A direct assessment of creep life based on small punch creep test

Abstract

The uniaxial creep test (UCT) has generally been used to identify creep properties of materials. However, it consumes an enormous amount of testing time and requires a substantial specimen volume. The small punch creep test (SPCT) is introduced as an alternative test method to overcome the shortcomings of the UCT. It uses a fingernail-sized sheet as a specimen (10 × 10 × 0.5 mm3) and a spherical punch ball to apply load to the specimen. Despite these positive aspects, SPCT has rarely received much attention in industrial fields due to a complex interpretation process that requires additional processes such as UCT, FEM, and CEN code. In this paper, a direct assessment method for SPCT is proposed to evaluate creep life directly from the SPCT. In this method, a stress formulation was derived to convert force into one equivalent stress. Moreover, a non-dimensional correction factor was defined to consider the friction effect between the punch ball and specimen. To demonstrate this method, STS316L stainless steel was used as a test specimen for both the UCT and SPCT at 650 °C. A comparison of Larson-Miller models derived from each test result showed good reliability of the proposed direct assessment method.