True Stress–Strain Curves Test and Material Property Analysis of API X65 and API X90 Gas Pipeline Steels

Abstract

In this paper, true stress–strain curves of two natural gas pipeline steels, API-5L X65 and X9, were studied using the three-dimensional digital image correlation (3D-DIC) technique. During the test, the steels were continuously strained hardening up to the fracture, and true stress–strain curves were obtained. It was found that the true stress increases with strain after necking, which is notably different from the results obtained by the conventional method. Using the FEM model, the true stress–strain curves were validated. Based on the material’s inherent properties, the material property analysis was implemented. The moduli of elasticity in the elastic region obtained by the proposed method and conventional methods were nearly equal. It was also found that the strain-hardening capacities of gas pipeline steels could be better characterized by the hardening exponent than yield-tensile ratio. Besides, the strain-energy density analysis based on the true stress–strain curve demonstrated that before the onset of necking, only 7.3% of the strain energy density of API-5L X9 was used.