Fatigue Crack Propagation under Service Loading (The 2nd Report)

Crack extension by fatigue may sometimes cause a catastrophic failure of structure. For the safety assessment of structure, therefore, it is indispensable to estimate fatigue crack growth under service loading. In most cases actual service loads are random, and the estimating method for fatigue crack growth under random loading must be established.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 convenient as a conventional and routine test. Many investigations have been carried out in order to clarify the relationship 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 has not been clarified yet.In this study, fatigue crack propagation tests under random loading-narrow band random for its simplicity-are carried out as well as constant amplitude loading tests for comparison. Crack closure phenomena of the specimens are automatically recorded over a long period of fatigue cycles using electric potential method 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.