An Investigation of static COD and dynamic COD

Abstract

It is generally accepted that KIC criterion is usefull for initiation of brittle fracture, but this method has a weak point that the linear elastic fracture mechanics approach must be applied only when a yield zone arround a crack tip is very small compared to crack length. Brittle fracture of a structure after large scale yielding, however, is observed sometimes at comparative high temperature.As an approach to this phenomenon, the crack opening displacement (COD) concept available in large scale yielding was proposed by A. A. Wells about ten years ago. After that, many papers about mathematical theory of COD concept or engineering criterion of fracture have been published. Nevertheless some problems to be solved have been left still. One of them is how to decide the critical COD for brittle fracture at comparative high temperature, since at such temperature a ductile crack mostly propergates before initiation of cleavage fracture.For this reason, in this paper the initiation of ductile crack was investigated precisely using single-notched and double-notched specimens of 60 kg/mm2 high strength steel in static and dynamic pure bending COD test. As a result, it was made clear that the critical COD for the initiation of ductile crack is independent of temperature and strain rate. Moreover, an influence of notch distance of double-notched specimen on critical COD and distorsion behavior of specimen was studied experimentally and numerically, since double-notched specimen have been used sometimes in COD test. And the most suitable notch distance was obtained, of which the double-notched specimen shows small interaction of notches and distorsion behavior close to single-notched specimen.