Materials Science and Engineering, A 176 (1994) 263-269 263 Molecular dynamics study of crack processes associated with dislocation nucleated at the tip Hiroshi Kitagawa, Akihiro Nakatani and Yoji Shibutani Department of Mechanical Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565 (Japan) Abstract Atomic scale changes of structure around a crack-tip in an f.c.c, crystal under in-plane shear (mode II) loading are analyzed by molecular dynamics simulation (MD simulation), and its counterpart in the continuum crystal plasticity model is discussed. A ductile fracture process involving dislocation nucleation from the stressed tip is always observed. The nucleated dislocation is driven away from the crack and a dislocation-free zone develops in the near-tip region. Time averages of local stress in the near-tip region, both before and after dislocation nucleation, coincide well with the linear elastic prediction. The critical stress intensity factor estimated by MD simulation agrees well with Rice's theoretical prediction derived from the unstable stacking energy concept. The temperature dependence of the critical factor can be explained primarily as a thermally activated process. 1. Introduction The fracture process around the crack tip is sensi- tive to the local atomic configuration and mechanical action on the crack due to the applied load. One of the elemental processes which appears in the atomic struc- ture around a crack, which is followed by ductile frac- ture of crystalline material, is dislocation nucleation at the tip and emission of the crack out of this. Several computer simulations based on molecular dynamics simulation (MD simulation) have been carried out to study such an atomistic process of cracking, and details of the atomic structure and the dynamics appearing around the crack tip have gradually been explained (see for example ref. 1 ). In a previous work [2], it was shown that in mode I cracking in an f.c.c.-crystal, crack tip deformation accompanied by dislocation activation is not necessary and is rather rare. A typical rare case is a crack front with [101] direction and surface on the (010) plane [2]. A ductile crack opening with tip blunting is recognized in this case, which has in good agreement with results of the continuum model obtained by finite element method (FEM) analysis based on a crystal plasticity theory [3]. However, ductile deformation with emis- sions of screw dislocations from the tip is observed in every case of mode III (anti-plane shear) loading [4], and we found an interesting counterpart in a theoreti- cal result of localized crack tip deformation in a contin- uum model given by Rice and Nikolic [4, 5]. In this paper, dynamic transition of the atomic struc- ture around the tip in an f.c.c, crystal under mode II (in- plane shear) loading is studied by MD simulation. The "N-body" potential proposed by Finnis and Sinclair [6] is used to describe the interatomic interaction. The critical condition of dislocation activation and its tem- perature dependence are discussed, and the interest- ing correspondence concerning microdeformation mechanisms in the near-tip field predicted by FEM analysis is found in an atomic scale structural re- arrangement involving dislocation activation at the crack tip. 2. Atomic model for molecular dynamics simulation A crack is assumed to be in an f.c.c, single crystal and is subjected to mode II loading as shown in Fig. 1. The atomic model for MD simulation has size 20~f2~a~ x 20/,[2ao in the x-y plane (a0 is the lattice constant) as seen in Fig. 2, which is regarded as two layers out of the periodic structure in the z-direction. The material is supposed to be copper single crystal, the interatomic interaction of which is described by the "N-body" potential proposed by Finnis and Sinclair [6], together with the material properties presented by parameters given by Ackland et al. [7]. Three models are introduced for analysis. In each model the initial crack has its front and its plane as follows: model 1, in [110] on (001); model 2, [ 110]( ] 11 ); model 3, [ 110]( ] 10). The crack is idealized as a vacancy sheet, the details of which are different for each model as shown in Fig. 3. The symbols D and F are used to indicate the type of boundary condition. 0921-5093/94/$7.00 © 1994 - Elsevier Sequoia. All rights reserved ,%'DI 0921-5093(93)02520-D