Multi-scale approach of the instrumented indentation technique on the fracture toughness estimation

Instrumented Indentation Technique (IIT) is widely used to determine the mechanical properties of materials. The elastic modulus is usually determined by applying the methodology proposed by Oliver and Pharr [1] who supposed that its value is independent of the indentation depth. However, some authors [2, 3] have observed a decrease of the elastic modulus when the indenter displacement increases which allowed them to introduce a continuous damage theory used afterwards to estimate the fracture toughness of ductile materials. The assumption made by the authors is that a damage in the region very close to the bottom of the indent results in the formation of microvoids which leads to the variation of the elastic modulus as a function of the indenter displacement. Starting from this observation, Lee et al. [2] proposed an energy model based on the Griffith’s theory and the continuous damage mechanics (CDM) which states that the elastic modulus variation is related to the fraction void volume through a variable damage, introduced by Kachanov [4], related to the surface density of the microdefects. On the other hand, the works carried out over ductile materials by Li et al. [3] have been performed only with nanoindentation data preventing a discussion on the scale-­‐effect. (...