Cracking in polymer glasses and evolution at zero stress: Highlighting   discrete long time scale relaxations
  Fracture initiation in glassy polymers with no notch is studied together with the evolution at zero stress in the glassy state. Confocal microscopy observations and auto-correlation methods are used to characterize specimens of polymethyl methacrylate (PMMA) loaded at room temperature and subsequently unloaded. The evolution of the morphology and the location of the cracking submitted to elongation rate up to 8% are reported and analyzed during the zero-stress relaxation. The crackling (longitudinal crack, transverse crack and crazes) takes place mainly within a 10micron thickness layer from the surface and does not extend in the bulk. It is shown that the strain field continues to evolve without stress, and that it can be described by an intermittent retraction of the displacement field. Correlatively the number of crazes or of micro-cavities is increasing in the post-loading state at zero-stress relaxation. The timescales involved in the retraction are of the order of several days reporting thus on so far unknown very slow relaxation timescales. 
S. Mbarek, P. Baroni, L. Noirez
cond-mat.mtrl-sci cond-mat.soft