塑性变形首先发现在裂纹顶端，当工作载荷超过了普通屈服时，则传播到物体中。
Plastic deformation takes place first at the crack tip and spreads across the solid as the applied load is increased beyond general yielding.
在准脆性断裂情况下，裂纹顶端存在一个不能忽略其影响的小范围塑性区。
In the case of sub-brittle fracture, at the crack tip there is a small plastic region of which effect should not be neglected.
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.
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