Nonsteady crack and craze behavior in PMMA under cyclical loading: III. Effect of load history on cohesive force distribution on the craze

Measurements of crack opening and craze profiles are made under a range of loading histories including cyclical deformations that lead to nonsteady cr ack propagation histories. Of particular interest is the comparison of the distribution of traction transmission of a newly formed craze relative to a cyclically stressed one as it approaches the slow-down phase. Real time, i nterferometric measurements provide precise and multiple craze profiles dur ing individual cycles. Cyclic deformations reduce the stiffness of a craze in its center resulting in a stress drop as part of the craze strength evol ution; also, its thickness changes nonuniformly during the acceleration/ret ardation phases of the advancing craze/crack. The implications are, that fo r quasi-statically formed crazes the craze material can be reasonably well characterized by a stress-strain relation, while that is no longer readily true for a cyclically deforming crack, since rate dependent characteristics of the craze and bulk material intervene. Cracks unloaded as part of cycli cal deformation histories exhibit crack closure (compression) near the trai ling end of the craze.