Inelastic Analysis of the SENT Specimen of X80 Pipe for Strain Capacity Evaluation

Strain capacity of the X80 line pipe with girth weld has been frequently evaluated from the SENT (Single Edge Notch Tension) test for the strain-based design (SBD). Also, it can be predicted from CWPT (Curved Wide Plate Test) and full scale test, which demand much larger scale test environment and higher costs in comparison to SENT test. On this scenario, well-developed numerical scheme can be a very useful tool for the strain capacity evaluation. The numerical scheme should be equipped with appropriate material models and inelastic analysis procedure to simulate the nonlinear behavior of the SENT specimen; in fact, the SENT specimen is expected to properly describe the defect in the girth weld part by using a notch. In this paper, for the validation of tensile strain capacity (TSC) of X80 line pipe, a phenomenological model, which is based on the GTN (Gurson-Tvergaard-Needleman) model, is developed and verified through the comparison with experimental results. The material model is implemented in the commercial finite element program ABAQUS with the aid of the user-defined material module. Calibration of material parameters expressing elastic and plastic behaviors of base and weld metals are done on the basis of the round-bar and full thickness tensile test results, and then the finite element simulations for SENT tests are carried out to predict the TSC. To ensure the reliability of the nonlinear procedure, the predicted strain capacities are compared with the test results as well as those obtained by the conventional design formulae, which are established from statistical analysis of numerous line pipe test data; then application of the nonlinear procedure is extended to the CWPT simulation. In some cases, damping effect was introduced to improve the convergence of iterative nonlinear solution. As a result, the developed nonlinear procedure acquires reliability and accuracy enough to be applied to the strain-based design process. Thus, it is highly expected that the procedure can be used in various numerical evaluations of strain capacity of line pipes with girth welds.