Formation of Factory-Roof in Torsional Fatigue Specimen with Circumferential Notch

The formation behavior of factory-roof in circumferential notched specimen was examined. To achieve an observation of cracks in internal of material, two plastics were used for testing. One was acryl and the other was polycarbonate. Cyclic torsional tests were performed with and without application of static tension. With the polycarbonate, a factory-roof was not formed in fracture surface. However, using acryl, that was observed when cyclic torsion tests were conducted with static tension. Therefore, the discussion was focused at the results in acryl. When the factory-roof was formed, many small cracks were initiated and grew by shear mode, followed by coalescence and branching of them. After that, the crack growth mode was changed from shear mode to mode I. During mode I crack growth, the formation of the factory-roof was observed.