The distribution of strain about a running crack

Abstract

In semi-brittle fractures in heavy-section mild-steel structures the propagating crack is accompanied by a moving field of elastic-plastic deformation. This field (in particular the extent of the plastic deformation) determines the effective value of the surface energy, γ, an important variable in the modified Griffith criterion for brittle fracture. The nature of this field can vary with the conditions of testing (the temperature, stress state and crack velocity) for any particular material, and may be related to the properties of the material itself by means of dislocation dynamics. It is difficult to calculate this field theoretically, and in this paper an account is given of some experimental measurements of the distribution of strain around running cracks. A photoelastic coating is applied to the surface of a steel plate, and the strain-birefringence in this coating is measured by means of high-speed photography. The results are related to the appearance of the fracture surface, and to microsections taken from near the crack.