Mobile dislocations meet some obstacles, then appear stress concentration .When the stress becomes greater than interatomic bonding force ,cracks nucleate, propagate, finally lead to fracture .

• 运动的位错遇到了某种障碍；就产生了应力集中；应力大到可以破坏原子间的键合力时；裂纹开始形核；裂纹长大导致断裂.