Determination of Creep Properties from Small Punch Test with Reverse Algorithm

Abstract

In the early 1980’s, the small punch technique was developed in USA and Japan. This technique has been applied to nuclear reactors, electric power plants for safety assessments. European researchers have carried out the pioneer work on SP creep testing in the 1990’s. European Code of Practice (CoP) documents have become available in 2006, which provides a guide line to perform small punch tests for metallic materials, and introduces methods for estimation of tensile properties, fracture toughness and creep properties. Since then several conferences and symposiums have been held to exchange ideas and experiences, and several efforts have been made to improve the approaches for interpreting the material properties from test results. However, due to the complexity of the small punch test itself and lack of systematic test data for verification, especially in the creep region, up to now there is no common acceptable method to interpret creep properties. This paper discusses some of the problems commonly encountered in the small punch creep test (SPCT) analysis, such as the uniqueness in reverse creep analysis for identification of material parameters, how to consider effects of large deformation and strain hardening, how to separate deflection into a part caused by loading and a part caused by creep, and how to estimate the elastic-plastic properties of a material in the creep temperature region. Based on these considerations, several existing approaches for interpreting creep properties have been re-evaluated. In addition, a software package, based on the optimization toolbox of Matlab has been developed for identification of material parameters automatically. Verifications are performed by checking the agreement between properties derived by small punch test and uni-axial tests. Discussions on problems of existing approaches and how to improve them further, are described in the paper. Experimental data are provided by JRC Petten, Netherlands, and Henan Electric Power Research Institute, China. Recently a working group led by JRC Petten is engaged in upgrading the CoP to a European standard. It is expected that in the future more data will become available for further verification.