Stress Strain Curve: Explaining Beyond UTS

[Nemo's]

Homework Statement

Explain in terms of stress and strain what happens to the stretched material beyond the ultimate tensile stress.

Homework Equations

A curve similar to this

The Attempt at a Solution

I can see that the curve beyond UTS represents increasing stiffness but i can't really explain why or relate this to strain.( Does the zero gradient at uts mean zero stiffness ?)

 

[haruspex]

I can see that the curve beyond UTS represents increasing stiffness

Decreasing (and negative)?

but i can't really explain why or relate this to strain.( Does the zero gradient at uts mean zero stiffness ?)

Yes. Stiffness would be the slope, no?
But the question doesn't mention stiffness. It just asks you to discuss what's happening to the sample in terms of stress and strain.

 

[Nemo's]

But the question doesn't mention stiffness. It just asks you to discuss what's happening to the sample in terms of stress and strain.

So Can I just say that as the strain increases the stress decreases at an increasing rate?
I thought I had to mention the quantity represented by the slope in these curve-related questions.
I only said stiffness because I know that the Young Modulus is only related to the linear part of the curve.( Honestly I don't know which name to give the slope in this part)

 

[haruspex]

So Can I just say that as the strain increases the stress decreases at an increasing rate?

Not sure that makes physical sense. Think of the test set-up. You, the experimenter, supply the stress. The test sample cannot control the stress on it. I would say that beyond the tensile strength (i.e. the peak stress) the stress level required to increase the strain diminishes increasingly rapidly.

I thought I had to mention the quantity represented by the slope in these curve-related questions.
I only said stiffness because I know that the Young Modulus is only related to the linear part of the curve.( Honestly I don't know which name to give the slope in this part)

From what I read, referring to stiffness does not solve that. That also refers to elastic deformation. This link is useful: http://www.etomica.org/app/modules/sites/MaterialFracture/Background1.html

 

[Nemo's]

I would say that beyond the tensile strength (i.e. the peak stress) the stress level required to increase the strain diminishes increasingly rapidly.
Yes this makes a lot more sense.
Thanks a lot :)

 

[Nemo's]

I would say that beyond the tensile strength (i.e. the peak stress) the stress level required to increase the strain diminishes increasingly rapidly.

Yes this makes a lot more sense.
Thanks a lot :)