VALIDATION OF A STATISTICAL CRITERION FOR INTERGRANULAR BRITTLE-FRACTURE OF A LOW-ALLOY STEEL THROUGH UNIAXIAL AND BIAXIAL (TENSION-TORSION) TESTS

Criteria initially developed by Beremin [1-5] for brittle cleavage fra cture are applied to intergranular brittle fracture of a MnNiMo steel submitted to a temper embrittlement heat treatment. These models which are based on the maximum principal stress as a damage loading paramet er and on the Weibull statistics are firstly reviewed and discussed. I n particular the effect of plastic strain and of temperature on the in tergranular fracture stress is emphasized. Then the results of fractur e tests on smooth tensile specimens and on notched bars are used to id entify the parameters of the models. Finally these models are applied to multiaxial tension-torsion tests carried out on thin tubular specim ens. In these tests the observation of the orientation of the fracture surfaces clearly shows the importance of the maximum principal stress . These tests underline also the importance of one of the basic assump tions of the models which is the necessity of plastic deformation to i nitiate brittle fracture. In particular the existence in the tau-sigma plane of two domains for fracture, one controlled by the maximum prin cipal stress, the other by plastic yielding is discussed. The results of further tests in which the material was submitted to a predeformati on at room temperature before being tested at -196-degrees-C are also presented and used to discuss the effect of plastic deformation on int ergranular fracture. In all cases it is shown that the Beremin criteri a account very well for the scatter in the results and for the size ef fect observed in the experiments.