Prediction Research On Fracture Of High-strength Steels With Prestrain By The Local Approach

Due to their outstanding mechanical properties, economic properties and service performance, steels have been widely applied in the civil and industrial facilities of ships, bridges, pressure vessels, power towers, high-rise buildings, oil and gas pipelines, etc. It has become an important material in the national economic construction and national defense construction. However, it may suffer from effects of the temperature and plastic deformation before and during operation. For these two factors, taking high-strength structural steel Q420 and pipeline steel X80 as examples, this paper systematically studied effects of temperature and prestrain on tensile and fracture toughness of steels, and evaluated the fracture behavior based on the local approach.Firstly, according to the impact test results, the ductile-brittle transition temperatures of high-strength structural steel Q420 and pipeline steel X80 were obtained. Then, tensile tests were conducted on the virgin and prestrained materials at different temperatures. The test results revealed that the yield stress and tensile strength of steels were increased with the decrease in temperature, but it reduced the plasticity. Furthermore, the yield stress and tensile strength of steels increased in steels with tensile prestrain due to work hardening and decreased in compressed steels due to the Bauschinger effect, but they all reduced the plasticity. In addition, the effect of prestrain could be more significant for the yield stress.The fracture toughness tests were performed under three-point bending loading conditions for the virgin and prestrained specimens of high-strength structural steel Q420 and pipeline steel X80 at different temperatures. The test results revealed that temperature significantly influenced the fracture toughness of steels and that fracture toughness was significantly reduced with the decrease in temperature, resulting in brittle fractures. Prestrain also reduced the fracture toughness of steels, and further increased the probability of brittle fractures. Meanwhile, specimen fracture morphologies of different fracture modes were also analyzed.Based on the finite element analysis, it was found that the near-tip stress/strain fields near the crack tip of different specimens were elevated by temperature and prestrain. Such activation of the near-tip stress fields facilitated the brittle fracture of steels. It can be concluded that the fracture toughness of elastic-plastic material mainly depends on the stress/strain field near the crack tip and its micro-fracture process.The local approach was used to study the fracture behaviors of the prestrained materials of structural steel and pipeline steel. The research found that the Weibull stress for all specimens were identical under the same fracture probability when the fracture initiation occurred for the virgin and prestrained specimens at different temperatures. Based on the local approach, the fracture toughness probability distributions of the prestrained specimens at different temperatures had been predicted from the test results of the virgin material specimens. Furthermore, this paper proposed a simplified reference temperature△T_ P evaluation method. The fracture toughness at the service temperature T with prestrain could be displaced by the fracture toughness of the virgin material at a lower temperature T△σ_T~P. The temperature shift△T_P was provided as a function of the flow stress elevation caused by the prestrain. The estimated△σ_ P~f-△T_ P curve was almost consistent with the experimental results.