In situ fracture experiments in b.c.c. metals

Abstract

Crack propagation and the structure of the plastic zone formed ahead of a crack have been investigated during in situ electron microscope fracture experiments of the b.c.c. metals molybdenum and tungsten. Most frequently, the cracks in polycrystalline specimens propagated along the grain boundaries. In some cases, the cracks were preceded by thin twins. When the cracks propagated transgranularly, the plastic zone ahead of the cracks consisted of an inverse pile-up of dislocations. In these cases, the relationship between the plastic zone size and the crack tip displacement was compared with the predictions of the theory of Bilby, Cottrell and Swinden (BCS). It is concluded that the plastic zone size agrees with the predictions of the BCS theory if the theory is modified to include the surface effects in thin foils.