Yield Point

What is a Yield Point?

The yield point of a material is defined as the point where the material no longer obeys Hookean properties after this point. The figure below shows a stress strain graph for a steel sample. Note the initial part of the graph in red - there is a linear relationship between stress and strain. After a point the material begins to behave differently this point is the yield point. While stress is below the yield point then the material is assumed to act elastically.

Note not all material feature a stress strain graph that is so distinctive. For these materials another method can be used such as 0.2% strain limit.

Why is a Yield Point important?

The yield point is often used to determine what stress is deemed allowable for the material. For example an engineering standard might specify that 2/3rds yield will define the maximum allowable stress for a materiel for a given application.

Can the Yield Point be changed?

Material processes can have an effect on a materials Yield Point, for example heat treating a steel can increase the yield point of it. Also local effects like shot peening or case hardening can increase the strength of the materiel at the surface. This is useful for applications where the surface of the materiel will contact another such as gears.  

Yield Point vs Strength of the material?

The Yield point is considered a measure of the materials strength. Other factors can be important when considering the strength of a material. The Young modulus or the ultimate stress of the material might be more important if the component is designed to fail for example.  

Is a higher Yield Point Better?

Once more this depends on the application. Often with steels a higher yield point will be a more brittle material. This can have issues when compared to a ductile material. Ductile materials are often able to handle shock loads better and they also show a visual sign before failure.