The Elastoplastic Response of Poly(Methyl Methacrylate) to Indentation

This paper describes an experimental and analytical study of the contact compliance and hardness of a glassy polymer, poly(methyl methacrylate), which has an elastoplastic response to indentation using rigid conical and spherical indenters. The experimental method and the associated intrinsic errors are described. The nature and importance of these errors are reviewed and a variety of corrections are applied. The potential errors addressed include the zero error, the indenter tip defect correction and the problems associated with various extrapolations and interpolations of the experimental data necessary in order to abstract the material characteristics of the polymer. The analytical description of the data has been facilitated by an adaptation of the Box-Cox transformation (Box, G. E. P. & Cox, D. R. 1964 J. R. Statist. Soc. 26, 211), which is shown to be a most effective way of abstracting the required material characteristics data and minimizing the influence of the intrinsic errors inherent in the compliance method. The deduced material response characteristics of the polymer, in particular the Young's modulus and the yield stress, are shown to be consistent with data obtained by conventional mechanical testing methods.