A plane strain problem (in $\text{X - Y}$ plane) must satisfy the condition:
- $\sigma_{z z}=0$
- $\epsilon_{z z}=\epsilon_{x z}=\epsilon_{y z}=0$
- $\sigma_{x x} \neq \sigma_{x y} \neq \sigma_{x z} \neq 0$
- $\epsilon_{x x} \neq \varepsilon_{y y} \neq \epsilon_{x y} \neq 0$
