**Strain energy** is the energy stored in a material or structure when it undergoes deformation due to an applied load. It is a result of the internal forces and stresses that develop within the material as it deforms elastically. In structural engineering, strain energy plays a significant role in understanding how materials and structures behave under different loading conditions.

### Key Concepts of Strain Energy:

1. **Elastic Deformation**:
- Strain energy is only stored when the deformation is elastic, meaning the material returns to its original shape after the load is removed. If the deformation is plastic (per