'Micro-Meso-Process Theory' for Fatigue Source Evolution in Metallic Parts

This paper is an overview of our previous works [1-15]. A “micro-meso-process theory” [1, 3, 7] for quantitative analyses of the critical conditions of fatigue source evolution in metals and metallic parts has been established under comprehensive considerations of the processes occurring in individual weak grains (called as “micro-processes”) and the cooperative processes occurring in their surrounding grains (called as “meso-processes”), as well as the macro-scopical features of the fatigue behaviour. All these processes have stochastic and harmonizing characters, then it is concluded, that the most important process for fatigue source evolution is the formation of “cyclic meso-yielding zones” (CMYZs) under the cyclic loading, in which dislocation motions occur during loading not only within some weak grains, but also within rather many grains surrounding them and, whereafter, during unloading or reverse loading, back motions of dislocations can occur along intersected slip plans in individual weak grains. The critical condition for fatigue source evolution is that the CMYZs get over some critical sizes and the apparent fatigue limit of metal or metallic part is a stress / load needed for formation of such critical CMYZs at the position of weakest link. According to this consideration, a concept of surface / internal fatigue limit of metal is put forward [5, 6, 11, 12]; furthermore, the relationships of internal fatigue limit of metals to their small strain flow stress [8] are established and the procedures for prediction of the apparent fatigue limits of un- and surface-hardened smooth/notched specimens are proposed [9-15].