A series of fatigue crack growth experiments was conducted on a transparent polymer (PMMA) using a surface flaw geometry in bending. A Newton interferometric technique was used to measure crack opening displacements and closure for four different loading histories. Results are presented for conditions of constant applied stress-intensity factor along the crack face free surface, at the crack tip in the depth direction, for constant applied load range, and for a block loading sequence.
Three distinct crack geometries are identified and are found to be a function of crack size. The crack opening displacement patterns are found to be strongly influenced by the formation of a void internal to the crack. Three distinct closure loads are identified and measured for surface flaws of various crack depths and aspect ratios. The various closure loads are used to define effective stress-intensity ranges. Life predictions, crack growth rates, and aspect ratios are used to evaluate the applicability of the various closure loads at different locations. It is found that the same closure load does not determine the best value of effective stress intensity at all points along the crack tip in the surface flaw geometry.