Studies on Fatigue Damage Caused by Stresses below the E durance Limit : Probability Distributions of Crack Initiation Lives at Stresses near the Endurance Limit

概要

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In order to investigate the probability distributions of crack initiation lives N_l under stresses near the endurance limit, S50C specimens having two small transverse holes were tested under cantilever type rotating bending stress. Through the analysis of the data the following conclusions are obtained. (1) The growth of the fatigue crack under the stresses near the endurance limit is very slow, but the crack propagates even after 10^8 cycles, repeating acceleration and deceleration. (2) The S-N_l curve, the relation between the stress σ and the crack initiation life N_l, is thought to be composed of three H^^--, F^^-- and S^^--curves, each of which represents the S-N_l relation resulting from a different crack initiation mechanism, where H^^--curve shows the S-N_l relation at stress levels well above the endurance limit σ_W, F^^--curve appears at stress levels near σ_W, and S^^--curve at still lower stress levels. The apparent scatter of N_l under stresses near σ_W can be explained as a result of the superposition of the respective distributions around these curves.
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