Determination of J-initiation toughness using pre-cracked small punch test specimens

Abstract

Determination of fracture properties is important to assess the in-service degradation of nuclear structural materials subjected to thermal fluctuations and irradiation and to calculate the residual life of the component. The pre-cracked small punch test is an alternative method for the determination of fracture properties in case of limited availability of materials and is insufficient for conducting conventional standard tests. In this paper, pre-cracked small punch test (denoted as p-SPT) specimens are used to obtain the fracture J-initiation toughness of nuclear structural steel 20MnMoNi55 and T91.The size of the p-SPT specimen is 10 x 10 x 0.5 mm and wire EDM technique is used to generate through thickness crack profile from mid-point of one side to a point just passing the center of the specimen, producing a crack length to specimen width ratio (a/W) as 0.4. The tests are conducted to get load v/s displacement data. Elastic-plastic finite element analysis of the p-SPT specimen is carried out and numerically obtained load v/s displacement data are compared with the experimental results. This analysis also helped to compute J-integral near the crack tip as a function of load. FE analysis is then repeated using the micromechanical Gurson-Tvergaard-Needleman model to know the load at which crack is initiated. J-initiation is then calculated using J-integral v/s load data. Computed J-initiation using micromechanical GTN model has good matching with the value quoted in the literature, which shows the viability of the method. The methodology described in this paper has the potential to determine fracture initiation toughness of aged nuclear materials using pre-cracked small punch tests.