建立了一种确定金属材料塑性性能的方法, 即利用有限元数值模拟对纳米压入过程进行反演分析, 确定金属材料的屈服极限和应变强化指数.首先在不考虑材料加工硬化的情况下, 对纳米压入过程进行反演修正模拟,当模拟曲线同正向分析曲线相吻合时, 确定金属材料的代表性应力;其次在考虑不同应变强化指数的情况下, 采用相同的方法确定金属材料的代表性应变; 最后结合量纲分析确定金属材料的应变强化指数,继而确定金属材料的屈服极限. 经过实验验证, 该方法具有较高的精度.

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 stress ε_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 strain "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, ν) expressing the elastic–plastic behaviors of the universal engineering metals. Fom the computational results, a dimensinless function ∏_u is constructed, and then the strain hardening exponent idetermined. At last, substituting the strain hardening exponent n into the power law constitution, the yield stress σ_y of metals is acquired. The examination of 5 kinds of metals from the forward analysis metal materials indicates that the dimensionless function ∏_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 applcation of this technique, the test error of nanoindentation should be maximally reduced.