Gérard Michot

Three dimensional analysis of the interaction between a crack and a dislocation loop

Abstract In this paper is calculated the stress intensity factor induced by a dislocation loop emitted at a crack tip in a three dimensional system. The numerical calculations, performed at experimentally studied configurations, indicate that a growing dislocation loop induces a shielding effect and that this effect is imposed by the loop portion closest to the emission point. On the other hand, sections far from the crack tip have an anti-shielding effect. This conclusion is contrary to the one derived from a two-dimensional approach. It must be pointed out that a dislocation loop with a Burgers vector parallel to the crack plane can induce a mode I shielding. Examples are presented for both the stress intensity factors distribution along the crack tip and the evolution of their maximum and average values with the loop radius.

A model of dislocation multiplication at a crack tip: influence on the brittle to ductile transition

Abstract It has been shown long ago experimentally that primary nucleation of dislocations in silicon takes place heterogeneously on defects along the crack tip. More recently, it has been observed that a source is easily activated at the intersection point of the crack front and of an attracted dislocation. The authors offer and discuss a source multiplication mechanism based on this stimulated emission process. One of the dislocations emitted at a primary source on the plane of maximum resolved shear stress cross-slips to a plane where it is attracted by the crack: the intersection event gives rise to a secondary source. Because shielding is very low at this point, there occurs an emission of a new bundle of dislocations. The process then starts again giving rise to an ‘avalanche multiplication’ of dislocations which strongly shield the crack. Soft/sharp brittle to ductile transitions (BDT) observed in semi-brittle materials result from such a high shielding rate coupled with a low/high threshold stress intensity factor for the activation of the primary sources.

Dislocation emission at crack tips in silicon under mixed mode loading

Dislocations emitted at crack tips in silicon are characterized by X-ray topography. It is shown that a controlled mixed mode of loading can be achieved in double-cantilever-beam samples thanks to warping of the crack and that activated slip systems are different when a mode III component is superimposed on the usual mode I component.

Stress relaxation at crack tip: An alternative to blunting in configurations close to that of the two dimensional approach

Due to geometrical limitations of available slip systems by crystallography, it appears statistically that plastic activity is more likely to take place on planes which cut the crack front (jogging planes) than on planes which contain the crack front (blunting planes), as usually put forward by the 2D models. This paper emphasises the fact that in the absence of increase in the crack tip radius, an enhanced shielding can be achieved through a cross-slip mechanism.