Determination of local true stress-strain response of X80 and Q235 girth-welded joints based on digital image correlation and numerical simulation

The girth-weld joint of a pipeline can be divided into three parts: base metal, heat affected zone, and weld metal. This study investigates the local mechanical properties of X80 and Q235 girth-welded joints. Microscopic observations and micro-hardness measurements were carried out, and it was observed that the metallographic structure and hardness gradients of each region were evidently different. Tensile tests of the base metal and girth-welded joint were conducted using digital image correlation (DIC) technology, and the strain distributions on the specimens and local true stress-strain curves of different regions were obtained. A parameter calculation method for the Ramberg-Osgood model considering strain was proposed. Subsequently, model parameters and mismatch coefficients were obtained and analyzed, respectively. The yield strength and hardening property of welded joints can be determined using this method, which is of great significance in calculating the strength-matching relationship of girth-welded joints. Finite element analysis based on the Gurson-Tvergaard-Needleman model was used to study the strain field and local stress-strain curves of girth-welded joints. The results obtained by the finite element analysis were in line with the results obtained by the DIC method. This demonstrates the feasibility of the simulation method and that the proposed material model is reasonably accurate in predicting the stress-strain response.