Yield Strength vs Ultimate Strength: Difference and Comparison

Key Takeaways

1. Definition and Behavior: Yield strength refers to the maximum stress at which a material begins to deform plastically, meaning it won’t return to its original shape after the load is removed. In contrast, ultimate strength indicates the maximum stress a material can withstand while being stretched or pulled before breaking or failing.
2. Material Behavior Point: A material transitions from elastic deformation (temporary) to plastic deformation (permanent) at the yield strength. Ultimate strength, on the other hand, represents the point at which the material experiences complete failure, after significant plastic deformation.
3. Practical Implications: Engineers consider the yield strength when designing structures or components to ensure the material won’t undergo permanent deformation under expected loads. Ultimate strength provides a safety margin, guiding engineers on the absolute limits before catastrophic failure. Both are vital for safety and durability considerations in design applications.

What is Yield Strength?

Yield strength is defined as the point at which a material starts to deform without an increase in strain. It is among one of the essential mechanical properties shown by the material. It is the point at which the material transitions or changes from elastic deformity to plastic deformation. 

When the load is applied to a material exceeding its yield strength, it causes it to show permanent change. This property helps the engineers find the best material and design it to withstand normal operating conditions without any further permanent changes.

What is Ultimate Strength?

Ultimate strength is said to be the maximum stress a material can thwart before breaking. It is also known by the name ‘tensile strength’. In other words, it can be said that the property shown by the different materials where they can no longer hold any load applied to them. 

To know the ultimate strength or, say, the tensile strength of a material, engineers measure it by a few tensile tests where various load is applied until it breaks off. It is crucial to know the tensile strength of any material, which will later help in selecting appropriate material and designing structures with required safety margins and strength.

Difference Between Yield Strength and Ultimate Strength

1. Yield strength is defined as the point at which a material starts to deform without an increase in strain. While in contrast, ultimate strength is said to be the maximum stress a material can thwart before breaking.
2. The behaviour shown by the material in yield strength is as follows – it regains its original shape after the stress is removed from the material. At the same time, the behaviour shown by the material in the case of ultimate strength is as follows – it leads to permanent changes if the stress exceeds the yield point.
3. The symbol representing yield strength is sigma y, i.e., σy. And on the contrary side, the symbol representing ultimate strength is sigma u, i.e., σu. 
4. The significance of yield strength is that it shows the limit to which a material can safely hold loads without getting deformed in shape. While comparatively on the other hand, the significance of ultimate strength is that it talks about the maximum capacity of carrying load before any failure occurs.
5. The relationship of yield strength is that it is lower than ultimate strength. On the other hand, the relationship of ultimate strength is comparatively higher than yield strength.
6. Yield strength is considered in spaces where deformation should be minimized, for example – machine components, buildings, bridges, etc. At the same time, ultimate strength is considered in conditions where maximum strength is required, for instance – automotive components, safety-critical aerospace, etc. 
7. In engineering designs, importance is given to yield strength, which is essential for making structures, ensuring that the given material remains within its elastic limit in normal conditions. Whereas, on the contrary side, in engineering designs, importance is given to ultimate strength, which is essential for creating structures to resist the maximum loads and prevent them from any catastrophic failures.

Comparison Between Yield Strength and Ultimate Strength

Parameter of Comparison	Yield Strength	Ultimate Strength
Definition	The point at which a material starts to deform without an increase in strain	It is said to be the maximum stress a material can thwart before breaking
Behaviour	It regains its original shape after the stress is removed from the material	It leads to permanent changes if the stress exceeds the yield point
Symbol	σy	σu
Significance	It shows the limit in which a material can safely hold loads without getting deformed in shape	It tells about the maximum capacity of carrying load before any failure occurs
Relationship	Generally lower than the ultimate strength	Higher than the yield strength
Importance of Property	They are used in spaces where deformation should be minimized, for example – machine components, buildings, bridges, etc	They are used in conditions where maximum strength is required, for example – automotive components, safety-critical aerospace, etc
Engineering Design	It is essential for making structures, ensuring that the given material remains within its elastic limit in normal conditions	It is essential for creating structures to resist the maximum loads and prevent them from any catastrophic failures.

References

1. https://link.springer.com/article/10.1007/S11665-008-9225-5
2. https://www.mdpi.com/1996-1944/15/3/832