13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- between two grains are defined. The tilt component produces only a deviation of the crack front. The twist component produces a segmentation of the crack front and the formation of ridges, as observed in Fig. 6a and as schematically depictured in Fig. 6b. Complete crossing of the grain boundary by the cleavage crack in grain 1 necessitates in these conditions that the intergranular “triangles” are also broken. The = global? cleavage stress is thus also dependent on the local composition of the grain boundaries. Figure 5. Notations for tilt and twist grain boundaries. Figure 6. a) SEM observations showing the formation of bifurcations when a cleavage crack crosses a grain boundary. Four ridges are formed. The values of the tilt and twist angles determined by quantitative stereoscopy are given; b) Sketch. A model was developed to analyze this phenomenon of crossing of twisted grain boundary cleavage cracks. It is out of the scope of the present paper to describe in detail this model which is schematically shown in Fig. 7. The grain boundary crossing process includes four steps. The cleavage crack in grain 1 is approaching the grain boundary in step 1. Cleavage microcracks are assumed to nucleate on (001) planes in grain 2 (step 2). These microcracks propagate in the second grain and break the “triangles” remaining along the grain boundary and which belong to the grain boundary itself (step 3). This generates a perturbed crack front which is still attached to the grain boundary by remaining ligaments. In step 4 the perturbed crack front propagates and leads to final fracture. The model is based on an energy balance between the driving energy corresponding to the strain energy release rate of the perturbed crack and the dissipated energy. This energy includes a) b)
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