EXTRACT THE PLASTIC PROPERTIES OF METALS USING REVERSE ANALYSIS OF NANOINDENTATION TEST

Using traditional methods to evaluate mechanical properties of bulk materials is not applicable for metal surface studying and metals with very small volume.Nanoindentation testing at very low load is a new successful technique for study of mechanical properties on small scales or near surfaces.However,so far there is not a robust approach to determine plastic properties of metal materials using nanoindentation test.The aim of this paper is to present a method for determining the plastic properties,e.g.the true plastic stress-true plastic strain relation of metals combining nanonindentation test and finite element simulation.This methodology contains three main parts.Firstly,considering the special case of metals without strain hardening,the representative stressσ_r is determined by varying assumed representative stress over a wide range until the reverse and forward loading curves are consistent.Then,also by comparing the reverse and forward loading curves,the representative straine_r is obtained,but with different values of strain hardening exponent n, which are in the range of 0—0.6.Secondly,a series of simulations are performed for 124 combinations of each parameter(E,σ_y,n,v) expressing the elastic-plastic behaviors of the universal engineering metals.From the computational results,a dimensionless functionΠ_u is constructed,and then the strain hardening exponent is determined.At last,substituting the strain hardening exponent n into the power law constitution,the yield stressσ_y of metals is acquired.The examination of 5 kinds of metals from the forward analysis metal materials indicates that the dimensionless functionΠ_u has generality and the strain hardening exponent has stability and uniqueness.The accuracy of this method is also examined by comparing the elasto-plastic properties of practical metal AISI 304 steel obtained from nanoindentation test and finite element simulation with the tensile test results.In order to make the reverse analysis results get higher precision,in the practical application of this technique,the test error of nanoindentation should be inaxinially reduced.