Nonsteady crack and craze behavior in PMMA under cyclical loading: I. Experimental preliminaries

This is the first of three papers devoted to the study of nonsteady crack p ropagation under cyclic loading in polymers, specifically PMMA. Drawing on experimental tools and measurement methods offering high spatial and real-t ime resolution at the subcycle level, we find that the change in craze beha vior with load history is more complex than believed to date. At room tempe rature the change in fatigue cycle frequency on the order of slightly more than a decade gives rise to considerably different fracture surface morphol ogies, which are load history dependent. These observations are categorized and deductions for the variable cohesive force transmission by the craze a re considered. In this first paper we outline the design of a special loadi ng device which allows ultra-precise load or displacement control commensur ate with the high resolution measurements of the crack tip material respons e. This development illustrates the precision of control for the experiment s described in the subsequent papers. In addition, we delineate the softwar e control and data acquisition methods to highlight the spatial resolution which provides the micron accuracy for measuring sub-cyclical crack propaga tion necessary for the later accounts.