The propagation of fracture in mild steel

The isochromatic strain patterns on the surfaces of mild steel plates containing moving brittle fractures have been determined by using strain birefringent coating and high speed photographic techniques. The major part of these isochromatic patterns may be described by a formula for the isochromatic pattern near a stationary crack in an elastic medium. The variations from this formula are caused by, shock waves in the specimen, inertia effects due to the high velocity of the crack, and yielding near the crack tip. The inertia effects cause a region of biaxial tension ahead of the crack on the plate surface. A. crack model which predicts the extent of the plastic zone near the crack tip has been proposed. The predictions from this model and other theories are compared with experimental estimates of the plastic zone depth. The results of a microstructural examination of fracture surfaces supported Tipper's (1957) explanation of the mechanism for crack propagation.