弹性的定义：
Mase的定义是：Elastic behavior is characterized by the following two conditions: (1) where
the stress in a material is a unique function of the strain, and (2) where the
material has the property for complete recovery to a “natural” shape upon
removal of the applied forces.
所以弹性并不紧紧指变形可以恢复，另一个重要的特点是应力应变的一一对应
