Study on Fatigue Strength Design of Welded-joint under random loading

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The fatigue crack propagation tests using structural model specimens are carried out under constant loading and the "Storm Model" block loading. Also study on the reliability of the current fatigue assessment methods and the application of fracture mechanics to fatigue assesment are made.Moreover, for the purpose of proper understanding of the fatigue crack opening/closure phenomenon, detailed observations of change in the strain near fatigue crack tip using "High-precision Compliance Measurement System" and CCT specimens are carried out.As a result, the following four main points are obtained.1) The fatigue assessment based on S-N diagram obtained from constant loading fatigue tests can not take into account the effect of past loading history. So the accuracy of this assessment turn out to be extremely lowered under some loading patterns. (for example, the loading pattern in which retardation is remarkable)2) The fatigue assessment based on S-N diagram obtained form block loading fati … More gue tests can take into account the effect of the loading pattern within one block. So the accuracy of this assessment is higher than the above mentioned method 1) but it even can not take into account the effect of interaction of different block, then further high accuracy of this assessment depend on how quantitatively the block loading model reflects the actually loading pattern.3) The fatigue assessment based on fracture mechanics can take into account the effect of past loading history, so the accuracy of this assessment could be higher than above mentioned method 2) if proper fatigue crack propagation model was established. Also further high accuracy of this assessment depend on proper understanding of the fatigue crack propagation behavior especially of the fatigue crack opening/closure phenomenon.4) Successful measurement of the hysteresis curve of load vs.strain with high accuracy was made and, introducing the curve of compliance and the change rate of compliance during one cycle we succeeded in the automatically measurement of the Closing load and RPG load. Less