How do we measure, and what is the Shear Strength of a fastener?

The Fastener Training Minute is a co-production of The FCH Sourcing Network and The Fastener Training Institute and is presented by Carmen Vertullo of the Carver FACT Center and the AIM Test Lab in San Diego, California.

Originally published on Fully Threaded Radio, Episode 145, 10/15/2019

Today's topic is a very important property of fasteners called Shear Strength.

Now strangely enough, it's a very unusual property in that we very seldom actually test for the Shear Strength of fasteners.

It's important for engineers to know the Shear Strength.

How can they find this out? How can you as a supplier help them know what the shear strength of a particular fastener is when it doesn't show up in any of the standards regarding those Fasteners?

Actually shear strength is a property of all materials. One of the mysteries about Shear Strength is that when it comes to fasteners, very few Fasteners are actually tested for Shear Strength in the commercial world.

That is, our ASTM and and our SAE and our ISO standard Fasteners are typically tested for a strength property called Tensile Strength. That means that if we pull them apart in a long ways direction, how much stress can they handle before they break.

Shear strength as you can imagine is just the way it sounds, it is the strength property of the faster when we try to cut it by shearing it, like we were cutting a wire or cutting the fastener with a pair of scissors to shear through the fastener.

Now fasteners aren't rated for Shear strength, but Engineers often want to know what the shear strength of a fastener is.

Certain other types of fasteners, typically non threaded fasteners such as pins and rivets are actually tested for this property, and some Aerospace fasteners are tested for Shear Strength.

So in our commercial world, if we wanted to know the Shear Strength of a fastener, obviously we could just test it to find out, but if we want to make a good estimate we can assume that the shear strength of a fastener through the threads (which is going to be the weakest point) is going to be at least 50% of the Tensile Strength.

So for example, a half inch grade 8 bolt has a tensile strength of about 20,600 lbs, so it's sheer strength through the threads in single Shear (That's one Shear plane), would be about half that or a little bit over 10,000 lbs.

Now often times when we test Shear strength, we test Double Shear, which means that we put the faster in a very tricky fixture and it actually gets loaded in two planes in the shear direction. If we want to know the shear strength of the material we would divide that by 2. If you really want to have a good resource for helping your customers understand Shear strength of their fasteners, the Industrial Fasteners Institute's IFI Technology Connection in the printout that comes with their Fastener analysis feature includes Shear strength. It's based on the formula that I just told you half of the tensile strength of the fastener through the threads.

Now oftentimes fasteners are loaded in sheer not through the threads but through the body since this is the preferred way to load the faster.

So what would the strength through the body be? Well the IFI Technology Connection printout gives you the shear strength through the body of the fastener and through the threads assuming it's not a fully threaded fastener.

That's really about all there is to know about Shear strength of fasteners.

It's not that mysterious and it's really easy to get a straightforward answer even though it may not be in the Fastener specification.