Numerical analysis of the creep of the contact and recovery of the imprint on amorphous polymer surfaces

This article attempts to analyze the viscoelastic behavior of an amorphous polymer during a microindentation test. The viscoelastic behavior of an amorphous polymer (poly(methyl methacrylate), PMMA) is derived from different relaxation tests performed under different applied true strains. A generalized Maxwell model is then used to identify the mechanical parameters of the viscoelastic behavior. The numerical results display good correlation with experiments during the creep phase. The uniaxial relaxation test used to identify the viscoelastic behavior is chosen in relation to the experimental conditions of indentation. The results obtained for the recovery phase allow a first analysis of the strain and von Mises equivalent stress fields during indentation test. The recovery of the imprint left on the surface seems to depend on the location of the strain maxima. If a strain level of 10 % or more reaches the surface of the deformed volume, a permanent imprint is obtained. Otherwise the residual imprint may be considered to be completely healed even if the subsurface has partially yielded during the loading phase or creep time.