The role of influence coefficients in hardness measurements: A case study in Rockwell hardness measurements

In the field of hardness measurements, a problem arises when trying to understand the effect of different measurement parameters (i.e. speed of the indenter, force, thermal drift) on the measurement itself. Since the mathematical models defining hardness scales do not consider such factors, additional influence parameters are introduced linearly via influence coefficients, obtained experimentally and thus characterized by uncertainties. However, uncertainties of the influence coefficients have never been considered in the evaluation of the com-bined standard uncertainty of hardness measurements. In this paper, the law of propagation of uncertainty is applied taking into account the uncertainty contribu-tions of the influence coefficients. We apply such a procedure to a case study that shows how the presented method can be used to determine the tolerance limits of the testing cycle parameters given in the related standards and verify that the actual tolerances assure hardness variations inside the expected uncertainty of the method.