Difference between Elastic Limit & Yield Point

In summary, there are two points on a stress/strain curve for ductile materials that are commonly referred to as the yield points. The first point is the elastic limit, which is the point where the material behaves elastically and returns to its original shape without permanent deformations. The second point is the yield point, which marks the transition from elastic to plastic behavior. However, not all materials have a well-defined yield point and it is often defined as 0.2% plastic strain. In some ductile materials, there may be two yield points, known as the upper and lower yield points. The exact location of the yield point can vary and it is not always the same as the elastic limit.
  • #1
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Hi All

I am trying to understand a stress / strain curve for a ductile material.

But I am struggling with understanding the difference between the Elastic Limit and the Yield Point. I define these terms as:-

Elastic Limit - Is the point on the stress/strain curve where the material will behave elastically i.e. will return to its original shape without leaving any permanent deformations.

Yield Point - The yield point is the point on a stress–strain curve that indicates the limit of elastic behaviour and the beginning of plastic behaviour.

Assuming that you agree with these definitions then the Elastic limit and Yield Point should be the same point on the stress strain curve - But they are NOT?

To help me better understand I created a stress strain curve for a ductile material and labelled them.

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I understand everything up to point E which is the elastic limit.

I have been told that Y1 and Y2 are yield points but why are there two of them and why is E and Y1 on different points on the curve?

What is happening between E and Y1?

Can anyone help explain?

Thank you.
 

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  • #2
I might be wrong but I thought the yield point was the point at which strain started to increase without much or any extra stress being added. So ductile materials have wider region between the two points than brittle materials.
 
  • #3
Some materials do not have a well defined yield point, so the yield point is defined as 0.2% plastic strain. That could be point E on your curve. Points Y1 and Y2 apply to some (not all) ductile materials, where the stress decreases immediately after yielding. They are the upper and lower yield points. So "the" yield point could be point E, point Y1, or somewhere between those two points.

Wikipedia has a good article: https://en.wikipedia.org/wiki/Yield_(engineering)
 
  • #4
Yes, it is possible for a material to have a higher elastic limit than yield point. This means that the material can withstand greater stress without permanently deforming, but once it reaches the yield point, it will begin to deform plastically.

but don't plastic deformation and permanent deformation mean the same thing?
 

1. What is the difference between the elastic limit and yield point?

The elastic limit is the maximum stress that a material can withstand without becoming permanently deformed. The yield point is the stress at which a material begins to deform plastically, meaning it will not return to its original shape once the stress is removed.

2. How are the elastic limit and yield point determined?

The elastic limit is typically determined through a series of stress-strain tests, where the stress is gradually increased until the material begins to deform. The yield point is determined through the use of a yield strength test, which measures the stress at which the material starts to show plastic deformation.

3. Can a material have a higher elastic limit than yield point?

Yes, it is possible for a material to have a higher elastic limit than yield point. This means that the material can withstand greater stress without permanently deforming, but once it reaches the yield point, it will begin to deform plastically.

4. What factors can affect the elastic limit and yield point of a material?

The elastic limit and yield point of a material can be affected by factors such as temperature, strain rate, and the presence of impurities or defects in the material. These factors can alter the strength and behavior of the material under stress.

5. Why are the elastic limit and yield point important to understand in materials science?

The elastic limit and yield point are important concepts in materials science because they help us understand the behavior and limitations of different materials under stress. This knowledge is crucial in designing and engineering structures and products that are safe and durable.

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