Abstract
Low cycle fatigue tests and fatigue crack growth rate tests were conducted at ambient temperature on fine and coarse-grained IN 718 of which grain size numbers were ASTM No. 10 and ASTM No. 5, respectively. In the low cycle fatigue tests on fine-grained material at maximum stress of 1180 MPa and R=0, fatigue lives of ASTM standard cylindrical specimens ranged from 50000 to 70000 cycles, and every fatigue crack was initiated from the defect on the surface of specimen, i.e., nonmetallic inclusion or carbide. The Fatigue crack growth rate test showed that small surface cracks in the coarse-grained material were propagated considerably faster than those in the fine-grained material. If the small surface cracks started to be propagated from the defects immediately after the start of cyclic loading, the difference in fatigue life between fine-grained material and coarse-grained material resulted from the difference in the small surface crack growth rate.
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