A Probabilistic Approach to the Fatigue Crack Propagation under Non-stationary Random Loading

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Crack extention by fatigue may sometimes cause a catastrophic failure of structure. For the safety assessment of structure, it is indispensable to estimate fatigue crack growth under service loading. In most cases actual service load are random, and the estimating methor for fatigue crack growth under random loading must be setablished.For practical use, it is very preferable that the prediction of fatigue behavior in real structures under random loading could be made using the data obtained by constant amplitude loading test which is simple and routine test. Many investigations have been carried out in order to clarify the releationship between fatigue behaviors under random loading and constant amplitude loading, recently based on the concept of "crack closure" proposed by Elber. In these investigations, however, pure random load has not been applied because of its complexity, and the relationship nas not been clarified yet.In this study, fatigue crack propagation tests under random loading are carried out as well as constant amplitude loading tests for comparison. Crack closure phenomna of the specimens are automatically recorded over a long period of fatigue cycles aided by micro-computer system. It is shown that consideration of crack closure is essential for examination of fatigue crack growth and fatigue behavior under random loading may be predicted from constant amplitude test. Furthermore, fatigue tests are carried out under the condition of the same constant stress amplitude on many specimens. The results are analyzed using Bogdanoff's probabilistic model (B model) based on Markov chain concept. It is shown that the B model has the possibility for proveding a useful tool for examining the statistical properties of fatigus crack growth.