Volume 19, Issue 3 (May 1991)
Effects of Microcracks and Artificial Surface Cracks on Fracture Ductility of a Torsional Prestrained Specimen
The diminution of fracture ductility of smooth specimens and small cracked specimens on surfaces subjected to torsional prestrains was investigated. For smooth specimens, the transition of tensile fracture ductility after a critical torsional prestrain is independent of the existence of microcracks and attributed to the embrittlement of the layers of the specimen surfaces due to the formation of anisotropic helical structures. For cracked specimens, the degree of fracture ductility depends on the crack area and the magnitude of torsional prestrain. Where torsional prestrain is lower than a critical value, the surface crack starts in the direction close to the maximum shear stress, and ductility is somewhat lost from virgin state. If the torsional prestrain exceeds a critical value, the specimen is broken in a brittle manner, and the crack extension shows a helical pattern. This behavior is due to the embrittlement of the specimen surface layer.