In this article, we鈥檙e talking about yield strength 鈥斅爋ne of the many measurements and specifications a fastener needs to meet to qualify for a particular grade or property class.
We鈥檒l talk about what yield strength means, how it鈥檚 measured, and how it relates to some other common fastener terminology.
What Is Yield Strength?
Yield strength is the load that鈥檚 carried at the point where a fastener permanently deforms. It鈥檚 one of the measurements set by a particular standard (ISO, ASTM, etc.) that a fastener must meet to qualify for a certain grade or property class.
Here鈥檚 a better explanation, perhaps:
When subjected to enough force, steel will begin to stretch. If the amount of force is low enough, the steel will elastically return to its original shape when the force is removed. But if the force becomes strong enough, the steel will stretch and not return to its original shape. This amount of force is the yield strength.
If you鈥檇 like an example, here are some of the most common and what the specification requires their yield strength to be:
| ASTM A354 Grade | Yield Strength |
| Grade BC (nominal diameter 录 – 2 陆鈥�)
Grade BC (nominal diameter 2 鈪� – 4鈥�) |
109,000 psi
94,000 psi |
| Grade BD (nominal diameter 录 – 2 陆鈥�)
Grade BD (nominal diameter 2 鈪� – 4鈥�) |
130,000 psi
115,000 psi |
Keep in mind that these are just some examples and specifications. Yield strength always changes between fastener size and grade (or property class).
How to Calculate the Yield Strength of a Fastener
Compared to some other fastener specs like proof load and tensile strength (more on those later in this article), yield strength is the one that鈥檚 tested the least. When yield strength is called for, it鈥檚 often in place of the proof load test. It just depends on the spec. For example, states that testing yield strength is an alternate option for meeting the proof load requirement.
Although it鈥檚 not as common as some others, yield strength still an important test to understand if your spec does require it. We鈥檒l describe the procedure below.
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Yield Strength Testing Procedure
A fastener鈥檚 yield strength is determined using a tensile machine, which looks like this:
Here鈥檚 a general overview on how it works.
The fastener is placed in the middle slot of a tensile machine, then the load is applied for a specified amount of time (usually seconds).
ASTM F606 allows for multiple methods of determining yield strength, but the most common used is the 鈥渙ffset method.鈥� The offset method involves measuring fastener elongation and plotting the results on a stress-strain diagram. The stress is determined at an offset, most commonly at 0.2% of the length of the fastener. Wherever the curve deviates on the stress-strain diagram is equivalent to the yield strength of the fastener.
When looking at yield strength values calculated using the offset method, you鈥檒l see the percentage offset in parenthesis after the term 鈥測ield strength,鈥� similar to this:
- Yield strength (0.2% offset) = 57,000 psi
Why Is Yield Strength Testing Important?
Yield strength testing is important because it helps us know how much force can be applied to a fastener before there is risk of deformation. The test verifies that the fastener meets the minimum requirements to qualify for a particular grade or property class.
From there, this information can help us understand if the fastener is safe enough, strong enough, and well-suited for a particular application.
Yield Strength vs. Tensile Strength vs. Proof Load
Yield strength is related to tensile strength and proof load, but there are key differences between them. An understanding of these terms, how they relate to one another, and how they differ from one another is important for safe and effective fastener production.
Here鈥檚 a look at each of their definitions, so you can see the differences for yourself:
- Yield strength: the load that鈥檚 carried at the point where a fastener permanently deforms.
- Proof load: the minimum amount of force that a fastener must be able to withstand for a given amount of time without permanently deforming.
- Tensile strength: the maximum amount of force a fastener can withstand before fracturing.
So there are really three levels of force we鈥檙e discussing here, and yield strength falls somewhere in the middle. If you鈥檇 like a better idea of how yield strength, tensile strength, and proof load compare and contrast with each other, check out this blog.
And if you鈥檙e still a little lost on these terms, or possibly some other technical fastener terminology, don鈥檛 hesitate to head to our glossary.
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