  Polymer glasses have numerous advantageous mechanical properties in
comparison to other materials. One of the most useful is the high degree of
toughness that can be achieved due to significant yield occurring in the
material. Remarkably, the onset of plasticity in polymeric materials is very
poorly quantified, despite its importance as the ultimate limit of purely
elastic behavior. Here we report the results of a novel experiment which is
extremely sensitive to the onset of yield and discuss its impact on measurement
and elastic theory. In particular, we use an elastic instability to locally
bend and impart a \textit{local} tensile stress in a thin, glassy polystyrene
film, and directly measure the resulting residual stress caused by the bending.
We show that plastic failure is initiated at extremely low strains, of order
$10^{-3}$ for polystyrene. Not only is this critical strain found to be small
in comparison to bulk measurement, we show that it is influenced by thin film
confinement - leading to an increase in the critical strain for plastic failure
as film thickness approaches zero.
