The use of small punch (Bulge) tests to estimate fracture stress in the lower shelf regime

The small punch fracture stress σf(SP) and fracture toughness KIc of embrittled turbine rotor steels have been evaluated at temperatures corresponding to the lower shelf regime. The lower bound values of σf(SP), determined by statistically analyzing the data, are found to be relatively independent of test temperature range from −196 to −170°C for rotor M steel and from −196 to −150°C for rotor T steel. The relationship of σf(SP) to fractographic features was also investigated. The lower bound values of σf(SP) were found to be comparable with values of the microscopic fracture stress σ*f. This result indicates that small punch tests might be used successfully to measure σ*f of materials at lower shelf temperatures.