Considerations in non-propagating crack of pure titanium

To verify the existing theory that non-propagating cracks (NPC) do not exist in titanium (Ti), NPC detection in industrial pure Ti was tried out. The research showed that the NPCs were discovered in specimens that have artificial defect. Two types of specimen; sharp-notched (the root radius is 0.02 mm and the depth is 0.2 mm) and micro-pitted specimens (the radius is 0.25 mm and the depth is 0.25 mm), endured the stressing of fatigue up to 107 cycles. It is considered that the existing theory with respect to the definition of fatigue limit of Ti should be modified. However, we could not detect the NPC in specimen without defects. This phenomenon would be due to the presumption that the NPC is extraordinarily short or crack initiation does not take place. The fatigue limit of Ti is found to not correspond to the threshold stress of crack initiation but to critical stress of crack growth. Furthermore, the measurement on the crack opening displacement (COD) of NPC in Ti reveals that the crack tip was surrounded by the region that had compressive residual stress. The crack tip was opened after stress relief annealing. Consequently, the compressive residual stress around the crack tip induced during crack growth is considered to be the prime factor causing the NPC.