Research Of The Meso-damage Evolution Behavior Of Pure Copper And Copper Alloy Sheets

Plastic damage and fracture of metal material and microstructure are closely related to the microstructure.Up to now,studies on the damage and fracture of metallic materials mostly focus on micro holes nucleation,growth and aggregation,etc of microholes,the evolution theory in local deformation stage revealed the damage and fracture mechanism of ductile metal materials and so is of great importance.In fact,the plastic nucleation microholes of plastic metal materials mainly is resulted from in cracking of interface(grain boundary)in plastic deformation process.Grain boundary is one of the typical defect(surface defect)in materials,which has a great influence on the mechanical behavior of materials.However,the mesoscopic damage evolution of grain boundary in plastic deformation has not been paid great attention.Therefore,the present work studied the grain shape change and the relationship with the macroscopic deformation for copper and copper alloys through a quasi-static uniaxial tensile test and a quantitative metallographic analysis based on assumptions that the grain boundary is a damage source and a material parameter of statistical grain shape factor that describes the grain shape change is a mesoscopic damage variable.Moreover,the on-lineapparent deformation and corresponding relationship with macroscopic deformation ofthe materials was also studied through quasi-static uniaxial tensile test combining a digital image correlation method(Digital Image Correlation,DIC).Thus,the mesoscopic damage evolution rule,damage equation and constitutive model were established,which truly disclosed the damage form and damage evolution on surface and internal material in mesoscopic scale.The conclusion is of great importance for further understanding the microstructure evolution and microstructure damage of plastic metal materials.Based on the quasi-static uniaxial tensile test of copper and copper alloys,the mesoscopic damage evolution of shape factor was studied with different grain sizes.Grain of two metal materials presented the trend of the longitudinal elongation and transverse shortening with the increase of plastic deformation,and the scale of which gradually decreased.The shape factor and the relative shape factor increased with plastic deformation increasing,which showed obvious increase when the strain was 0.24 and 0.2 respectively,and which corresponded to elongation or shortening of grain.The mesoscopic damage evolution of the materials with different grain size was studied by annealing.Evolution of grain shape factor was less affected by the increment of grain size,which increased with plastic deformation increasing.However,the large-scale grain size of maerials resulted in late plastic deformation,long plastic deformation process,and slow rate of damage deformation.The distance of the critical strain points were postponed to 0.34 and 0.28 respectively,which of copper alloys had longer distance and slower damage speed than that of pure copper.Based on the results of uniaxial tension experiment,by the normalized shape factor as damage variables,the damage evolution equations were established,where a,b and c were constants related to the materials.The different materials(alloy type,grain size)have smaller effect on value of a,b,which influence value of c.With the increase of grain size,value of a and b declined slightly,and the value of c showed obvious decreasing trend.After alloying,the valued of a,b and c showed increasing trend.The equation revealed the microscopic damage evolution of the metal materials quantitatively,which can be extended to the microscopic damage evolution characterization of other metals and alloys(such as Al,Zn,Ni,Ti,etc.).Based on the normalized shape factor as damage valiable,the damage constitutive model is established by Ramberg-Osgood model.The hardening coefficient and hardening index of the model were closely related to change of material(alloy type and grain size).With grain size of the materials increasing,the hardening coefficient and the hardening index decreased gradually.After alloying,the hardening coefficient and the hardening index increased.Based on the Cockcroft-Latham criterion and the damage constitutive model,the critical damage factor was consistent with the experiment,which can better predict rapid damge process of materials.The prediction error of the model were less than 10%,which can accurately predict the tensile flow stress of plastic metal materials(alloy type,grain size).By the method of digital image correlation(DIC),surface damage evolution and damage evolution equation of the materials were researched.Based on strain damage factor as the damage variable,the surface damage constitutive models of two kinds of materials were established.The results showed that the surface damage evolution was similar to grain-boundary damage evolution,and damage evolution equations and the critical strain points were approximately the same.The damage constitutive model and the critical damage factor on the surface and grain boundary were approximately the same.The method was proved to be feasible by the ductile damage evolution quantitative characterization of metal materials,to provide supplementary for mesoscopic damage evolution of the whole analysis.