Effect of stress-triaxiality on void growth in dynamic fracture of metals: a molecular dynamics study

The effect of stress-triaxiality on growth of a void in a three dimensional single-crystal face-centered-cubic (FCC) lattice has been studied. Molecular dynamics (MD) simulations using an embedded-atom (EAM) potential for copper have been performed at room temperature and using strain controlling with high strain rates ranging from 10^7/sec to 10^10/sec. Strain-rates of these magnitudes can be studied experimentally, e.g. using shock waves induced by laser ablation. Void growth has been simulated in three different conditions, namely uniaxial, biaxial, and triaxial expansion. The response of the system in the three cases have been compared in terms of the void growth rate, the detailed void shape evolution, and the stress-strain behavior including the development of plastic strain. Also macroscopic observables as plastic work and porosity have been computed from the atomistic level. The stress thresholds for void growth are found to be comparable with spall strength values determined by dynamic fracture experiments. The conventional macroscopic assumption that the mean plastic strain results from the growth of the void is validated. The evolution of the system in the uniaxial case is found to exhibit four different regimes: elastic expansion; plastic yielding, when the mean stress is nearly constant, but the stress-triaxiality increases rapidly together with exponential growth of the void; saturation of the stress-triaxiality; and finally the failure.Comment: 35 figures, which are small (and blurry) due to the space limitations; submitted (with original figures) to Physical Review B. Final versio

A New Model for Void Coalescence by Internal Necking

A micromechanical model for predicting the strain increment required to bring a damaged material element from the onset of void coalescence up to final fracture is developed based on simple kinematics arguments. This strain increment controls the unloading slope and the energy dissipated during the final step of material failure. Proper prediction of the final drop of the load carrying capacity is an important ingredient of any ductile fracture model, especially at high stress triaxiality. The model has been motivated and verified by comparison to a large set of finite element void cell calculations.

Numerical Simulations of Void Linkage in Model Materials using a Nonlocal Ductile Damage Approximation

Experiments on the growth and linkage of 10 μm diameter holes laser drilled in high precision patterns into Al-plates were modelled with finite elements. The simulations used geometries identical to those of the experiments and incorporated ductile damage by element removal under the control of a ductile damage indicator based on the micromechanical studies of Rice and Tracey. A regularization of the problem was achieved through an integral-type nonlocal model based on the smoothing of the rate of a damage indicator D over a characteristic length L. The simulation does not predict the experimentally observed damage acceleration either in the case where no damage is included or when only a local damage model is used. However, the full three-dimensional simulations based on the nonlocal damage methodology do predict both the failure path and the failure strain at void linkage for almost all configurations studied. For the cases considered the critical parameter controlling the local deformations at void linkage was found to be the ratio between hole diameter and hole spacing

Approximate Models for Ductile Metals Containing Non-spherical Voids—Case of Axisymmetric Prolate Ellipsoidal Cavities,"

The aim of this paper is to extend the classical I Introduction Metals often contain microvoids which significantly alter their mechanical behavior. The growth of such microvoids was studied in the pioneering works of The choice of axisymmetric ellipsoids can serve to model a wide range of void shapes, ranging from penny-shaped cracks to needles. Such a choice was made in several recent papers. In a recent paper (Gologanu et al., 1993), we proposed a constitutive model for ideal-plastic materials containing aligned or randomly oriented prolate ellipsoidal cavities. This model was obtained by extending The aim of the present work is to extend this analysis to the case of oblate cavities. Two fresh difficulties will be encountered. The first one is that while in the prolate case, both of Gurson's classical models for spherical and cylindrical voids could be used as references, only the first one is relevant to the oblate case. The second difficulty arises when one looks for an approximation of the two-field criterion leading to an analytic, Gurson-like expression. While in the prolate case it was possible to neglect the "crossed term" proportional to the product of the two velocity fields in the expression of the plastic dissipation without significantly altering the accuracy of the estimate, the same approximation reveals impossible for oblate voids, as will be detailed below. As in the prolate case, we shall consider an ellipsoidal cavity embedded in a finite volume (this allows to study arbitrarily great porosities), namely, a confocal ellipsoid. Such a geometry is primarily chosen because of mathematical tractability, but it is not a completely unreasonable approximation of an elementary cell in a periodic medium, just as Gurson's hollow sphere. Attention will be restricted to ideal-plastic behavior and axisymmetric loadings. Conditions of homogeneous boundary strain rate (v = D.x, where v denotes the velocity, D the overall strain rate and x the current position) will be enforced on the outer surface. The reason for imposing a homogeneous strain rate rather than a homogeneous stress is that the space of admissible velocity fields is smaller in the former case, so that the search for a