Experimental and analytical studies in crack initiation under fatigue: Part II — Analysis

The experimental results on damage evolution before crack initiation reported in Part I are analyzed. An energy release rate due to damage growth before crack initiation is evaluated with the use of a semi-empirical method for energy calculations and experimental measurements. Correlation of the elementary movements of the damage zone with the energy release rate shows that the rate of damage growth decreases monotonically. This is consistent with the growth behavior of an average damage density within the zone. The experimental data have shown that crack initiation occurred when damage density within a core zone ahead of the notch tip reached a critical level, independent of loading conditions. Assuming that damage evolution is a stress-temperature driven process, it is shown that a first order reaction equation describes reasonably well damage growth within the core zone. The rate constants are evaluated by invoking principles of kinetic theories of fracture and experimental results on crack initiation times. An activation energy for defect nucleation is found to be half the energy for thermal destruction and twice the enthalpy of activation for secondary chain motions below glass transition temperature. This result indicates that chain motions and chain scission are possible rate determining processes leading to crack initiation. This is consistent with the observation that dense crazing precedes crack initiation.