Fracture And Damage Simulation Of Low Carbon And High Strength Steel Based On CDM Model And Appliction To T-stub

Under extreme loads,ductile fracture of metal becomes an unavoidable problem due to the large deformation and nonlinearity.With the increasing development of metals in steel structure,the high strength steel has been widely applied.However,steel structure subjected to earthquake,possesses strong load-bearing capacity but low ductility,once the ultimate bearing capacity is exceeded,metal fracture will occur quickly.It is assumed that the presence of existing cracks and concentration stress and local strain at the initial crack location in traditional fracture mechanics,which formulates J integral with path independence and stress intensity factor K.However,this theory cannot be applied to the normal structure,because there usually exists less initial imperfections,and ignores the microscopic mechanism in the material level,Therefore,it is difficult to predict the damage state in the process.Therefore,some scholars proposed a micro-mechanism based method to discuss ductile fracture of metal structure.Without considering the damage,the traditional elastic-plastic model tends to overestimate the load-displacement response of the component greatly,so that the ultimate value cannot be captured and the actual load-displacement curve cannot be well simulated.Metal fracture is a complex damage process.The occurrence of damage,material deterioration and ultimate fracture are all related to the development of material plastic deformation.Uniaxial tensile fracture and shear fracture are two typical fracture modes in structural components.In the ductile fracture processes of metal considering,the stress triaxiality,equivalent plastic strain,damage factor D and other key parameters are closely related to the damage-induced fracture.Therefore,two kinds of metal materials were selected in this paper,,including low carbon steel Q235 and high strength steel Q690.And different specimens of tensile and shear groups were designed.In the tensile group,different notches are made for the tensile specimen to construct different stress triaxial states.Then the DIC technique was used to measure all specimens to obtain accurate images of load-displacement curve,stress,strain and loading process.The experiment was carried out in the mechanics laboratory of the comprehensive laboratory building of Chongqing University.The universal SANS servo hydraulic tensile testing machine was selected for the tensile current experiment.The tensile loading speed of this experiment was 1mm/min.All specimens were loaded until complete fracture,in order to obtain the entire loading process and complete recordings with DIC.Abaqus was used to model test specimens.A calibrated method based on CDM theory was used to simulate the specimen.Firstly,the no-damage model based on the traditional elasto-plastic constitutive method was compared with the experimental results.The bifurcation points of the two compared curves were clearly observed and the starting point of the damage got determined.It was found that the initial point of damage is located at the ultimate point of load-displacement curve in this paper,except for the standard notched tensile specimen,while the initial point of damage in other groups is located at the moment of yield,that is,when the specimens enter the plastic stage,the damage begins.Then,the damage rules of each group of specimens were calibrated by means of "trial and error".Finally,a curve completely fitted was obtained according to the experiment data,thus the different damage rules of each group of specimens were obtained.Due to the different distribution states of stress triaxility,the equivalent plastic displacement of discriminant groups are completely different.Meanwhile,it is shown that the damage factor D of each group for Q235 steel is consistent at the fracture point,while the damage factor D of Q690 steel is different from each group at the fracture point.High strength steel Q690 also has a higher degree of damage than that of Q235 steel on the whole.The FE model was divided into different mesh sizes,and the effect of mesh sizes on the damage fracture model is also discussed.And the damage rule based fitting formula related to the mesh characteristic length is proposed.Finally,the damage rule of Q235 steel is applied to the T-stub connection for FE simulation.In the T-stub experiment,it was found that the tearing fracture mainly occurred in the region adjacent to the fillet welds.Therefore,the meshsizes of the welding seam and its surrounding area were set intensely,and the damage law curve obtained above and the traditional elastoplastic constitutive structure obtained from the standard specimen were introduced,which finally shows a great consistency with the experimental results.Thus,it is proved that this damage theory is correct and feasible to discuss metal ductility damage based on CDM model,which is of profound significance for the further research and scientific extensions in the future.