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