Which row best defines elastic and plastic behaviour of a material?

Q#7 (Past Exam Paper – November 2009 Paper 11 Q20 & Paper 12 Q19)

Which row best defines elastic and plastic behaviour of a material?

Solution:

Answer: D.

An elastic material returns to its original shape and size when the deforming force is removed.

Up to the limit of proportionality, the extension of the material is proportional to the force. The curve of the force-extension graph is linear. The material is said to obey Hooke’s law up to the limit of proportionality.

Beyond the limit of proportionality P is a point called the elastic limit. From extension = 0 to the P, the extension is proportional to the force and the material regains its original shape when the force is removed.

Beyond P up to the elastic limit, the extension is no longer proportional to the force (Hooke’s is no longer obeyed) BUT the material still regain its original shape when the force is removed.

Thus, as long as the elastic limit is not exceeded, the material have an elastic behaviour. For the extensions up to the limit of proportionality P, the extension is proportional to the force (i.e. it obeys Hooke’s law – the graph is a straight line). 

Even beyond P, the material would behave elastically (regain its original size and shape) as long as the elastic limit is not reached. [A incorrect]

As for a plastic material, as the force is removed, it does not regain its original size and shape – it suffers permanent deformation. If a large enough force is applied, it could break. [C incorrect]

A horizontal force-extension curve would mean that for the extension does not depend on the force applied. Meaning that for any given force, the material would extend continuously. [B incorrect]