A Proposed Mechanism of Brittle Fracture Initiation influenced by Overstressing Techniques in Terms of Dugdale Model

Abstract

Features of stress and displacement around a crack are discussed in terms of Dugdale model for on-load, off-load and re-load conditions. The size of compressive yield zone ahead of the crack tip introduced by on-and-off loading cycle plays a role of threshold, within which the size of tensile yield zone formed by the subsequent loading is confined unless the re-load reaches the pre-load level. When the re-load reaches the pre-load level, the size of tensile yield zone coincides with that given by the first on-load. Further magnitude of re-load provides the same size of tensile yield zone as that given by the direct loading. Reduction in risk of brittle fracture by means of pre-loading is well interpreted if the size of tensile yield zone is regarded as the fracture criterion. On this basis, fracture stress of notched specimen or welded structure bearing a crack can easily be predicted as a function of temperature and pre-loading condition. Some experimental confirmation is supplemented. A good agreement of the experimental with the theoretical is shown. Practical significance of COD measurement is nevertheless reserved.