Explain the difference between plane stress and plane strain conditions in FEA.

Plane stress applies to thin plates where σz = 0 but εz ≠ 0 (free to expand in thickness). Plane strain applies to thick sections or long bodies constrained in one direction where εz = 0 but σz ≠ 0. Plane stress is used for sheet metal stamping analysis, while plane strain is used for dam cross-sections, tunnels, and long cylinders. Wrong assumption leads to significant errors in FEA results.