Nonsteady crack and craze behavior in PMMA under cyclical loading: II. Effect of load history on growth rate and fracture morphology

This is the second of three papers devoted to time or frequency effects on crack propagation in PMMA under cyclic loading with high resolution measure ments of crack and craze growth in the micron range and with subcycle tempo ral resolution. It is demonstrated that the breakdown rate of the craze lea ds to quasi-periodic spurts of propagation and growth retardation in depend ence on both the magnitude of the stress intensity factor and on the impose d load frequency. The surface morphology is governed more by the time requi red to build and deteriorate the craze (process time) than by the amplitude of the stress intensity factor. Three different growth ranges are distinguished: At the highest load levels (K approximate to 1 MPa m(1/2)) multiple crazes occur at various distances from the major crack plane which coalesce and give rise to an incoherent s urface structure not characterizable in terms of a single craze, although t he front of the craze/crack is still represented by a smooth curve; crack a dvance occurs over domains that are larger than a typical craze length. At lower stress levels (K approximate to 0.6 MPa m(1/2)) the craze breaks at d ifferent levels within the craze structure, the spatial breakdown of the cr aze being limited to dimensions smaller than the craze length but with a co rrelation or memory effect that generates terraces ton the fracture surface ) on a scale larger than the craze length. In a transition between this sta ge and the lowest stress level considered, both features may exist simultan eously on a fracture surface, one increasing or decreasing relative to the other. Increase/decrease of one phenomenon relative to the other does not o ccur at a fixed stress intensity range but depends on the past history. Whe n a change in the loading produces a change in the fracture mode, it is pos sible to associate, during the transition, decreasing crack growth rates wi th increasing stress intensity factors. At the lowest loading (K approximat e to 0.35 MPa m(1/2)) a craze breaks essentially through its center leaving a submicroscopically smooth surface; the associated crack growth and craze break down occurs with more or less periodic increments that are each smal ler than the full 'equilibrium' craze length.