Ductile Materials Indentation Model And Test Method Based On Elastic And Plastic Energy

Stress vs.strain relations,elastic modulus and strength(yield strength,tensile strength)are the basic relations and performance indexes of structural design and safety evaluation.For aeroengines,nuclear reactor pressure vessels and other special structures,welded structures and materials with the size of nanotrace,it is difficult to achieve the traditional tensile test by intercepting or processing standard tensile samples,and with the development of the miniaturization of parts,higher requirements are put forward for the test of material properties.Therefore,the test method of pressing hard alloy indenter into material surface with spherical,conical and column flat indenter(indentation method)has attracted more and more attention because it can realize the testing of material mechanical properties with small damage.In this paper,based on energy density equivalence,for ductile metal materials whose constitutive relations conform to Ramberg-Osgood law(R-O law),with the help of finite element analysis(FEA)and experimental means,The theoretical models and test methods of conical indentation and flta indentation are studied,and the main work is as follows:(1)The semi-analytical model between strain energy U_e and pure linear elastic displacement h_e and load P under linear law pure elastic condition and between strain energy U_p and pure plastic displacement h_p and load P under power law pure plastic condition are established for conical and column flat indentation.Assuming a proportional superposition relationship between U_e,U_p and elastic-plastic strain energy U,a load-displacement model of conical and column flat indentation based on proportional superposition of elastoplastic energy(LDM-PS)is proposed by introducing a proportional factor of energy superposition λ.A semi-analytical method is proposed to obtain model parameters based on given mechanical properties constants and geometric parameters.(2)Based on LDM-PS,the conical indentation based on elastoplastic proportional superposition test method(CI-EPS)and flat indentation based on elastoplastic proportional superposition test method(FI-EPS)were proposed to obtain the R-O law parameters of ductile metal materials.(3)For 60 materials with different constitutive relationships,the load-displacement curves predicted by LDM-PS are in close agreement with the load-displacement curves calculated by finite element method for given stress-strain relationship(positive verification).The load-displacement relationship was obtained by FEA for the stress-strain data set conforming to the R-O law,and the stress-strain curves predicted by CI-EPS and FI-EPS were in close agreement with the curve preset by finite element method(reverse verification).The double-conical indentation test was performed with 10 kinds of metal materials(4 kinds of aluminum alloy,6 kinds of steel),and the flat indentation test was performed with 6 kinds of metal materials(3 kinds of aluminum alloy,3 kinds of steel).The goodness of the stress-strain relationship predicted by CI-EPS method and FI-EPS method and the uniaxial tensile test is higher than 0.96,the errors of the predicted elastic modulus and the tensile results are within 5%,and the relative errors of the strength and the uniaxial tensile test results are small.