Experimental-numerical Study On Ductile Damage Of DP900 Sheet Considering Strain Path And Strain Rate

In order to meet the lightweight trend,increasing higher attentions are paid to the properties of materials in the industrial production,especially on their strength.Therefore,the proportion of high strength alloy used in aerospace,railway transportation,automobile manufacturing and many other fields has increased year by year.However,the ductility of materials tends to decrease with the increase of strength,so the plastic processing of high strength alloy is difficult to control.How to accurately predict the forming limit of high strength alloy under various stress states and prevent its failure in processing is one of the research hotspots in recent years,which has guiding significance for industry production.Strain path and strain rate are important factors affecting damage evolution and forming limit of materials.In this paper,the high strength double phase steel DP900 sheet is selected as the research material,its plasticity,damage evolution and fracture strain under different strain paths and strain rates are studied by mechanical experiment and finite element simulation.The main research contents are as follows:(1)Construction of uncoupled damage models considering the effects of strain path and strain rate.The stress triaxiality and Lode parameter are introduced to describe the influence of strain path on the damage evolution,and the MMC model and LOU model are constructed based on that.Meanwhile,three uncoupled strain rate dependent damage models based on LOU model are constructed by transforming its damage threshold a constant to a strain rate dependent function.(2)Mechanical experiment.The evolution laws of hardening and damage for DP900 sheets under different strain paths and strain rates are obtained through mechanical experiments(uniaxial tensile test,pre-notched tensile test,butterfly test,cyclic shear test and twin bridge shear test).Meanwhile,the DIC three-dimensional speckle system is used to monitor the strain field and obtain the fracture strain of DP900 under different stress states.(3)Identification of constitutive parameters.According to the stress-strain curves of various mechanical experiments and fracture strains of materials obtained by DIC system under different stress states,a set of constitutive parameters which can be used to study the macroscopic mechanical properties of DP900 under different strain paths and strain rates are obtained through inverse identification based on the least square method.(4)Finite element simulation.The finite element simulation model is established,and the uncoupled damage models and the constitutive model parameters obtained by inverse identification method are applied to numerical simulation.The plastic and damage evolution law of DP900 sheet under different strain paths and strain rates are studied.Meanwhile,the accuracy of damage models established in this paper is verified by comparing the finite element simulation results with experimental results,the applicability of each damage model to DP900 is discussed as well.