Research On Indentation Characterization Method Of Residual Stress Of Metallic Materials

Residual stress will be introduced during the manufacturing,processing,and surface treatment processes.These introduced residual stresses not only affect the material’s serviceability（crack strength,fatigue strength,corrosion resistance,static strength,etc.）,but also have an important influence on the dimensional stability of the material components and the safety of the structure,so it is of great significance to accurately measure the residual stress on the surface of the engineering material.The main contents of this paper are as follows:（1）A two-dimensional numerical model was established,and the relationship between the ratio of residual stress and yield stress（σ_r/σ₀）and the ratio of the elastic recovery depth after unloading and the maximum indentation depth(h_e/h_max)was obtained.The parameters of the elastic modulus,yield strength and hardening index of the material were calculated and the relative errors are within 6%.（2）The residual stress loading device was designed and fabricated to realize the uniaxial and equal-biaxial stress loading,which laid the foundation for the simulation of various residual stress states.Experimental results show that in the direction of the principal stress,the pile up height is proportional to the size of the stress.（3）A finite element model was established to calculate the relationship between the uniaxial and equal-biaxial residual stress and the maximum pile up height,and the effect of the material yield strength,hardening index and contact conditions on the indentation morphology was analyzed.The calculated results are consistent with the experimental study.The average error is about 10%,indicating the validity and reliability of the model.（4）An unequal biaxial residual stress measurement method based on the indentation pile up morphology was proposed.The variation of in-plane indentation morphology is studied when the in-plane residual stress ratio is changed,and the relationship between the maximum pile up height ratio and the stress ratio is established.The mechanism study shows that the compressive residual stress will suppress the reduction of the pile up height.Unloading pile up height is more sensitive to compressive residual stress.