Measurement of Fracture Toughness of Materials With Non-Uniform Microstructure Based on Spiral Notch Torsion Test

Materials such as welds consist of highly non-uniform distribution of microstructure. It is typically difficult to measure fracture toughness of those materials using the conventional testing approaches such as ASTM compact tension test, since the non-uniform microstructure can induce irregular crack propagation front. In this study, the spiral notch torsion test (SNTT) is enhanced to measure the fracture toughness of welds. The test specimen resembles a round tensile bar with a spiral-shaped notch on the circumference. To introduce nonuniform microstructure, the heat treatment with a Gleeble system is used to rapidly heat and quench the specimen. The heating and cooling rates applied are typical of those experienced in welding. The hardness distribution in the axial direction of as-heat-treated specimen has a bell shape centered in the middle of the specimen. This distribution resembles that in a transverse section of a weldment. During loading, the specimen is twisted about its axis. It is possible to achieve a Mode I crack front that is long enough to meet the plain strain state and other fracture mechanics requirements for testing the local toughness of weld region. Fracture toughness calculation based on finite element analysis is performed to convert the recorded load-displacement into the fracture toughness K_Ic.