Papers by Author: Yu Kui Gao

Abstract: 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].

Abstract: The characteristics of compressive residual stress fields induced by shot peening in 40CrNi2Si2MoVA, 16Co14Ni10Cr2Mo, 30CrMnSiNi2A and 0Cr13Ni8Mo2Al ultra-high strength steels, which are used widely in aeronautical industry were investigated, and the change of surface integrity including surface residual stress, surface roughness as well as its effects on fatigue properties were investigated. The results show that the fatigue limits of ultra-high strength steels can be increased by shot peening because the surface integrity can be ameliorated by shot peening, and that for a given steel there is a appropriate peening intensity under which the fatigue property of this steel is optimum. Finally, a judgement for the optimization condition of shot peening process is proposed based on a theory of micro-meso processes of fatigue crack initiation and experimental results. The technique should be considered to be optimum, if the fatigue crack source of shot peened specimen has been moved to the internal matrix metal region beneath the hardened layer; and its apparent fatigue limit has been improved and got to a value, which is near to that predicted according to the concept of surface/internal fatigue limit.