Determination of the true stress-strain relations of high-grade pipeline steels based on small punch test correlation method

Abstract

The small punch test (SPT) has been widely used to evaluate the mechanical properties of small regions in pipeline steels. However, its application has thus far been limited to the determination of elastoplastic mechanical properties such as the ultimate tensile strength, and few works on estimating he true stress-strain curves. This paper aims to propose a new method for determining the true stress-strain relations of high-grade pipeline steels based on the SPT results, and apply it to the welded joints of X80 grade pipeline steel where the traditional mechanical characterizations cannot be carried out. Mechanical properties of several high-grade pipeline steels were obtained by empirical formulas that were determined by the correlation relationships between the conventional tensile tests and SPT results. Then the Ramberg-Osgood(R–O) power law which can describe the stress-strain relations of high-grade pipeline steels was simplified and expressed by four determined mechanical property parameters so that the true stress-strain curves could be generated using these parameters. Meanwhile, the SPT correlation method was used to estimate the true stress-strain curves of the heat-affected zones and weld zones of the welded joints of X80 steel and the accuracy was verified by numerical simulations. The results demonstrate that the true stress-strain curves determined by the SPT correlation method are in good agreement with the uniaxial tensile results for base materials of high-grade pipeline steels and the predicted results of different areas of the welded joints of X80 steel are reliable.