LEDGE EFFECTS ON DISLOCATION EMISSION FROM A CRACK-TIP - A FIRST-PRINCIPLES STUDY FOR SILICON

Extensive recent efforts to understand the intrinsic brittleness or du ctility of materials have focused on crack-blunting mechanisms using t he Peierls stress concept. So far, the effects due to newly created le dge surface during crack propagation have been either ignored or inclu ded within some semiempirical approximation. Using silicon as a protot ypical brittle solid, we show that the energy associated with a newly created ledge surface at a crack tip can be obtained from first-princi ples calculations. We incorporate these results into a continuum theor y of critical loading of cracks by assuming an evanescent force law fo r the surface effects. Our results indicate that inclusion of surface ledge effects can change the values of the critical load by up to 20%.