Waves An understanding of colour from Cambridge IGCSE/O Level Physics or equivalent is assumed.
Elastic and Plastic Behaviour
What is Elastic Behaviour?
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.
Exam Tip
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–Strain Diagram
| Stress (σ) | Strain (ε) | Region |
|---|---|---|
| 0 – σy | 0 – εy | Elastic |
| σy – σu | εy – εu | Plastic (Permanent deformation) |
| > σu | > εu | Fracture |
What is Plastic Behaviour?
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:
- Yield Strength (σy) – stress at which permanent deformation begins.
- Ultimate Strength (σu) – maximum stress the material can withstand before fracturing.
- Fracture Point – where the material breaks.
Exam Tip
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.
Practical Examples & Analogies
- 🧪 Rubber Band: Elastic – stretches and returns.
- 🔧 Steel Rod: Elastic up to a point, then plastic deformation before breaking.
- 📐 Paperclip: Bending it once – plastic deformation; bending it back – elastic.
- 🎯 Bridge Design: Engineers use elastic behaviour for safety margins but must account for plastic deformation under extreme loads.
Key Takeaways for the Exam
- Write the equation of Hooke’s Law and explain each variable.
- Draw a clear stress–strain diagram and label elastic, plastic, and fracture regions.
- Define yield strength, ultimate strength, and explain their significance.
- Use analogies to explain why materials behave elastically or plastically.
- Always check units – stress in Pa (N/m²) and strain is dimensionless.
Revision
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