Extrinsic plastic hardening of polymer thin films in flat punch indentation

Confined geometries offer useful and experimentally amenable mechanical testing arrangements in which to study the molecular micro-structural processes govern plastic yield stress environments dominated by hydrostatic pressure over shear. However, changes macroscopic strain behaviour that result from switching an unconfined mode such as uniaxial compression a confined one are often overlooked display surprising level of complexity, even for simple elastic constitutive models. Here we report confinement induced hardening effect polystyrene thin films achieved through repeated loading with cylindrical flat punch whose diameter is many times initial film thickness. This high aspect ratio combines constraint provided material surrounding contact generate state indented region, rendering deformation dimensional. By into domain, achieve 66% increase stress, 0.3 GPa 0.5 GPa. Through finite element simulation analytic modelling principal stresses strains, show this arises not intrinsic structure material, but rather residual imparted during loading. We contrast glassy physical ageing thermal cross-linking.