Study On Uncoupled Ductile Fracture Criterion For High-Strength Steel Q690

In many fields such as building structures,metal forming,automobile and aerospace manufacturing,if a ductile fracture occurs in the metal component in use,it may lead to immeasurable consequences.A ductile fracture means that the metal material experiences significant plastic deformation before fracture(area reduction is usually not less than 5%).In the field of the steel building structure,the ductile fracture of steel is common,such as the shear-out failure of bolt connection,the fracture failure of welding,and the fracture failure of the metal damper.When studying the mechanical behavior of structures involving fracture,accurately predicting the occurrence and development of fracture is helpful to judge the loss of component bearing capacity accurately and then more accurately analyze the mechanical behavior of the whole structure after a local fracture.It should be emphasized that the premise of the study mentioned above is based on a reasonable ductile fracture model of steel.The academic community favors the uncoupled ductile fracture criterion among all kinds of fracture models because of its simple calibration process and high fracture simulation accuracy.However,most uncoupled ductile fracture models are based on experience without physical support.Meanwhile,the study on the uncoupled ductile fracture model under high temperatures and high strain rates is limited.Conclusions about ductile fracture drawn from quasi-static conditions at room temperature may not be applicable to high temperature and high strain rate conditions,such as fire,shock,and explosion.To make up for the above shortcomings,the main research contents of this paper are as follows:(1)The ductile fracture experiments of Chinese high-strength steel Q690 are conducted under quasi-static conditions and room temperature.The tested specimens include smooth plates,notched plates,and simple shear specimens to cover a wide range of stress states.A new ductile fracture model is proposed based on dislocation theory and micro void development mechanism.The model has a simple calibration process,high simulation accuracy,and a clear physical basis.(2)Because of the shortcomings of the existing incremental damage accumulation rule,a full-form damage accumulation rule is proposed considering the influence of void coalescence.Compared with the former,the damage accumulation rule proposed in this paper matches well with the fracture criterion calibration method,which has better simulation accuracy and more evident physical significance.(3)The applicability of five uncoupled ductile fracture models to fracture prediction of typical metal materials is compared.The metal materials include Q355 steel,Q690 high-strength steel,8.8s bolt material,and 2024-T351 aluminum alloy material.(4)The core idea of Hillerborg’s fracture energy is extended to the uncoupled ductile fracture model.The energy expression method suitable for the uncoupled ductile fracture model is deduced.By introducing the effective plastic displacement into the fracture criterion,the number of elements in a finite element model is reduced,and the minimum element size is increased to reduce the simulation time.(5)The ductile fracture tests of Q690 steel at different temperatures focus on the relationship between stress state,ductility,and temperature.A finite element modeling method is proposed for steel ductile fracture under high temperatures.(6)The ductile fracture tests of Q690 steel under different strain rates are performed.The hardening behavior and ductile fracture properties of Q690 steel under different strain rates are systematically studied especially steel’s positive ductile strain rate effect.Then the uncoupled ductile fracture criterion considering high strain rate is proposed.(7)Based on the research results of this paper,the fracture phenomena of bolt connections and T-stub connections under the quasi-static condition at room temperature,high temperatures,and high strain rates are simulated,which verifies the universality and effectiveness of the uncoupled ductile fracture model considering the effects of stress state,temperature and strain rate.