Materials: Force - Extension Graph Query

S.H.Rahman

In a Force - Extension graph we usually have a point labelled ' The limit of proportionality' where beyond this point, Hooke's Law is no longer obeyed. Slightly after this, we have another label called 'Elastic Limit'.

My question is:

Why don't we get plastic deformation during the stage between these 2 and is the gradient of this region the same as when Hooke's law was being obeyed? I think my textbook just doesn't have a clear enough diagram as from what I know it makes sense for it to start curving once the limit of proportionality has been reached.

Thanks in advance for any replies!

Reply 1

ambershell27

The point is that the limit of proportionality and elastic limit are different. An object can pass the limit of proportionality (not a straight line graph) and still return to its original state.

Reply 2

S.H.Rahman

Original post by ambershell27

The point is that the limit of proportionality and elastic limit are different. An object can pass the limit of proportionality (not a straight line graph) and still return to its original state.

Why is that?

Reply 3

ambershell27

It just depends on the material. For example, an elastic band has no linear regions at all, but has elastic behaviour. Brittle objects may follow Hooke's law up until breaking point.

Reply 4

S.H.Rahman

Original post by ambershell27

It just depends on the material. For example, an elastic band has no linear regions at all, but has elastic behaviour. Brittle objects may follow Hooke's law up until breaking point.