Issue 38

R. Pezer et alii, Frattura ed Integrità Strutturale, 38 (2016) 191-197; DOI: 10.3221/IGF-ESIS.38.26 192 problem is not only due to the immense complexity of the dislocation dynamics coming from atomic degrees of freedom but also because it is governed by phenomena at multiple length and time scales. In last two decades we witness substantial improvement of computing power and accompanied development of the sophisticated physical models [2,3] providing us with possibility to complement information from simple tensile test with numerical simulation. This fact calls for computational experiments that give us direct control over the whole range of scales involved so that we can avoid difficulties like short length ultraviolet divergences in standard crack dynamics laws. Crystals are among the most prominent examples of the decisive role of the sub micrometer scales governing the dislocation dynamics. Another prominent example is crack propagation accompanied with crystals surface production energy ("Fracture Energy") that is orientation dependent so that crystal response can be highly anisotropic even in a peace of material that is completely isotropic on the macroscale. At the atomic level in metal systems, interactions that capture complexity that accounts for fatigue and fracture must include terms beyond pairwise interactions. Such realistic potentials exist [4, 5] and are constantly improved to provide us with detailed information about non-equilibrium dynamics including crystal structure defects like vacancies and dislocations. The task to develop successful dislocation theory to model plastic phenomena in metal materials proved to be very difficult one. Indeed, a great deal of theoretical work has been expended in the past 5 decades in attempts to explain the phenomena of metal plasticity by means of dislocation theory yet no comprehensive theory has been achieved. There has been substantial progress in modeling nucleation and growth of fatigue cracks under multiaxial stresses like Fatemi-Socie [6] parameter applied to steel specimens. However, at the fundamental atomic scale it turns out that the full resolved stresses play also very important role [7, 8]. Here we utilize MD simulations to help elucidate how the principal and resolved stress components on the primary slip plane(s) impact dislocation nucleation in FCC Cu and Al perfect crystals at room temperature. In contrast to prior studies of those systems here we perform both multiaxial and fatigue tests where we quantitatively examine sensitive dislocations effects that govern materials response under the plastic deformation. We are focused mostly on the yielding deformation zone where dislocation nucleation starts at the very high rate. In this sense, Cu and Al (apart from applicative interest in itself) are interesting FCC candidates. Those metals share crystal structure but behave very differently: soft Cu contains almost no discernible straight part while Al experience very graduate transition from straight to the curved zone of the stress-strain diagram. In short the goal of this work is to examine correlation of the loading axis orientation, stress components resolved onto the {1 1 1} primary slip plane and dislocation density dynamics. Here we have found that dynamic properties in FCC single crystals critically depend on the magnitude and loading axis orientation. For the basic response, we have deformed Cu and Al single crystal according to simple loading path including cyclic fatigue harmonic tensile deformation. In order to further study complex patterns of fracture effects in crystal sample we prepare whole range of different loading-paths. This way we are able to specifically probe anisotropic response in the sample subject to different loading directions and look for the signatures of the multiaxial stress states at the atomic scale. In order to show distinct features of stress-strain relationship for several loading paths first we have simulated evolution by deforming entire model system at the usual ps time scale isothermally at 300 K. The strain rate is several orders higher than we usually see in LAB or industry setting but is due to the intrinsic limit of the MD time propagation of the atomistic system dynamics. However, since we are following trajectories at individual atoms level the time scales separation is part of the complex process of information transport to meso and macroscale. What is the real information at the atomic level that survives and govern tensile test in the experiment is one of the important questions in solving mechanical properties puzzle. SIMULATIONMODELS ANDMETHODS Embedded Atom Method he semi-empirical embedded-atom method (EAM) potentials are energy functions and govern the interaction among the neighboring atoms. It is commonly used approach for metals because it captures main features of the metallic bonding. The potential proposed by M. S. Daw and M. I. Baskes [9, 10], was based on the quantum mechanical density functional theory. They combined theoretical considerations with a fitting of parameters to the main properties of the bulk crystal. Their approach leads to the following expression for the total potential energy of a crystal:     ij i h i i j i E V r F tot , 1 2       (1) T