纯弹性断裂总是极少的，因之在加载应力作用下，裂纹顶端总是存在着或大或小的塑性区，存在着裂纹与位错的交互作用。
Pure elastic fracture is always rare, so th at under the action of stress, a plastic zone(large or small), and interactions between crack and dislocations always exist at the crack tip.
材料疲劳断裂主要是因为试样在交变应力的作用下上产生滑移最后致使位错塞积而导致的。
The main reason for the fatigue fracture of material is that the specimens under action of alternating stress engender slip, finally resulting in dislocation stacking.
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