Effects of crack tip geometry on dislocation emission and cleavage: A possible path to enhanced ductility

Abstract

We present a systematic study of the effect of crack blunting on subsequent crack propagation and dislocation emission. We show that the stress intensity factor required to propagate the crack is increased as the crack is blunted by up to thirteen atomic layers, but only by a relatively modest amount for a crack with a sharp 60 • corner. The effect of the blunting is far less than would be expected from a smoothly blunted crack; the sharp corners preserve the stress concentration, reducing the effect of the blunting. However, for some material parameters blunting changes the preferred deformation mode from brittle cleavage to dislocation emission. In such materials, the absorption of preexisting dislocations by the crack tip can cause the crack tip to be locally arrested, causing a significant increase in the microscopic toughness of the crack tip. Continuum plasticity models have shown that even a moderate increase in the microscopic toughness can lead to an increase in the macroscopic fracture toughness of the material by several orders of magnitude. We thus propose an atomic-scale mechanism at the crack tip, that ultimately may lead to a high fracture toughness in some materials where a sharp crack would seem to be able to propagate in a brittle manner.When the crack is loaded in mode II, the load required to emit a dislocation is affected to a much higher degree by the blunting, in agreement with the estimates from continuum elasticity. In mode II the emission process is aided by a reduction of the free surface area during the emission process. This leads to emission at crack loadings which are lower than predicted from the continuum analysis of Rice.

Experimental investigation on ductile stable crack growth emanating from wire-cut notch in AISI 4340 steel

“…In most cases sharp crack has been considered to begin with. Schiøtz et al (1996Schiøtz et al ( , 1997 and Fischer and Beltz (2001) have reported the effects of crack tip radius on the brittle and ductile crack growth and extension. Recently, Hampton and Nelson (2003) have characterized the SCG through thin sheets in terms of CTOA and the results have been applied to predict the crack tearing and instability in pressurized cylinder of the same thickness.…”

“…They have used the elasticity solutions to solve for the thresholds to emit a fully formed dislocation on a slip plane intersecting the crack tip. The reduction of sharpness at the crack tip by providing a radius or blunting of crack tip increases the force required to propagate it (Schiøtz et al, 1996(Schiøtz et al, , 1997. Some conditions have been stipulated to predict whether a sharp crack tip will blunt leading to a ductile fracture, or continue to cleave in a brittle fashion (Fischer and Beltz, 1997).…”

Ab initiostudy of tension-shear coupling in NiAl

“…The dislocations shield the stress field of the crack and may prevent further crack propagation. 2,3 The competing processes are commonly characterized in terms of G c , the critical release energy rate for cleavage, and in terms of G d , the critical energy release rate for dislocation emission: the process with the lower release energy rate is supposed to dominate. While cleavage at an atomistic level can be conveniently described within Griffith theory in combination with ab initio calculations, 4-7 the modeling of the dislocation emission is much more complex.…”

Coarse-graining description of solid systems at nonzero temperature

“…Fully atomistic treatments by means of molecular dynamics ͑MD͒ and Monte Carlo ͑MC͒ have been applied to isolated defects in solids since the late 1960s. [1][2][3][4][5] The fairly recent advent of parallel computing, to which molecular simulation is ideally suited, has permitted multi-million-atom MD studies of the behavior of defective crystals. In particular, investigations by researchers at the Louisiana State University [6][7][8] and at IBM 9,10 have revealed valuable insight into atomic mechanisms of microcracking in silicon.…”