Evaluation of mechanical properties of structural materials by small punch yest

Abstract

Small punch(SP) test method was applied to evaluate the mechanical properties of structural steels for nuclear power plant components. The yield strength of several nuclear structural steels was estimated based on the detailed examination of initial deformation characteristics during the SP test. In addition, the ductile-brittle transition temperature(DBTT) of heat-treated SA508 Cl. 3 nuclear pressure vessel steel was correlated with the temperature dependence of the SP characteristics. Finally, the SP test was performed on duplex stainless steel to estimate the extent of thermal aging embrittlement. Initial deformation behavior during small punch test was investigated using two SP specimen sizes, 10×10×0.5㎣ and 10×10×0.25㎣ by finite element analysis and by experiments with heat-treated SA508 Cl. 3 and 12Cr steel. The increase of specimen thickness resulted in a deviation from the initial linearity of the load-displacement curve at higher load. The deviation was attributed to different causes, namely loss of the constraint by the surrounding material and radial propagation of plastic bending deformation for 0.5 and 0.25mm specimens, respectively. The causes for the transition of deformation mode were identical irrespective of material strength and work-hardening rate in the range of yield strength about 400 to 900 MPa. Based on the invariance of the deformation mode, a distinct linear relationship between the load at the deviation from the linearity and the yield stress was predicted by finite element analysis and compared with the experimental results. The validity of using the SP test for the DBTT evaluation of SA508 Cl. 3 steel was investigated. It was clearly observed that the variation of load-displacement curves with temperature was accompanied by the change of fracture mode and appearance. The temperature dependence of the SP characteristics well described the toughness degradation which was made by heat treatment. The SP energy was selected to correla...