Understanding Yield Strength & Stress Strain Curve

[tomtomtom1]

Hi all

I was hoping someone could shed some light on the following:-

I am trying to understand what Yield strength is and understand the exact limit of where elastic and plastic deformation occurs on a stress strain curve.

Correct me if I am wrong but I define:-

Yield strength as the amount of stress a material can undergo before it deforms plastically.

Yield Point is the point on the stress strain graph where Elastic deformation ends and Plastic deformation begins.

Would you agree with the above definitions, the reason why I ask is because on a stress strain curve (as shown below), we have 2 x Yield Stress Points shown as B and C.

1) what is the difference between these points.
2) where would the elastic and plastic limits commence from, the limits in Red or the limits in Blue?

 

[AZFIREBALL]

I think the portion of the curve after point B is caused by work hardening of the material during elongation.

 

[jack action]

This graph from Wikipedia, might help you understand it better:

Where:

• 1: Ultimate strength
• 2: Yield strength (yield point)
• 3: Rupture
• 4: Strain hardening region
• 5: Necking region
• A: Apparent stress ($F/A_0$)
• B: Actual stress ($F/A$)

Pass point 2 (B on your graph), you are in the plastic deformation zone (thus, the limits in red on your graph would make sense). It is interesting to see the same stress-strain curve based on the actual area $A$ instead of the initial area $A_0$ (blue line B). Even though the relationship is not as linear in the plastic zone, it still requires an increase of the actual stress to get an increase in strain.

I can't explain why they wrote the limits in blue on your graph.