Research On Mechanical Properties And Residual Stress Evaluation Technology Of Ductile Metals Based On Micron Indentatio

Traditional mechanical property evaluation methods require the production of macroscopic specimens,which are destructive testing methods.For in-service equipment,destructive sampling is not possible,and for welding line and additive manufacturing and other micro-scale multi-structures,macroscopic specimens is hard to get.Therefore,the evaluation of mechanical properties in similar scenarios is an industry problem that has not yet been effectively solved.Micro indentation technology provides a technical approach for nondestructive evaluation of metal mechanical properties and residual stress.This method has achieved many results and shows great application potential.However,the macro mechanical properties and residual stress evaluation theory and technology based on micro indentation technology still need to be improved and developed.This thesis using dimensional analysis combine with finite element simulation to improve and develop the evaluation methods of uniaxial tensile properties,fracture toughness and residual stresses based on micrometer indentation,as follows.（1）A simple method to distinguish power-hardened metallic materials from linearly hardened metallic materials is proposed based on the representational stress-representational strain model.The pile-up and sink-up phenomena of different instantonal models are investigated by means of dimensional analysis and finite element simulations,and formulas for calculating the new pile-up and sink-up coefficients of the materials under different instantonal models are given.Experimental validation of the modified model using five different materials showed that for the four power hardening metal materials,the maximum error in the calculation of yield strength was 7.2%and the maximum error in the calculation of tensile strength was 11.4%;for the linear hardening metal materials,the errors in the calculation of yield and tensile strength were larger.To address this problem,this thesis quantitatively analyses and establishes the dimensionless function of the stress constraint factor in the representational stress-representational strain model and the plasticity parameter of the linear hardening material.By using the modified stress constraint factor to calculate the yield strength and tensile strength errors of the linear hardening metal material as 4.19%and 7.77%respectively.（2）The stress states below the indenter in the ball indentation test and at the crack tip in the conventional fracture test are analysed by finite element simulation,and the results showed that the damage mechanisms of the two materials are not the same.It is found that the material below the indenter is shear fracture,and the material at the crack tip is microporous aggregation fracture.Although the damage mechanisms are different,the damage causes of the two materials are the formation and penetration of pores,and the strain energy release rate to reach the critical damage is the same.Therefore,there is a certain theoretical basis for calculating the fracture toughness of materials by using the spherical compression test of notchless specimens.CIE model,ERR model and CSS model are used to calculate the ball compression test data of Q345 and 304,and the corresponding fracture toughness values are obtained.The results are compared with the conventional fracture test results.The results show that the CIE model,whether National Standard or American Standard,REE model and CSS model have large calculation errors for different materials,and can not meet the requirements of fracture toughness test accuracy.（3）A theoretical analysis of the residual stress under the indenter is carried out based on the indentation test theory,and the interconversion relationship between the equal biaxial residual stress and the uniaxial residual stress is explained;the functional relationship between the dimensionless parameter（7）c-c₀（8）/s_y and the dimensionless parameter variables_r/s_y,n,E/s_y,was established by the combination of dimensional analysis and finite element simulation,and the correlation coefficients in the functional relationship were obtained by SPSS fitting.Finally,the calculation accuracy of the model is verified with two materials,AL2024 and Copper C11000.The results show that the average calculation accuracy of the residual stresses obtained by this model is higher than that of the Lu model,and the residual stress calculation method proposed in this thesis can meet the accuracy requirements in engineering applications.