Testing Of Residual Stress In Surface Modification Layer Based On X-ray/Layer Removal Method And Indentation Method

According to statistics,about 80% of mechanical components in the engineering were damaged by fatigue.After surface strengthening processes such as heat treatment and shot peening,the surface layer of sample will form a surface modification layer to improve the fatigue life of mechanical components.The residual stress in the surface modification layer has an important influence on the fatigue life,crack initiation and propagation of mechanical components.Tensile stress can promote the initiation and propagation of fatigue cracks and reduce the fatigue life of mechanical components;compressive stress can inhibit fatigue cracks and improve the fatigue life of mechanical components.At the same time,the coating also has varying degrees of residual stress.The residual stress in the coating has an important influence on the mechanical properties such as the interface bonding strength.The interface stress mismatch caused by the residual stress will reduce the bonding strength and cause the coating to break and fall off.Therefore,the accurate characterization of the residual stress in the surface modification layer and coating has an important role in studying its mechanical properties and service life.X-ray diffraction method combined with layer removal method(X-ray / layer removal method)is a more widely used method to measure the residual stress in the surface modification layer.Layer removing will cause the redistribution of internal stress in the material.At this time,the measurement result of the X-ray / layer removal method is not the initial residual stress value in the material.Accurate measurement of the residual stress has an important influence on the determination of the structural rigidity,the dimensional stability of the component and the fatigue life.Therefore,it is necessary to study the correction method of the residual stress results measured after removing.Through the finite element simulation of two stress states: equal-biaxial residual stress and planar non-equal-biaxial residual stress,a finite element matrix correction method for planar non-equal-biaxial residual stress is given,and the finite element matrix correction method was verified.The residual stresses in the modification layer of the heat-treated and shot peened samples were measured by X-ray / layer removal method,and the measurement results were corrected by the finite element matrix correction method of the equal-biaxial and planar non-equal-biaxial residual stresses.The indentation technology is an important method to measure the residual stress of the coating.Determining the residual stress and the thickness of the coating(that is,the thickness of the residual stress layer)has an important role in studying the mechanical properties of the coating and judging the failure mode.Based on the degraded uniform residual stress calculation model of Zhao et al.,the relative loading curvature of the loading section of the indentation curve is used as the analysis parameter to calculate residual stresses of single crystal copper and copper coating.Compared with the applied residual stress and the measurement results of the X-ray / layer removal method,the calculation results of the residual stress are accurate.Then,the layer thickness of the electrodeposited nickel coating was calculated based on Zhao et al.'s model for determining the residual stress layer thickness,and the factors affecting the results were discussed.