The Effect of Specimen Geometry on JIc

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Abstract
Rigid-plastic slip line field analysis shows that fully plastic flow fields and hydrostatic stress elevation are greatly influenced by geometry. This argues against a one parameter fracture criterion, such as JIc, which purports to work in the plastic range. Two specimen geometries, a center cracked panel and a bend bar were tested to provide a critical evaluation of the JIc concept. An intermediate strength Ni-Cr-Mo-V rotor steel was used in the investigation. Results show that JIc is a consistent fracture criterion for plane strain behavior for essentially elastic to fully plastic conditions. This work supports the contention that a plastic crack tip singularity is a dominant consideration for crack initiation even in fully plastic bodies. Bend bars of two thicknesses were tested to show that nearly plane strain behavior can be achieved with conventional geometries as much as an order of magnitude smaller than sizes required for linear elastic plane strain fracture toughness tests. The method used for calculating the J integral for experimental load versus deflection curves is explained in detail.

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