Research On The Residual Stress Around An Indentation And The Elimination Of Its Influence

In order to meet the safety requirements of industrial production,more and more attention has been paid to the mechanical properties of materials tested by indentation tests.Spherical indentation test does not require sampling and can be tested in-situ and has advantages over traditional methods.However,this method is not a real non-destructive testing method.The residual pits generated by the indentation test cause stress concentration,and the additional residual stress exacerbates the stress concentration.Under static load,stress redistribution occurs during the plastic deformation of ductile materials before failure,so indentation generally has no effect on the static strength of the structure,but the effects on fatigue and corrosion resistance cannot be ignored.Especially,the surface damage of the impellers and blades of high-speed rotating machinery such as steam turbines,gas turbines,flue gas turbines,and centrifugal compressors can cause serious side effects on the performance of the impellers and blades.Therefore,it is of great significance to deeply study the residual stress of the indentation,find out its impact on the subsequent service performance of the material,and propose a method to eliminate its impact.The size of the residual pits is in the micron level,but with the current residual stress detection technology,it can only be fine to the millimeter level and cannot be used to test the residual stress of the indentation.Therefore,the author of this paper carried out research work mainly by using the finite element method,and verifies the reliability of the finite element simulation results by available experimental methods.Details are as follows:Taking two kinds of steels for pressure vessels(SA508 and S30408)and two kinds of steels for impellers(FV520B-S and KMN-S)as research objects,the material properties were obtained through uniaxial tensile tests.Ball indentation test was simulated by finite element software ABAQUS and the residual stress around the indentation was obtained after unloading.Residual pits were obtained by performing automatic ball indentation experiments.The indentation strain field was measured using digital image correlation technology.The comparison of the strain fields ensured the reliability of the finite element simulation,which can be regarded as a new method for acquiring residual stress at microscale.The basic law of indentation residual stress distribution was analyzed using contact mechanics related theories.The changes of the microstructure during the indentation were observed,and the residual stress of the indentation was explained from a microscopic perspective.The influence of material and indentation parameters on the indentation residual stress,including material properties,indenter radius and indentation depth,were analyzed.The superimposed distribution of residual stress and unidirectional/bidirectional working load was studied,and the degree of stress concentration caused by indentation was revealed.A cyclic load was applied to the residual pits and it was found that the release of residual stress could be ignored.This part of the work reveals the impact of indentation residual stress,and lays the foundation for the subsequent proposed elimination methods.Two schemes are proposed to eliminate the effect of indentation residual stress.The method of killing activating elements was used to simulate the removal of material.The change of residual stress during grinding was analyzed.Micro-milling tests were adopted to simulate that using grinding method to smooth out or slow down the indentation pits in actual engineering.The residual pits were milled with a ball-end micromill,and the strain field was tested again using digital image correlation technology to ensure the reliability of the finite element results after material removal.Finally,based on the stress concentration under the working load after milling,the optimal milling parameters of the two schemes were determined,and the method for different indenters are proposed.The research work is of great significance for the safe application of Spherical indentation test.