickness of thin films in micro-bending and uniaxial tension; (d) the grain size of nano-crystalline materials; (e) void size in nano-porous media; (f) the indentation depth in micro/nano indentation tests, etc (see Abu Al-Rub and Voyiadjis [1, 2] for a complete list of references). Therefore, it is well-known by now through intensive experimental studies that have been performed at the micron and nano length scales that the material mechanical properties strongly depend on the size of specimen and the microstructural features. The classical continuum mechanics fails to address this problem since no material length scale exists in its constitutive description. On the other hand, nonlocal continuum theories of integral-type or gradient-type have been to a good extent successful in predicting this type of size effect. However, they fail to predict size effects when strain gradients are minimal such as in the Hall-Petch effect. This problem is the main focus of this work. The effect of the material microstructural interfaces increase as the surface-to-volume ratio increases. It is shown in this work that interfacial effects have a profound impact on the scale-dependent plasticity encountered in micro/nano-systems. This is achieved by developing a higher-order gradient-dependent plasticity theory that enforces microscopic boundary conditions at interfaces and free surfaces. These nonstandard boundary conditions relate the microtraction stress at the interface to the interfacial energy. Application of the proposed framework to size effects in shear loading of a thin-film on an elastic substrate is presented. Three film-interface conditions are modeled: soft, intermediate, and hard interfaces.