A SIMPLIFIED METHOD FOR ESTIMATION OF COD AND PREDICTION OF BRITTLE FRACTURE INITIATION

Crack Opening Displacement (COD) concept, an engineering criterion for the initiation of brittle fracture in low carbon steel, has proved itself so effective that proposals have been made on a standard test procedure for determining the critical COD delta sub c as a fracture toughness value, and on the basis of the aforementioned concept defect evaluation standards have been established. However, several matters have to be settled. That the relation between the applied stress and COD can be evaluated correctly only in an ideal state is among them. For this reason, difficulties are often encountered when an assessment of degree of safety against brittle fracture of structural members is attempted on the basis of the COD concept. In this paper, the author proposes a simplified method for evaluating the relationship between applied stress and COD under a unified process as a breakthrough for this problem. By the use of the proposed method, estimation of COD under complex conditions, analytical approach to which has been unavailable so far, and in reverse, evaluation of fracture strength from delta sub c value are attempted whereby the adequacy of the proposed method is discussed. Additional discussions are made on the relationship between the COD obtained by the proposed method and the overall strain.