Evaluation On Fracture Performance Of Low Carbon Steel Base Metal And Welded Joint Based On GTN Model

Ships and offshore platforms are always fabricated with welding,while mechanical performance and fracture strength of welded joints will significantly influence the strength and service life of whole ship and offshore platform considering the impact of actual working condition and external loading.The stress-strain curve can be obtained through uniaxial tension test to evaluate the fracture performance of welded joints.However,the preparation of tension specimens is a destructive method and is time-consuming and labor-intensive;through the method of numerical simulation,the evaluation of the fracture performance of welded joints is currently a relatively common method,which has high calculation efficiency and significant engineering application value.Firstly,the engineering stress-strain curve of different strength grades of low carbon steel base metal were obtained through uniaxial tension test;butt joints of the low carbon steel that meet the requirements according to the shipyard welding specification were producted,and the tension specimens were made to obtain the stress-strain curve to evaluate its fracture performance.The test results showed that the fracture performance of the welded joint was slightly reduced compared to the base metal,while the plastic hardening properties also changed.Based on the GTN damage model,the numerical simulation analysis process was completed by a C++program;combined with the particle swarm optimization algorithm to solve the optimal parameters of the GTN model of the material,the linearly decreasing inertia weight and asynchronous learning factors were used to improve the stability of the algorithm convergence.The optimal GTN model parameters of each strength low carbon steel base metal were obtained in a shorter calculation time.Compared with the exhaustive method,the calculation efficiency of the program is greatly improved.The corresponding numerical simulation stress-strain curves are highly consistent with the measured curves through tension test.For low carbon steel butt welded joints that meet the welding specifications,the influence of the welding process on the internal micro-defects of the material and the influence of the welding residual stress on the plastic hardening process of the material should be considered.It was proposed that the initial void volume fraction f₀ in the GTN damage model and the power function plastic strengthening parameters of the material could be modified to predict the stress-strain curve of the welded joint and evaluate its fracture performance.The numerical simulation results of the GTN model with modified parameters are in good agreement with the stress-strain curves obtained from the tensile tests.Based on the numerical simulation results of three different strength grades of low carbon steel in this study,an empirical formula for modifying the parameters of the GTN model of welded joints is proposed.Based on the above research,the fracture performance of a certain high-strength steel welded joint under equal-strength matching conditions is predicted.