Stress Indentation Theory Model And Test Method For Ductile Materials

Material properties and stress（residual stress,working stress）testing of in-service structural materials are the basic tasks of structural integrity evaluation.However,to accurately obtain the stress and mechanical properties of a material by indenting the surface of the component with a specific indenter is challenging,especially by small-invasive indentation.Therefore,the development of the indentation theory and test method has important theoretical value and engineering significance for the safe evaluation of service structures.Based on the energy density equivalence,material equivalence,and deviatoric stress balance assumptions,we conducted systematic theoretical and experimental studies on the power law（Hollomon law,H law）for material stress indentation of conical indenter and cylindrical flat indenter.The main work is as follows:1.For a semi-infinite solid subjected to flat and conical indentations,a unified stress indentation model was proposed to describe the relationship between energy/load,H-law parameters,geometric parameters of the indenter,and displacement based on the energy density equivalence.Through the numerical calibration of finite element analysis,the deterministic flat indentation and conical indentation semi-analytical models of load-displacement relationship for the zero-stress state were established,namely,zero-stress state flat indentation FI_ZS model and cone indentation CI_ZS model.Furthermore,a flat indentation test method for testing the stress-strain relationship was proposed.2.For 120 and 40 preset H-law materials containing common ductile materials,the load-displacement curves of zero-stress flat indentation and conical indentation were obtained using the finite element analysis.We conducted the inverse numerical verification of the stress-strain relationship predicted by the zero-stress flat indentation model and the forward numerical verification of the load-displacement curve predicted by the zero-stress conical indentation model.The results showed that the predicted results of the model are closely consistent with the provided results of the finite element analysis.Flat indentation tests were conducted on 15ductile metal specimens,including P92,1Cr12Mo,30Cr1Mo1V,and 6061-T6511.The results showed that the goodness of fit between the stress-strain relationship predicted by the zero-stress flat indentation FI_ZS model and uniaxial tensile test results is higher than 0.96.3.For the equibiaxial and uniaxial stress states,the loading exponent of the load-displacement relationship of flat indentation is sensitive to stress.A stress flat indentation depended on the exponent increment or ratio(SFI_EI and SFI_ER)model for solving stress was proposed,as well as test methods based on them.For 32 H-law materials with 11-level stress,the indentation load-displacement curve was obtained according to the finite element analysis.The prediction results of SFI_EI and SFI_ER models were closely consistent with the 11-level stress preset by the finite element analysis,indicating that the SFI_EI and SFI_ER models with flat indentation have good accuracy for stress prediction.4.Under the equibiaxial and uniaxial stress states,the stress state conical indentation material is equivalent to the zero-stress state conical indentation"new material".Assuming that the relationship between the relative difference between the nominal yield stress of the equivalent and raw materials and the dimensionless stress conforms to the power law,a material-equivalence based conical indentation（MCI_S）model for stress state and a test method based on it were proposed.5.For the equibiaxial and uniaxial stress states,assuming that the normal deviator stress and the average stress（the ratio of the difference between the indentation load in the stress state and zero-stress state to the contact area）are balanced,a deviatoric-stress-balance based conical indentation（DCI_S）model for stress state and a test method based on it were proposed.6.Numerical and experimental verifications were conducted for 120 preset H-law materials,a variety of equibiaxial or uniaxial stresses,and 3 metal materials.The results showed that the stresses predicted by MCI_S and DCI_S models are in close agreement with the stresses preset by finite element analysis and that applied in experiments.