Materials: Force - Extension Graph Query

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S.H.Rahman
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#1
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#1
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!
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ambershell27
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#2
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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.
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S.H.Rahman
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(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?
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ambershell27
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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.
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S.H.Rahman
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(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.
Alrighty I'll just take your word for it.

P.S. I don't think brittle materials don't follow Hooke's Law lel
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ambershell27
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#6
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Sometimes they do. Because they withstand such a small strain, they have a linear relationship until that point. Look up glass, for example.
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