Quantitative characterization of intrinsic plastic properties in carburized 18CrNiMo7-6 steel via dual sharp indentation

: Residual stress significantly alters the load-depth response in sharp indentation, frequently resulting in inaccurate predictions of intrinsic mechanical properties. By incorporating the influence of initial stress on indentation curvature, an analytical framework is derived based on the energy density equivalence principle and the concept of representative strain, establishing an explicit relationship between representative stress, residual stress, loading curvature, and equivalent cone angle. Leveraging this theoretical foundation, a dual-sharp indentation method (DSIM) is proposed to identify the intrinsic plastic properties of metallic materials under equibiaxial residual stress. This framework operates on the premise of prior knowledge of the elastic modulus and residual stress, provided that the initial stress magnitude remains below the yield strength. The effectiveness and robustness of the DSIM are substantiated through systematic numerical simulations spanning a wide range of elastoplastic attributes, as well as experimental validations on four representative metallic materials. Furthermore, the methodology is implemented to characterize the carburized gradient layer of 18CrNiMo7-6 steel via layer-by-layer dual-sharp indentation, yielding the precise depth-dependent distribution of intrinsic plastic properties. The results demonstrate that compressive residual stress induces an overestimation of yield stress by up to 8.2% while concurrently leading to an underestimation of the strain-hardening exponent. This work achieves the mechanistic decoupling of the initial stress field from the constitutive response, providing a powerful tool for the localized micromechanical assessment of surface-modified components.