Fracture process and molecular kinetics of PMMA

The critical value of the stress intensity factor, K_1c, (fracture toughness) has been measured for poly(methyl methacrylate) (PMMA) over a wide range of testing speed (K ? 4 to K? ? 50 × 10⁶ lb/in^3/2-sec) and temperature (from ?197°C to +21°C) in air and inert gas, by use of single-edge notch, double cantilever beam (normal and compact types) and instrumented impact tests. Some features of the toughness curves were found to be subject to time–temperature shifts and were explained in terms of relaxation motions of parts of the polymer molecule, (second-order thermodynamic transitions). Correlation with published data on mechanical energy losses in vibration experiments, dielectric losses and NMR results provided an identification of the operative relaxations. The coincidence of the fracture mode transition in the glassy state with the peak of the β relaxation was observed and the trend of K_1c values in a transition region was attributed to the close relationship of K_1c to the complex modulus for the hard glassy state. Impact and propagating crack values of K_1c have been discussed in general terms, and the limitations of fracture mechanics in studying a time-dependent material property have been considered in the light of recent rheological studies on the fracture of polymers.