Basic Research On Deformation Damage And Plastic Deformation Parameters Of Materials In Spherical Indentation Process

Compared with traditional testing methods of mechanical properties of materials,indentation method has great advantages of almost non-destructive,in-situ operable,cross-scale properties measurable and convenient.The spherical indentation has become a new hotspot in the field of indentation identification due to its better performance in overcoming the problem of uniqueness of testing results.The major task in indentation identification of material mechanical parameters is the establishment of identification algorithm,whose rationality and reliability depend on the in-depth study of the plastic deformation mechanism of spherical indentation.At present,although various algorithms to obtain the stress-strain curve,parameters of creep property and fracture toughness have been proposed by many researchers,the understanding of relevant deformation mechanism in indentation process is still very limited,which leads to the limitations in rationality and universality of these algorithms.Deformation and damage mechanisms in spherical indentation process are studied by means of theoretical analysis,numerical simulation and experiment testing based on crystal plasticity simulation,strain gradient theory and plastic damage theory.The characteristic of convexity-concavity transition of spherical loading curve is found and its influencing factors and forming reasons are analyzed,which provides reference for the identification of mechanical property parameters.At the same time,based on the Nix-Gao model,quantitative relation of the strain gradient,indenter size and indentation depth is deduced,which provides a theoretical basis for the indentation identification of material strain length in gradient plasticity.In terms of damage reduced by spherical indentation,a theoretical model for predicting the damage distribution under spherical indentation is developed and applied to AA7075-T6.Meanwhile,the nucleation,shrinking,expansion and torsion of the voids in deformation region are observed and studied fron the micro-scale level.For single crystal material,damage behavior is found to be quite different from that of alloy materials.The source of damage comes from the microcracks formed at the pile-up region around the indentation print.An analysis to the damage mechanism is revealed with the help of crystal plasticity simulation and scanning electron microscope.Based on the studies on the mechanism of indentation deformation,a method of identifying material intrisice length using only single indentation test with single spherical indenter is proposed.At the same time,a calculation procedure based on intelligent algorithms is also developed to identify the parameters in stress-strain relation with multiple fitting freedoms.By comparing the loading curves of sharp indentation and spherical indentation,it was found that the shape of the loading curve of spherical indentation showed a characteristic of concavity-convexity transition,which means the front part of the loading curve presents concavity and the second half presents convexity.The effects of elastic modulus-yield strength ratio,poisson's ratio,friction factor,strain hardening index and indenter radius on the occurance of the charicteristic and the position of concavity-convexity transition point were studied by finite element simulations.It was found that strain hardening index palyed an important role.When n>0.1,the shape transition occurs and when n?0.1,the loading curve presents convexity without transition.The special shape characteristic of the loading curve of spherical indentation result from the change of the two different deformation patterns in spherical indentation deformation:vertical compression and radial expansion.The study of the shape characteristics provides a reference for the development of the identification method of macroscopic mechanical parameters of materials using the loading curve.The indentation method can not only determine the macroscopic mechanical parameters of materials,but also can characterize plastic deformation of materials in smaller scale.Strain gradient hardening plays an important role in small-scale plastic deformation.For spherical indentation,it is generally believed that the effect of strain gradient only depends on the size of the spherical indenter.However,in this study,it was analytically proved that indentation penetration influences strain gradient beneath spherical indenter especially when it is deep.Based on the analytical relation of strain gradient,indenter radius and penetration,a method for the determination of material intrinsic length with single spherical indentation was proposed,and the length scales of annealed anoxic copper,(220)single crystal copper,AA5052 and AA7050-T7451 were determined.By comparing the experiment results with the published ones,it was found that this method can give an accurate value of the material intrinsic length.For AA5052and AA7050-T7451,the intrinsic length determined by this method can be regarded as the average value of various micro structures of the alloy beneath the indenter.In addition to identifying the parameters related to the continuous plastic deformation,many researchers also try to identify the fracture toughness of materials by using spherical indentation method.However,these studies did not provide the distribution and detailed formation of damage beneath the spherical indentation.In order to investigate the damage in the view of mechanism,the initiation,evolution and influencing factors of damage of AA7075-T6 and f.c.c.single crystal beneath spherical indenter were discussed in this study.For AA7075-T6,the main forms of damage initiation are the crushing and disbonding of MgZn₂ particles,which is a regular damage mechanism of AA7075-T6.Based on ECM and Bonara damage models,an analytical method for predicting damage distribution under spherical indenter was developed.At the micro level,electron back scattering diffraction(EBSD)results showed that the strain gradient around the microvoids was much higher than the average level beneath spherical indentation.Through CMSG finite element simulations and nano-indentation test,it was found that the strain gradient varied around the edge of microvoid between the maximum and minimum.The above results indicate that the evolution mechanism of microvoid under spherical indentation is the shrinking,expansion and torsion behavior dominated by strain gradient hardening.In this study,a calculation procedure was also proposed to determine the paramters in a stress-strain relation which occupies multiple fitting degree of freedoms with the help of artificial neural network and gloabal optimization methods.Firstly,the database of 355 materials with various parameters was established and the spherical indentation of these materials were simulated to obtain the mapping of material parameters and the loading curves.Then the artificial neural network was optimized by genetic algorithm.Using the optimized artificial neural network and gloabal optimization algorithm,parameters of materials can be identified more accurately by using the loading curve only,the error between the optimized load-displacement curve and the target curve is less than 1%.At the same time,the so-called"mystery material",which means the materials with different stress-strain curves occupy very close loading curves,can also be found with this method,This method provides a good reference for the establishment of data-driven spherical indentation identification algorithm for material parameters.