FRACTURE PROPAGATION PATHS UNDER MIXED-MODE LOADING WITHIN RECTANGULAR BLOCKS OF POLYMETHYL METHACRYLATE

Mixed mode I+III loading of a fracture front results in out-of-plane p ropagation into echelon stepping fractures. Because a planar fracture geometry is the exception rather than the rule, and because the introd uction of even a minor component of mode II or III loading is known to promote out-of-plane propagation, an understanding of mixed mode frac ture growth is imperative to analyze fracture behavior. We have loaded cracks in mixed mode I+III within polymethyl methacrylate (PMMA or Pl exiglas) rectangular blocks resembling conceptual fracture mechanics m odels of mixed mode loading and have analyzed the resulting geometries . The observed angle of twist of echelon fractures from the parent cra ck plane increases with the ratio K-III/K-I and falls below theoretica l predictions. Fracture propagation paths depend not only on the load ratio applied but also on sample geometry, loading configuration, and interaction among growing fractures. Sample geometry and loading confi guration are approximately accounted for using analytical determinatio ns of the stress intensity factors. We propose that interaction among growing fractures may contribute to the discrepancy between theoretica lly predicted twist angles and those observed in these and other mixed mode I+III experiments. Analysis of these experimental results has mo tivated the design of a new sample and loading configuration to test t he propagation paths of uniformly loaded mixed mode I+III fractures.