---
title: "Short note: elastic and plastic behaviour"
subject: "Civil Engineering Materials"
chapter: 1
key: "semester_3/civil_engineering_materials:elastic-plastic-behaviour"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/elastic-plastic-behaviour
site: BCT Engineering
---

# Short note: elastic and plastic behaviour

Civil Engineering Materials · Chapter 1: Introduction to Civil Engineering Material

- Asked in: 2059 Poush (old course), 4 marks
- Video: [Elastic Deformation and Plastic Deformation | Mechanical Properties of Solids | Don't Memorise](https://www.youtube.com/watch?v=YKpvYF0hVDE) — Sri Chaitanya Academy NEET
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/elastic-plastic-behaviour
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-1

## Question

Write a short note on elastic and plastic behaviours of materials.

## Short answer

Elastic behaviour is reversible and follows Hooke's law below the elastic limit. Plastic behaviour is permanent, caused by dislocation slip beyond the yield point. Structures are designed for the elastic range; plasticity provides ductility and warning before failure.

## Answer

#### Elastic behaviour
A material is **elastic** if it fully regains its original shape and size when the load is removed. Within the proportional limit, stress is proportional to strain (Hooke's law), $\sigma = E\varepsilon$, and the deformation comes from stretching of atomic bonds without any slip of atoms. It occurs below the elastic limit. Steel below its elastic limit is a typical example; rubber is also elastic, but its stress-strain curve is non-linear.

#### Plastic behaviour
Beyond the elastic limit (yield point), the deformation remains permanently after unloading. It results from slipping of atomic planes by the movement of dislocations. Stress is no longer proportional to strain. Clay, lead and mild steel above yield are plastic.

```text
 stress
   |        ,-----  plastic
   |      ,'
   |    /  <- yield
   |  /  elastic
   +----------- strain
 on unloading, the line returns parallel to the elastic slope,
 leaving a permanent (plastic) strain
```

Structures are designed to work in the elastic range, while plasticity gives ductility and warning before failure.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Elastic deformation arises from
   A. slip of atomic planes
   B. stretching of atomic bonds without slip
   C. melting
   D. crack growth
   Answer: B. Elastic = bond stretching. (Key point: Elastic)

2. Plastic deformation results from
   A. bond stretching
   B. cooling
   C. slipping of atomic planes by dislocation movement
   D. creep only
   Answer: C. Dislocation motion. (Key point: Plastic)

3. On unloading from the plastic range, the line returns
   A. along the loading curve to zero
   B. vertically to zero strain
   C. flat
   D. parallel to the elastic slope leaving permanent strain
   Answer: D. Permanent plastic strain remains. (Key point: Unloading)

## Related in Civil Engineering Materials

- [Short note: types of plastic materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Atypes-of-plastic) — Civil Engineering Materials, chapter 10, asked 2×
- [Define thermosetting plastic](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Athermosetting-plastic-define) — Civil Engineering Materials, chapter 10
- [Define plastic material](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Adefine-plastic-material) — Civil Engineering Materials, chapter 10

## Related in other subjects

- [Limiting plastic moment is 1.5 times elastic moment](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Aplastic-moment-rectangular-1-5) — Strength of Materials, chapter 7
- [Elastic, inelastic, linear and non-linear behaviour](https://www.bctengineer.com/question.md?key=semester_4%2Ftheory_of_structures_i%3Aelastic-inelastic-linear-nonlinear) — Theory of Structures I, chapter 1
- [Hardened concrete properties; elastic behaviour](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Ahardened-properties-elastic-behaviour) — Concrete Technology and Masonry Structure, chapter 4
- [Plastic hinge versus elastic hinge](https://www.bctengineer.com/question.md?key=semester_5%2Ftheory_of_structures_ii%3Adefine-plastic-hinge-compare-elastic) — Theory of Structures II, chapter 5