JOURNAL OF THE MARINE ENGINEERING SOCIETY IN JAPAN

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In order to investigate bending fatigue fracture of crank shafts that occur quite rarely propulsion diesel engine, rotating bending fatigue tests were carried out by using large (60mm in diameter) and small (10mm in diameter) specimens taken from the crank shaft fractured in service.
Statistical analysis in the results evaluate lower confidence limits of bending fatigue strength of crank shafts.
The conclusions are summarized as follows;
1. In an aim to raise the reliability of crank shafts against bending fatigue fracture, it is more effective rather to decrease the deviation of the fatigue limit by using homogenious steel without macroscopic defects than to increase the mean value of fatigue limit.
2. If a mean value σ_W and a sample standard deviation √V of fatigue limit are obtained correctly by the fatigue tests, the failure rate of crank shaft due to bending fatigue can be presumed by r of the following expression;
r= (σ_W-σ_L) ⋅ (1/√V)
where σL is a half amplitude of bending stress at the fillet of the crank shaft. The failure rate may become more than several percents if r<2 and quite small if r>3.

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