The multistage strength degradation theory states that fatigue is caused by the sporadic sudden change of cracking behaviour in a system under cyclic loading, leading to intermittent strength reduction of the system and its eventual failure. To prove this theory, an experimental study on the load-carrying capacity of notched concrete beams was carried out first, in which the sizes of several notches were enlarged sequentially to simulate the growth of cracks under repeated loading. A multistage strength degradation relation between the maximum load and notch size was obtained. Next, numerical studies of the same problem using sequential loads for propagating multiple cracks in the beam were performed, and the results presented several multistage strength degradation processes based on the numerical simulation of crack propagation. This review paper introduces these experimental and numerical studies on this newly-established fatigue theory, with an emphasis on the latter.
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