| The short rod fracture specimen is used to examine the effects of prestrain, in the form of low cyclic fatigue and tensile predeformation, on the fracture behavior of plain carbon steels, of approximately 0.18% carbon content, and a series of tempers of 2024 aluminum alloy. In the case of cyclic predeformation, it is found that all materials used in this study cyclically harden, and this results in a decrease in fracture toughness. Tensile predeformations of 2%, in the case of the aluminum alloys, and 10%, in the case of a 0.15% C mild steel, result in no change in the measured fracture toughness.; A dynamic computer simulation utilizing dislocations configured in a dislocation cell wall substructure segment near a sharp crack tip is used to model the effects of the dislocation substructure produced by large amounts of plastic deformation on the fracture behavior of a model material. Plastic zone formation and fracture behavior are monitored under a wide range of experimental conditions. It is found when using this model, that the effective stress intensity at the crack tip, at any value of applied stress intensity, is constant and is controlled by the dislocation emission criteria. When holding the dislocation emission criteria constant, but varying other material constants, the plastic zone formation will vary to keep the effective K - applied K behavior invariant. Because of this behavior, the model indicates that no change will be seen in fracture toughness of a material when a dislocation substructure is introduced until the crack begins to propagate. The presence of the dislocation substructure results in added stability to crack growth over a small distance in which the crack is approaching and propagating through the cell wall.; Numerical computations are performed to explore the effects of experimental noise on crack growth stability during a short rod fracture test. It is found that except in special cases, the expected magnitude of the experimental noise is insufficient to cause crack growth instability. The apparent instabilities exhibited in the experimental section are attributed to interaction of the crack tip plastic zone with the end of the specimen. |
