Experimental measurement and numerical simulation of the plastic strain during indentation and scratch tests on polymeric surfaces

For elastic-plastic contacts, we propose a complete description of the plastic strain field beneath the indenter during indentation and scratch with a spherical indenter (with R, the tip radius), as a function of the testing conditions, defined by the geometrical strain, noted a/R (with a, the contact radius), and the local friction coefficient mu(loc). The main parameter of the description is the level of the plastic deformation imposed during test into amorphous polymeric surfaces, related in first approximation to the ratio a/R. An equivalent average plastic strain, noted (epsilon(P))(av), is calculated over a representative plastically deformed volume, both for indentation and scratch tests. The equivalent average plastic strain (epsilon(P))(av), is observed to increase with the ratio a/R, as predicted by the empirical Tabor's rule, but also with the local friction coefficient mu(loc), for a given ratio a/R, especially during scratching. The plastic zone dimensions and the plastic strain gradient developed beneath the moving tip are shown to depend both on the geometrical strain a/R and also on the friction coefficient mu(loc).