Elastic & Plastic Deformation
- Elastic deformation: when the load is removed, the object will return to its original shape
- Plastic deformation: when the load is removed, the object will not return to its original shape or length. This is beyond the elastic limit
- Elastic limit: the point beyond which the object does not return to its original length when the load is removed
- These regions can be determined from a Force-Extension graph:
Graph showing the elastic limit
Below the elastic limit, the material exhibits elastic behaviour and returns to its original shape. Above the elastic limit, the material exhibits plastic behaviour
- Elastic deformation occurs in the ‘elastic region’ of the graph. The extension is proportional to the force applied to the material (straight line)
- Plastic deformation occurs in the ‘plastic region’ of the graph. The extension is no longer proportional to the force applied to the material (graph starts to curve)
- These regions are divided by the elastic limit
Brittle and ductile materials
- Brittle materials have very little to no plastic region e.g. glass, concrete. The material breaks with little elastic and insignificant plastic deformation
- Ductile materials have a larger plastic region e.g. rubber, copper. The material stretches into a new shape before breaking
Brittle and ductile materials
Stress-strain curve for a brittle and ductile material
To identify these materials on a stress-strain or force-extension graph up to their breaking point:
- A brittle material is represented by a straight line through the origins with no or negligible curved region
- A ductile material is represented with a straight line through the origin then curving towards the x-axis
Worked example
A sample of metal is subjected to a force which increases to a maximum value and then fractures. A force-extension graph for the sample is shown. 
What is the behaviour of the metal between X and Y?
A. both elastic and plastic
B. not elastic and not plastic
C. plastic but not elastic
D. elastic but not plastic
ANSWER: C
- Since the graph is a straight line and the metal fractures, the point after X must be its elastic limit
- The graph starts to curve after this and fractures at point Y
- This curve between X and Y denotes plastic behaviour
- Therefore, the correct answer is C
Exam Tip
Although similar definitions, the elastic limit and limit of proportionality are not the same point on the graph. The limit of proportionality is the point beyond which the material is no longer defined by Hooke’s law. The elastic limit is the furthest point a material can be stretched whilst still able to return to its previous shape. This is at a slightly higher extension than the limit of proportionality. Be sure not to confuse them.
