1. As the material is deformed beyond the strain that elastic deformation persists, the stress is no longer proportional to strain, and permanent, nonrecoverable, or plastic deformation occurs.
   当材料的形变超出弹性形变发生的范围，其应力将不再与应变成正比，永久的、不可回复的形变发生，即为塑性形变。

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