Abstract
The low-cycle fatigue characteristics of two casts of Type 316 stainless steel have been examined at 400°C in air for specimens with machined and with electro polished surfaces. Fatigue lives of electropolished specimens were found to exceed those of specimens with either ground or turned finishes by a factor of four. Post-failure microscopic examination of these specimens and specimens from interrupted tests indicate that the fatigue life of specimens with either machined or polished surfaces is dominated by crack propagation processes. The difference in the lives of specimens with the two types of surface results from different crack initiation characteristics associated with the two surface finishes. For specimens with electropolished surfaces, crack nucleation takes place at discrete sites around the specimen circumference and leads to the formation of cracks with highly constrained semi-circular fronts. Crack initiation in specimens with machined surfaces occurs at the roots of machining marks and results in a continuous unconstrained front all around the specimen circumference. The growth rate of the highly constrained semi-circular cracks is some four times less than that of the unconstrained crack. It is this difference in crack growth rate which is principally responsible for the difference in fatigue life of the two types of specimen. Little cast-to-cast variation in fatigue properties were observed.
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