Abstract
Fatigue crack propagation experiments performed on thin, single edge notched, polystyrene specimens show that during slow crack propagation, an intensive zone of crazing surrounds and precedes the propagating crack. The system of the crack and the craze zone constitute a crack layer (CL). The part of the CL ahead of the crack tip, where crazing accumulates prior to CL growth, is called the active zone. The width of the active zone is found to increase by almost an order of magnitude during slow CL propagation. Fractographic analysis reveals two types of discontinuous crack growth bands: multicycle bands and single cycle bands. The point of transition from multicycle to single cycle bands is established and used to extract crack speed from fracture surface markings. The experimental results demonstrate that fracture propagates by the translation, isotropic expansion and distortion of the active zone. Analysis of craze distribution within the active zone reveals that the craze density decreases away from the crack. A considerable difference in the critical energy release rates is observed in specimens fatigued under different loading levels. This is attributed to the difference in the density of crazes at the critical crack tip.
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