Method Of Determining Residual Stress And Its Layer Thickness Based On Instrumented Indentation Testing

High-level residual stress can be formed in the surface layers of the parts manufactured by machining,surface modification,additive manufacturing or coating.Surface residual stress significantly influence the service performance of parts,such as fatigue life,brittle fracture,stress corrosion and dimensional stability.It is significant to measure surface residual stress for improving processing technique and analyzing the service performance of parts.There are many kinds of methods to measure residual stress,such as X-ray diffraction,Barkhausen magnetic noise and hole-drilling.Instrumented indentation testing(IIT)has been widely used in surface mechanical properties testing due to its advantages of micro damage,micro area testing and wide range of applicable materials.There are many mechanical models estimating residual stress based on IIT in theoretical research and scientific applications,such as Suresh Model,Lee Model,Carlsson Model and Lu Model.These models are based on the assumption that residual stress are uniform along the depth in the whole model.This thesis takes the influence of the surface residual stress layer thickness into account,and established the method of determining surface residual stress and its layer thickness.For the condition that the materials of surface layer and substrate are uniform,this thesis proposed the Homogeneous Material Surface Residual Stress Indentation Model and established the method of determining surface residual stress and its layer thickness by using finite element analysis(FEA).Based on dimensional analysis,the dimensionless function of relative loading curvature and material parameters and residual stress parameters was given.We proposed the piecewise fitting method which can obtain the characteristic curve of relative loading curvature and indentation depth through one indentation testing.The critical indentation depth can be obtained by normalized central difference method.When the indentation depth is shallow,the Homogeneous Model can be degenerated to the Uniform Residual Stress Model.The surface residual stress can be calculated by using the relative loading curvature of the platform of the characteristic curve,and the average calculation error is less than 10%.The surface residual stress layer thickness can be calculated by using the critical indentation depth,and the average calculation error is less than 5%.For the condition that the materials of surface layer and substrate are different,this thesis proposed the Film/Substrate Residual Stress Indentation Model and established the method of estimating the film residual stress and the film thickness(residual stress layer thickness)by using finite element analysis(FEA).When the indentation depth is shallow,the Film/Substrate Model can be degenerated to the Uniform Residual Stress Model.The film residual stress can be calculated by using the relative loading curvature of the platform of the characteristic curve,and the average calculation error is less than 12%.The film thickness can be calculated by using the critical indentation depth,and the average calculation error is less than 5%.