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:
Below the elastic limit, the material exhibits elastic behaviour
Above the elastic limit, the material exhibits plastic behaviour
- The linear portion of the graph represents the loads at which the force and extension are directly proportional
- The limit of proportionality marks the upper end of this proportional relationship
- Up to the elastic limit, the material will return to its original shape after the load is removed (elastic behaviour)
- Beyond the elastic limit, the material will not return to its original shape after the load is removed (plastic behaviour)
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
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 at point X?
A. both elastic and plastic
B. not elastic and not plastic
C. plastic but not elastic
D. elastic but not plastic
ANSWER: D
- The graph is a straight line at point X
- Therefore, the force and extension are directly proportional
- Point X is not beyond the elastic limit
- So the metal is behaving elastically
Examiner 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.
