| The classical quasi-static indentation testing technique has been widely studied and is gradually maturing due to its advantages of minor damage to the tested sample,easy fabrication and accurate data acquisition,while dynamic indentation testing has attracted much attention due to its loading method and the rate effect of the response,which is obviously different from quasi-static indentation.However,research on relevant testing,characterization techniques and data treatment methods is lacking,mainly focusing on the improvement of external sensing devices for dynamic indentation devices and simple comparative analysis of the quasi-static/dynamic indentation response of various materials.In practical engineering applications,functional materials and structures are often subjected to short-duration strong dynamic loads resulting in large local deformations at high strain rates.In view of the excellent development prospects shown by dynamic indentation techniques and the fact that the relevant fundamental issues are not yet fully understood,the study of dynamic mechanical properties of engineering materials characterized by dynamic indentation is particularly important.In this study,a two-dimensional finite element model of a dynamic indentation test device based on a split Hopkinson bar test system is built based on the symmetry of the device;and the effectiveness and applicability of the "three-wave method" for obtaining indentation response data(indentation depth and indentation load)are verified by numerical simulation of a wide range of material parameters and a variety of conical indenters based on the "direct method" of extracting dynamic indentation response data from the simulation.Quasi-static/dynamic indentation tests are carried out on three typical crystalline structures(body-centred cubic(FCC),face-centred cubic(BCC)and dense hexagonal(HCP))at various loading rates with three conical indenters,and the relationships between the mechanical parameters of the metal materials are effectively obtained by combining indentation test theory and unloading stiffness analysis.The results of the indentation tests under different loading rates are compared,and the rate sensitivity of the dynamic indentation response of metals under different loading rates is clarified;the method and results provide the foundation for the experimental characterization of the dynamic indentation test.Constitutive relationships are an invaluable aspect of the study of the mechanical properties of materials;a dynamic constitutive model(Johnson-Cook constitutive)containing strain-hardening,strain-rate strengthening and thermal softening terms is used as a model to investigate the indentation test data of three typical crystalline metal materials under three conical indenters and various loading conditions.Based on the results of quasi-static indentation tests and the inversion analysis of numerical simulations under two types of indenters,the parameters related to the strain hardening term in the Johnson-Cook constitution are obtained;the calculation method of the indentation strain rate for dynamic indentation tests based on the Hopkinson indenter is given,and the strain rate strengthening factor in the Johnson-Cook constitution is obtained based on the relationship between the results of dynamic indentation tests and the indentation strain rate under different loading rates.The effects of the adiabatic temperature rise on the indentation response during dynamic indentation are clarified by systematic numerical simulations;a method for obtaining Johnson-Cook constitutive parameters at room temperature for metal materials based on indentation test characterization is presented.The Johnson-Cook constitutive parameters of three kinds of typical crystalline metal materials are obtained by quasi-static/dynamic compression tests in a similar strain rate range,and compared with the relevant constitutive parameters obtained by indentation tests;the effectiveness of the method for obtaining metallic constitutive parameters based on indentation tests is verified.A new method for characterizing the mechanical properties of metal materials and obtaining the constitutive parameters is provided.A single-load dynamic indentation test based on a Hopkinson lever is carried out with the addition of a momentum trap device.The recovered samples after the dynamic indentation test are cut and surface treated in the axial direction,and the effect of the indenter angle and loading rate on the distribution to the gradient of residual plastic deformation for three metals is investigated based on the nanoindentation test;The normalization of the equivalent plastic strain in the residual plastic deformation region with respect to the radial/axial length is carried out by combining numerical simulations;The effects of loading rate and indenter angle on the normalized length of strain in different directions under quasi-static/dynamic indentation tests and the effects of metal material parameters on the contour of the plastic strain region have been analyzed.It provides certain ideas and methods for the study of large deformations and strain gradients under local loads. |
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