Indentation processes in poly(methyl methacrylate). I. Ball indentation

Abstract

The indentation of PMMA is shown to be highly dependent on indenter-PMMA friction and a correction for this effect is proposed. For penetrations greater than half a radius the pressure varies with penetration rate and ball size as predicted by the expanding cavity model of Puttick et al. (1977) provided the indentation diameter is determined under load and the friction correction is applied. This allows assignment of a value to the parameter plastic-zone radius/cavity radius, c/a. Observations of the growth and position of the region of anomalous refractive index beneath the indenter and the fracture surfaces shows the creation of a core of material beneath the indenter which can be identified with the cavity of the plastic-elastic model. This allows an alternative estimate of c/a and both methods give a value of 1.9.