Abstract. The multistage strength degradation theory, which has recently emerged from studies on the material and structural behaviour of concrete, provides a clear description of the mechanism of fatigue. According to this theory, 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. As metal is the main engineering material plagued most by fatigue failure, this newly-established theory needs to be experimentally verified on metal, which is the aim of this study. The obtained test results present strong experimental evidence for the existence of multistage strength degradation processes in metals under cyclic loading, and the strength degradation is clearly triggered by the abrupt change of cracking behaviour. These tests confirm the relevance of the multistage strength degradation theory on metal fatigue, and the engineering implications of the study are discussed.

Under cyclic loading, the material weakening processes in structural members inevitably involve multiple cracking originating from some of the spatially-distributed initial flaws and imperfections, and hence diverse cracking behaviors can be expected. It is known from previous studies on multiple cracks that, the cracking behavior in a structural member can abruptly change as a crack or a number of cracks reach a critical value of crack propagation, causing sudden strength degradation. In this study, by applying sequential loads at different locations of the same FE model of a notched beam, it is shown that this unique strength degradation mechanism can repeatedly occur as cracks propagate under sequential loads, leading to multistage strength degradation of the member. This result is in line with early experimental findings that the load-carrying capacity of a notched concrete beam under bending decreases in a similar fashion as the sizes of multiple initial notches are arbitrarily increased. This study has important implications for understanding the fundamental fatigue mechanisms of various engineering materials.