Slow crack propagation data in PMMA previously published by Atkins et al. are reanalyzed in terms of a thermodynamic model in which the activation area is inversely proportional to the crack extension force. Important thermodynamic activation parameters are derived from the data. A correlation between the activation area and the crack extension force is observed in many brittle materials; it is compared to Li's general correlation for activation areas in creep.
* As a first approximation, it is assumed that the modulus and Poisson's ratio are independent of rate at constant temperature and, therefore, ~ is proportional to K 2. ** Although R is numerically equal to the crack extension force qd during quasi static steady state crack propagation, we prefer to use qd, which has the meaning of a mechanical force, in the thermodynamic equations rather than R, which designates a material property. *** As it was observed by Atkins et al. [3], their data taken at temperatures of 273"K and below do not follow the same relationships as those taken at higher temperatures. This is probably an indication that a different rate controlling process is occurring at lower temperatures. We found that for the low temperature data at 273ยฐK and 253"K, a linear relation is obtained between log v and l/qd over the whole range of crack velocities, which, according to [I], corresponds to an activation area of the form A* = CI~ .2, where C is found to be independent of T.