Experimental Studies About Dynamic Responses And Fracture Criteria Of Q460GJ Structural Steel Considering Stress Triaxiality And Strain-rate Effects

With the development of high and super-high rise buildings,large and super-large span structures and green buildings,higher requirements on engineering structures have been suggested.As a high performance structural steel,GJ steel with superior seismic behaviour,good weldability and improved tearing resistance has been developed to adjust the trend.However,the studies of GJ structural steels are mainly based on structural members,residual stress,design parameter and reliability,and there are few investigations on the dynamic constitutive relationship and fracture criterion subjected to various strain rates and stress states.Thus,no material basis is available on the study of the resistance of GJ steel to earthquake,explosion,impact and progressive collapse of GJ structural steels.It also hinders the wide application of GJ structural steel in construction practices.Therefore,it is necessary and urgent to carry out research on the performance of GJ structural steel,especially high-strength GJ structural steel.Fupported by the Fundamental and Frontier Research Project of Chongqing(cstc2015jcyjBX0024),the thesis has studied not only the simulation of steel joints subjected to impact loadings but also the dynamic mechanical properties and fracture criterion of high strength Q460GJ structural steel.The main research works and achievements are summarized as follows:① Numerical simulations of steel joints:the numerical simulations and parametric analysis of steel joints under impact loadings were introduced by ABAQUS.And then the mechanism of axial force was determined subject to impacting.② Experimental studies of Q460GJ structural steel:Tension and compression tests were conducted on notched specimens of Q460GJ structural steel with various strain rates by the DIC method to obtain engineering stress-engineering strain curves,true stress-true strain curves and fracture strain both.under various stress triaxiality and strain rates It provided a basis on the development of dynamic constitutive relationship and facture criterion and enriched the material database of Q460GJ structural steel.③ Building dynamic constitutive relationship of Q460GJ structural steel:The strain-hardening effect of Q460GJ structural steel was determined through analysis and comparison between two aspects:engineering stress-engineering strain curves and true stress-true strain curves.In reference to the strain-rate effect of Cowper-Symonds and Johnson-Cook constitutive relationship,the strain-rate effect of Q460GJ structural steel was suggested based on the yield stress and ultimate stress,respectively.The stress triaxiality was introduced to the constitutive relationship and the contribution of stress triaxiality was also determined on the basis of both yield and ultimate stresses.Thus,the dynamic constitutive relationship of Q460GJ structural steel was built at ambient temperature.④ Suggesting fracture criterion of Q460GJ structural steel:The effect of strain rate and stress triaxiality on fracture strain of Q460GJ structural steel was determined in accordance with Johnson-Cook fracture criterion,in which the contribution of stress triaxiality could be divided into three regions.Therefore,the fracture criterion of Q460GJ structural steel at ambient temperature was suggested.Novelties:① Carrying out experimental studies on the dynamic mechanical properties and fracture criteria of Q460GJ structural steel at high strain rate and large range of stress triaxiality.② Obtaining the constitutive relationship of Q460GJ structural steel consdiering strain hardening effect,strain rate and stress triaxiality through experimental tests.③ Determining the fracture criterion of Q460GJ structural steel which considered the effects of strain rate and stress triaxiality.