Effects of Microcracks and Artificial Surface Cracks on Fracture Ductility of a Torsional-Prestrained Specimen

摘要

The diminution of fracture ductility of smooth specimens and small cracked specimens on surfaces subjected to torsional prestrains was investigated. For the 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 surface thin layers of the specimens due to the formation of anisotropic helical structures. For the cracked specimens, the degree of fracture ductility depends on the area of the crack and the magnitude of torsional prestrain. In the case where the torsional prestrain is lower than a critical value, the surface crack starts in the direction close to the maximum shear stress, and ductility remains within the specimen. If the torsional prestrain exceeds a critical value, the specimen is broken in a brittle manner, and the crack extension shows the helical pattern. This behavior is due to the embrittlement of the specimen surface layer.