Behavior of Fatigue Crack Propagation : In the Case of Repeated Torque

概要

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There are many problems on behavior of a fatigue crack that are not still revealed by many studies with an electron microscope. Macroscopic behavior of the fatigue crack cannot be completely explained by its electron microscopic one. Medium scopic observation between the macroscopic and the electron microscopic one should be made to fill the gap between them. In this investigation, the vicinity of the fatigue crack tip is observed every several cycles of stress by an optical microscope of 280 magnification and the fracture surface is observed by the electron microscope. The relation between an optical microscopic observation and an electron microscopic one is discusssed and the following conclusions are obtained : (1) It is observed that a fatigue crack intermittently propagates. So, there are dormant terms in a fatigue crack propagation process.(2) The dormant terms ΔN and a crack length Δl which intermittently propagates after a dormant term depend on material and applied stress. The former is determined by alternating stress and the latter determined by maximum stress.(3) Striations are found on the fracture surface on which a fatigue crack intermittently propagates.
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