Development of Local Ductile Failure Assessment Method and Its Application to Four Alloys—Part I: Stress–Strain Relation and Analysis Without the Effect of Lode Angle

Abstract In evaluating the margin against fracture under various loadings, resistance against initiation and propagation of fracture needs to be evaluated. Elastic-plastic finite element (FE) analysis serves as a powerful tool for such assessment but prediction of plastic deformation and stress state in structures needs stress–strain relations to be implemented in the analysis. For avoiding the necessity of testing and processing the data each time, true stress–strain relations whose constants can be determined from fundamental properties such as yield or proof stress, tensile strength, rupture elongation, and reduction of area were developed based on the results of uniaxial tensile tests on many kinds of alloys used in nuclear power plants. Tensile tests were also conducted on notched round bar and grooved plate specimens made from four representative materials at various temperatures below the creep regime. Conventional large strain elastic-plastic analyses were performed on these specimens to elucidate the necessity of incorporation of the effect of damage on deformation in addition to a failure criterion. Then the calculation of damage based on equivalent plastic strain rate and stress triaxiality factor was introduced with an aim of predicting failure and the gradual decrease in deformation resistance. It was found that, although the predictions became more realistic, the behavior of some of the specimens could not be predicted with a sufficient accuracy when the effects of the stress triaxiality factor were calibrated based on the analyses of other types of specimens, suggesting the necessity of introducing another parameter which will be addressed in a subsequent paper.