FRACTURE SURFACE INTERFERENCE EFFECTS IN MODE-III

In this experimental investigation, the effects of fracture surface in terference upon the torsional properties of steel alloys were quantita tively characterized. A cyclic compression pre-cracking technique was employed to produce microscopically fine crack fronts in circumferenti ally notched cylindrical test specimens. The abrasion and deformation that the resulting fracture surfaces underwent during the application of torsional loading was studied in detail. The interference of the fr acture surfaces was observed to enhance the measured torsional stiffne ss of a mode III specimen by as much as 70%. The most intense abrasion effects occur close to the crack tip where the separation between adj acent fracture surfaces is relatively small. Compliance techniques, wh ich are considered standard practice in the construction of crack resi stance curves in mode I loading applications, were applied to determin e their accuracy in mode III loading applications. These techniques, i n comparison with direct post-fracture crack Length measurements, were found to produce unreliable results in mode III due to the effects of fracture surface interference.