Fracture Mechanics of Metals: Some Features of Crack Initiation and Crack Propagation

Abstract

The main features are considered concerning the crack initiation and crack propagation events during the fracture toughness measurements at static and dynamic loading with respect to perfectly brittle, quasi-brittle and ductile behavior of material. The relationships are described between seven different stages of loading process and corresponding stages of the crack tip plastic zone evolution as well as the appearances of fracture surface of specimen. Quantitative stereofractographic analysis of the stretched zone is performed on the fracture surface of different steels. The effect of temperature, loading rate, stress state mode and specimen size on the stretched zone formation as well as on the crack initiation is studied. In particular, if plane-strain conditions are satisfied the stretched zone parameters are independent of the specimen size variation. A correlation is obtained of the stretched zone height and width with fracture toughness of materials. The stretched zone height being in better correlation with fracture toughness of the material than its width. In contrast to other methods of metals fracture toughness measurement, the stretched zone as a subject for investigation has a number of unique properties. The method of stereoscopic measurement of the stretched zone geometry by superimposing the mating profiles of fracture surfaces reveals the scattering of results comparable or even lower than a scattering at conventional fracture toughness measurements. The direction of initial crack propagation differs essentially from the main direction of crack growth, during testing of standard specimens for the fracture toughness measurement. This peculiarity of crack initiation is explained as initial crack propagation into one of two symmetrical plastic zone fans along the trajectories of maximal damage of material by the plastic deformations. One of the main conclusions was drawn: the fracture toughness dependence on temperature and loading rate can be quantitatively described within the framework of the interaction between thermally activated plastic flow inside of plastic zone and the athermal brittle fracture process involved the micro-crack nucleation at some characteristic distance ahead the crack tip. A plane strain transition temperature for a given specimen thickness is defined for ferritic steels as the temperature along the K Ic transition curve where the ASTM validity limit is reached. This temperature is linearly related to the specimen thickness logarithm under both static and dynamic loading. For a given steel, transition temperatures deduced from other tests were found to correspond to various characteristic values of the non-dimensional crack tip plastic zone size. The different relationships are described to deduce transition temperatures of various tests from the knowledge of either the K Ic transition curve or from two other transition temperatures. It could provide a method to decide when the stress relief is needed after welding plates of a given thickness. It should also find its use in studying irradiation effects