# Yield point

1. Jun 24, 2016

### foo9008

1. The problem statement, all variables and given/known data
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 ?

2. Relevant equations

3. The attempt at a solution
IMO , it's wrong.... can somone clarify ? if there's no force applied , how can the material elongate itself ?

2. Jun 24, 2016

### SteamKing

Staff Emeritus
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)

3. Jun 24, 2016

### 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.

4. Jun 24, 2016

### foo9008

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

5. Jun 24, 2016

### 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.

6. Jun 24, 2016

### foo9008

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

7. Jun 24, 2016

### PhanthomJay

Yes

8. Aug 17, 2016

### foo9008

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?

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9. Aug 19, 2016

### 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.