Theoretical Studies On The Characterization Of Uniaxial Mechanical Properties Of Metallic Materials By Indentation Method

The elastoplastic mechanical performance of metal materials is very important in engineering application and daily production life,such as bridge construction and the micro/nano materials.We usually use the uniaxial tension method to measure the stress-strain curve of the metal material,and then determine the elastoplastic mechanical performance of metal material.As the size of material becomes smaller and smaller,the traditional uniaxial drawing method can no longer satisfy the measurement of small size metal materials.Therefore,the mechanical properties of materials extracted by indentation method have been developed.From the indentation method,a series of mechanical parameters are obtained,such as hardness,elastic modulus,interface binding energy and so on.Some studies have tried to extract stress-strain curves from these parameters,but the relevant theoretical studies are complex and lack the feasibility of engineering application.In order to solve the problem of obtaining the stress-strain curve of metal materials by indentation method,this thesis starts from the dimensional analysis and obtains the quantitative relationship between the indentation process parameters and the stress-strain curve through the finite element numerical simulation.The innovative research results are mainly as follows:1. Through dimensional analysis,finite element numerical simulation?more than 1200 set of metal materials indentation theory calculation?and mathematical formula fitting,we successfully got the detailed expression of the three dimensionless functions of Vickers indenter which is widely used for the engineering application.The reliability of the inverse analysis is verified by numerical simulation,and the error can be controlled at about 5%.For different materials,even if the elastic-plastic index is completely different,as long as the dimensionless variables?_y/E and n are the same,the indentation morphology is completely the same..2. Starting from size effect of indentation hardness,we introduce the strain gradient plasticity theory into the ABAQUS subroutine,and numerically simulate three specific materials.Through three dimensionless functions?₁,?₂?₃ and inverse analysis,we successfully obtain the elastic-plastic mechanics performance of three kinds of materials considering size effect.Taking the indentation process of Ag as a typical case,the evolution relationships of elastic modulus,yield strength and hardening index with indentation scale were systematically analyzed.The results show that the sensitivity of elastic modulus and hardening index to indentation scale is relatively low,while the yield strength has strong scale sensitivity,which proves that the stress-strain curve also has size effect.Based on the relationship between indentation hardness and yield strength,the elastoplastic mechanical properties obtained by the Nix-Gao model were successfully described.3. From the theory of mechanics,we derive two inferences:the same morphology and the similar morphology.Combined with these two inferences,the law of load-displacement curve and indentation morphology is further studied,and the mechanical behavior in indentation process is further understood.By means of dimensional analysis and finite element simulation,the two inferences of the same and similar morphologies are tested.In addition,the function of unloading curve described in the literature is compared and discussed.The sensitivity of Poisson's ratio,a dimensionless parameter in the indentation method,is also studied.