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The cyclic deformation behaviors of single- and polycrystals were well documented in the past three decades. it has been recognized that there existed great difference in the fatigue damage mechanisms between single- and polycrystals, which can be mainly attributed to the effects of grain boundaries (GBs) and the crystallographic orientations. In the present work, a series of research work mainly on copper bicrystals with various GBs and different component crystals have been systematically investigated, including the macroscopic cyclic stress-strain responses and fatigue damage mechanisms in micro-scale. Firstly, direct evidence is offered to show the obvious strengthening effect caused by the large-angle GBs during cyclic deformation. The data of cyclic stress-strain responses will be presented to show the effects of the GBs and the crystallographic orientations in the macro-scale. Next, the influence of various GBs on fatigue cracking behavior will be considered for the crystalline materials. Clear evidences are shown that the interactions of persistent slip bands (PSBs) with various GBs play a decisive role in the intergranular fatigue cracking during cyclic deformation. it is suggested that the intergranular fatigue cracking strongly depends on the interactions of PSBs with GBs in fatigued crystals, rather than the GB structure itself. The underlying fatigue damage mechanisms were further discussed in terms of the different interactions between PSBs and three types of GBs. (c) 2008 Elsevier Ltd. All rights reserved.