Objective Numerical simulations represent an important approach to studying the damage evolution behaviors of oil and gas pipelines constructed using high-grade steel. However, the accuracy of simulation results is often restricted by the precision of damage model parameters. Accurately determining damage parameters, being a challenge in the analysis process, is vital to ensure the accuracy of simulation results.
Methods This study established a fitting relationship between different pore volume fractions and characteristic parameters reflecting material behaviors upon elastic-plastic deformations and damage fractures, through a finite element inverse calibration method integrating material tensile experiments, numerical simulations, and the response surface method, to effectively calibrate the parameters of the Gurson-Tvergaard-Needleman (GTN) damage model for the girth welds in X80 pipelines. The tensile experiment process was numerically inverted, using the calibrated parameters, and the precision of this model was subsequently verified by comparing the fracture characteristics and stress-strain curves between simulation and experimental results. Based on the above-mentioned GTN damage model, numerical simulations and result analysis were conducted for the single edge notch tensile and single edge notch bending experimental processes for the girth welds in X80 pipelines, verifying the applicability of this GTN damage model.
Results The accuracy verification of calibrated parameters indicated alignment between the fracture positions and fracture characteristics presented by simulations after parameter calibration and the experimental results, and high consistency of their stress-strain curves throughout the damage and fracture process. A comparison of the four characteristic values of tensile stress, tensile strain, fracture stress, and fracture strain resulted in a maximum error not exceeding 10%. In the applicability verification for the model, the apparent toughness values of single edge notch tensile and single edge notch bending specimens were recorded at 0.399 3 and 0.324 2 respectively, corresponding to both final conversion rates of about 1.23.
Conclusion > The parameter verification results prove the effectiveness of the GTN model and its parameter calibration method proposed in this paper in predicting the damage evolution behaviors of the girth welds in X80 pipelines. In addition, the numerical simulation results using the small-scale fracture toughness experiment model built based on the GTN model confirm the difference in fracture toughness among specimens due to the influence of crack tip constraint degrees. The experimental results further demonstrate the substantial application value of the GTN model in the damage and fracture field of X80 oil and gas pipelines.
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