Optimization and Verification of Ultra-Miniature Specimen for Evaluating Creep Property of In-Service Component Material under Uniaxial Loading

Abstract

Abstract A practical procedure of creep life prediction has already been applied to an in-service boiler steam pipe in Japan. The procedure is based on a few creep tests by a small punch creep (SPC) specimen machined from a thin coupon sample, which is successfully taken from the outer surface of the pipe by an electric discharge sampling equipment. However, a coefficient converting the applied load into stress is required in the application of the SPC test data to the life prediction. Thus, in this study, as an alternative specimen with a dimension equivalent to a SPC disc specimen (8 mm in diameter and 0.5 mm in thickness), a thin plate specimen referred to as the ultra-miniature creep (UMC) specimen (2 mm in width, 10 mm in length, and 0.5 mm in thickness) was devised for a uniaxial tension creep test. Compared to the creep and creep-rupture properties obtained by a standard round bar (φ6 mm) specimen machined from a replaced steam pipe material (ASME Grade 91 steel), the stress-rupture relationship of the UMC specimen was found to coincide with that of the standard one. However, the creep strain and creep rate of the UMC specimen measured by a pull-rod displacement were larger than those of the standard one. From an elastic-creep analysis by a finite element method, this was attributed to superimposition of creep deformation in a grip portion clamping the UMC specimen on the creep strain in the parallel portion of the specimen. So, from the analytical results, an adjustment factor was derived to modify the creep property of the UMC specimen. As a result, application of this factor was found to lead to coincidence of the creep property between UMC and standard specimens.