Imagine a rubber band stretched and then released – it snaps back to its original shape. That’s elastic behaviour. In physics, a material that returns to its original shape when the applied force is removed follows Hooke’s Law:
\$\sigma = E \varepsilon\$
Where \$\sigma\$ is the stress (force per area), \$\varepsilon\$ is the strain (relative change in length), and \$E\$ is the Young’s modulus – a measure of stiffness.
Analogy: Think of a spring or a rubber band – pull it, it stretches, but when you let go, it returns to its original length.
When drawing a stress–strain diagram, always label the x‑axis as strain \$\varepsilon\$ and the y‑axis as stress \$\sigma\$. Mark the elastic region clearly and note the yield point.
| Stress (σ) | Strain (ε) | Region |
|---|---|---|
| 0 – σy | 0 – εy | Elastic |
| σy – σu | εy – εu | Plastic (Permanent deformation) |
| > σu | > εu | Fracture |
When a material is stretched beyond its yield point, it deforms permanently – it’s no longer elastic. Think of bending a paperclip: once you bend it, it stays bent even after you release it.
In the plastic region, the relationship between stress and strain is no longer linear. The material may continue to deform until it eventually breaks.
Key Terms:
When asked to identify the elastic and plastic regions on a graph, look for the straight-line portion (elastic) and the curve that deviates from linearity (plastic). Remember to write the yield point and the ultimate strength.