Time-dependent deformation and craze initiation in PMMA: Volume effects

As part of a study of long-term deformation and failure in glassy polymers, the time-dependent tensile compliance and lateral contraction ratio have been simultaneously determined for PMMA as a function of applied stress at room temperature. In the time range 10² to 10⁶s, measurements of longitudinal and lateral strain were made using extensometers of novel design and a new optical interference technique was developed to serve as a check on lateral displacement data. At low stresses, the time-dependence of the tensile, shear and bulk compliance was derived over an extensive timescale range (10^?8 to 10⁶s) by combining the long-time data with transformed complex modulus and Poisson's ratio results. These data illustrated the large influence of the broad β retardation region on the room temperature creep behaviour and the onset of the glass-rubber or α region at long-times. With increasing stress a relatively large decrease in retardation times associated with the α process was principally responsible for the onset of non-linearity and resulted in an increased merging of the α and β regions. Stress-induced structural changes associated with this effect were indicated by a small initial positive contribution to the volumetric strain attributed to the partial erasing of previous physical ageing in the material. A subsequent decrease in volume might involve the mechanical enhancement of the original ageing. The substantial reduction in α retardation times with increasing stress was paralleled by a decrease in craze incubation times and this result is discussed in relation to craze initiation criteria based on considerations on non-linear viscoelastic response.