Yield Point: Why Can Material Elongate without Force?

[foo9008]

Homework Statement

I was told that yield point is a point which the material which will have an appreciable elongation of yielding without any increase in load . my question is If there's no increase load , how can the material elongate ?

Homework Equations

The Attempt at a Solution

IMO , it's wrong... can someone clarify ? if there's no force applied , how can the material elongate itself ? [/B]

 

[SteamKing]

Homework Statement

I was told that yield point is a point which the material which will have an appreciable elongation of yielding without any increase in load . my question is If there's no increase load , how can the material elongate ?

Homework Equations

The Attempt at a Solution

IMO , it's wrong... can someone clarify ? if there's no force applied , how can the material elongate itself ? [/B]

You probably should start asking for references for all this stuff you are being told.

Here is a discussion of what yielding actually consists:

https://en.wikipedia.org/wiki/Yield_(engineering)

 

[PhanthomJay]

There is a force applied, it is just that in the non elastic or idealized plastic region, it doesn't have to increase from the force required at yield (it doesn't follow Hooke's Law) and it sometimes decreases at the higher strains, but it doesn't go to the 0 or 'no force' condition.

 

[foo9008]

There is a force applied, it is just that in the non elastic or idealized plastic region, it doesn't have to increase from the force required at yield (it doesn't follow Hooke's Law) and it sometimes decreases at the higher strains, but it doesn't go to the 0 or 'no force' condition.

what do you mean by it here ? I'm confused

 

[PhanthomJay]

In the elastic region of stress and strain, you apply an increasing force to achieve and increasing deformation, per Hookes Law. Once you reach the idealized yield point, say at a force P_y, you then get into the plastic range, and Hookes law becomes invalid. At this point, you do not have to increase the load to get increasing strain. You now more or less can keep the force at a constant value P_y and the material will exhibit increasing strain. Your question asked why does it continue to stretch with no force applied. But there is a force applied, namely, P_y.

 

[foo9008]

In the elastic region of stress and strain, you apply an increasing force to achieve and increasing deformation, per Hookes Law. Once you reach the idealized yield point, say at a force P_y, you then get into the plastic range, and Hookes
law becomes invalid. At this point, you do not have to increase the load to get increasing strain. You now more or less can keep the force at a constant value P_y and
the material will exhibit increasing strain. Your question asked why does it continue to stretch with no force applied. But there is
a force applied, namely, P_y.

Do you mean the object will continue to stretch with the same force applied (constant) , but not increasing force?

 

[PhanthomJay]

Yes

 

[foo9008]

Yes

I have 2 graphs below, to show the yield point, in the first graph, the stress is constant from point B to C (C is yield point)(B and C are at the same level of stress), that means the force is constant,right?
For the second diagram, the yield point is slightly higher than the elastic limit point...Which of it is correct?

 

[PhanthomJay]

The first diagram is for materials like steel and the second is more like for materials such as aluminum. They are both more or less correct.