Materials like epoxies and phenolics come to mind here. The overwhelming void in understanding the behavior of these materials for mechanics related applications arises from their complex constitutive behavior. An important aspect of this is physical aging which deals with the changes of physical response as a result of past vitrification history. Closely related to physical aging is accelerated testing by which one attempts to predict long term response on the basis of short term testing. Such an acceleration is generally produced through elevating the test temperature.
Several issues arise with regard to failure.
It may also be the result of rate sensitivity under the elevated stress levels to which a crack tip is exposed. Certainly, this phenomenon will not be understood sufficiently well until progress is made in the constitutive modeling of the material. Steady or monotonic loading can produce time-dependent crack propagation. It is clear at this time that the nonlinear viscoelastic properties of any polymer play a dominant role in understanding or characterizing time dependent crack growth. The details of how they interact with failure in general, other than being precursors, is far from being understood.