The authors have already presented a simple and quick method for measuring elastic modulus by utilizing Hertzian contact theory. It is very important to examine the effect of surface roughness on a relative normal approach between contacting bodies for measuring the elastic modulus. In this work, a numerical analysis verified by several experiments, which takes surface roughness into consideration, was made of the relative normal approach from this point of view. The results show that the effect of surface roughness on Hertzian contact deformation becomes gradually small with a decreasing equivalent elastic modulus, with increasing the normal load, and with increasing the hardness of the contacting bodies. An equation that gives the measure of surface finish necessary to conduct the measurement of elastic modulus accurately is presented based on the analysis.
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