17. Power Spectrum Density Effects on Strain Cycling Fatigue Life
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概要
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The effects of power spectrum density of strains on the fatigue life of a pressure vessel steel were examined by random loading fatigue tests. Different three types of power spectrum density of strains, S_1 (symmetric, single peak shaped), S_2 (asymmetric, single peak shaped) and S_3 (two peaks shaped) were used. One problem in a comparative discussion of the fatigue life by random loading with that by constant amplitude loading is the definition or representative expression of the fatigue life under random loading. In this paper, a new expression N_<f(rep)> was defined with an idea of representative frequency. It was found that the fatigue life corresponding to different strain spectrum could be put in order in a lump with r.m.s. value of strains by using the N_<f(rep)>. In the same way, the constant amplitude fatigue data and the random fatigue data were compared with each other using N_<f(rep)> and r.m.s value of strains, showing a good agreement. A life estimating' method TYPE1 on the basis of r.m.s. value of strains and N_<f(rep)> was proposed. Mean hysteresis energy W^^- was newly defined. W^^- corresponding to a particular r.m.s. value of strains showed a little lower value (about 90%) in the random fatigue test than in the constant amplitude fatigue test. By putting the fatigue life N_<f(rep)> in order with the mean hvsterersis energy W^^-, each group of the random fatigue data and the constant amplitude fatigue data was located on a respective line on log-log coordinates. The two lines were almost parallel, and the ratio of fatigue life was found to be nearly equal to 0.8. A method for life estimation was newly proposed, TYPE2 obtained on the basis of W^^- and N_<f(rep)>. Comparing the two proposed methods, TYPE1 is much easier to apply, and TYPE2 needs more information although it gives more precise estimation. In addition, verification of the Miner's law was also carried out using three kinds of wave counting methods, i.e, the range count, the hysteresis loop count and the peak count methods. As a result, it was found that a proper counting method had to be selected according to the r.m.s. value of strains for application of the Miner's law.
- 社団法人日本船舶海洋工学会の論文
著者
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Iida Kunihiro
Emeritus Professor Of Thc University Of Tokyo
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TAKAHASHI Ichihiko
University of Tokyo
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Takahashi Ichihiko
University Of Tokyo:ship Research Institute