Determination of plastic properties of metals by instrumented indentation using different sharp indenters

Indentation testing is a common method to assess the mechanical properties of materials near their surface. The elasto-plastic properties may be determined from the force penetration curves measured in indentation using inverse methods. In this spirit, Dao et al. [1] (Acta Materialia, 49, 2001) have established a forward and a reverse analysis for engineering metals using the equivalent conical indenter of the Berkovich and the Vickers pyramids, which has an included angle θ of 70.3°. Extending Dao's approach, we studied, based on a finite element analysis on elasto-plastic materials, the influence of the included angle of conical indenters (θ=70.3, 60, 50 and 42.3°) and the friction coefficient on the force penetration curves. Based on this analysis, we suggest a more general method for determining the plastic properties of metals. The mechanical behaviour is modeled with the Young's modulus, E, the yield strength, σ_y, and the strain hardening exponent, n. We have shown that friction has a significant effect on the normal force measured on tips having included angles lower or equal to 50°. We have constructed, for each indenter geometry, a dimensionless function relating the characteristic parameters of the loading curve in indentation to the elasto-plastic parameters of metals. These functions have been generalized for any included angle. We show that the use of a second indenter with an included angle lower than θ=70.3° allows us to determine the strain hardening exponent with greater accuracy. Moreover, the sharper the indenter, the better the accuracy.