Requirement of a robust method for the precise determination of the contact point in the depth sensing hardness test

摘要

The fundamental quantities in the depth sensing hardness test are force and displacement. The penetration depth used for the calculation of a certain hardness parameter (for instance HU, Eq. (1)) makes it necessary to determine the contact point. As shown in the paper, fitting of the force-displacement curve, F(h), by a second order polynomial and extrapolating to F = 0 is not always the right way to achieve a sufficiently small uncertainty of depth. The feature of that procedure is analysed by a simple mathematical model. In dependence of the testing force, the scatter of the experimental data, and the required limit of depth uncertainty, a profound selection of the fitting range, the fitting function, and the extrapolation function should be done.