Below we’ll take a look at what tensile strength is, how it’s tested, and how it relates to other common fastener specs.

What Is Tensile Strength?

A fastener’s tensile strength is the maximum amount of force a fastener can withstand before fracturing. It’s also referred to as ultimate tensile strength, or UTS. It can be affected by factors like material composition, hardness, and size of the part.

Tensile strength is one of the measurements set by a quality standard — like ISO, ASTM, SAE etc. — that fasteners must meet to qualify for a grade or property class. Each standard has its own tensile strength requirements.

Let’s use , for example. The chart below shows three different ISO 898-1 property classes for bolts and what tensile strength they require.

How Do You Determine the Tensile Strength of Fasteners?

To determine the tensile strength of a fastener, the fastener must undergo a testing procedure called a wedge tensile test. Below is an overview of how that test works and why it’s important.

Wedge Tensile Test

During wedge tensile testing, a wedge is placed under the head of a fastener, then force is applied until the fastener breaks. The wedge is used because it puts extra stress on the junction of the head and the body of the fastener. This ensures the integrity of the junction.

If the fastener breaks at a force greater than the minimum tensile requirement in the spec, the fastener has passed the tensile test. However, the break must not occur at the junction of the head and the body of the fastener. If the break does occur there, the fastener has failed the tensile test regardless of the force at which the break occurred.

Why Is Tensile Strength Testing Important?

Tensile strength testing is important because it shows us how much force can be applied to a fastener before it will break. You can probably guess why this is an important piece of information to know. It helps solidify that the fastener is strong enough and safe enough to be used in specific conditions and for a specific purpose.

If you get fasteners that don’t have a high enough tensile strength for your application, you’ll have to deal with fastener breakage and failure issues. On the flipside, if your fasteners’ tensile strength is higher than necessary, you’ve probably wasted money on that added strength.

Tensile Strength vs. Proof Load vs. Yield Strength

Now, you may wonder how tensile strength relates to other industry terms like proof load and yield strength.

Tensile strength, proof load, and yield strength are all measurements set by a standard that fasteners need to meet to qualify for a particular grade or property class. They’re all related to the load a threaded fastener can bear when pulled perpendicularly from its head. The diagram below shows an example of this.

However, there are key differences between these terms and how they relate to fastener testing, production, and performance. Here’s a side-by-side look at their definitions.

• Tensile strength: the maximum amount of force a fastener can withstand before fracturing.
• Proof load: the minimum amount of force that a fastener must be able to withstand for a given amount of time without permanently deforming.
• Yield strength: the load that’s carried at the point where a fastener permanently deforms.

To summarize, there are different levels of force measured here. Tensile strength is the highest level, followed by yield strength, then proof load. For a closer look at how these terms compare and contrast, read this blog.

If you weren’t aware of any of these terms, and maybe a few other ones, too, this glossary-style blog is full of helpful fastener terminology.

RELATED: Need a specialty bolt, screw, or stud? ºÚÁÏ´óÊÂ¼Ç can help.

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At ºÚÁÏ´óʼÇ, per-spec and limited-run fasteners are our specialty. You hand us the blueprints and we’ll deliver the parts you need, in the quantity you need, and with the level of quality you need.

Give us a call at (800) 656-2658 or contact us online to learn more about our services.

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Basics of the Bolted Joint

Before we can talk about torque-tension testing, we should take a second and define torque, tension, and the basics of the bolted joint. To put it simply, a bolted joint is two or more things held together with a bolt or screw. For the purposes of our discussion, we’re going to be specifically referring to a bolted joint as an assembly consisting of three metal plates with existing holes being joined together by a bolt, nut, and washer. See Figure 1 above. The holes are not tapped and therefore are not interacting with the threads. Also, in this example, we’ll be tightening the nut, not the bolt.

In order to tighten the nut, you turn it. The force required to rotate the nut is referred to as torque. As the nut is tightened, everything in the joint squeezes together as the nut moves up the threads. The force of this squeeze is referred to as tension (can also be referred to as clamp load.) As you tighten further and further, the bolt itself will begin to stretch. When the steel in the bolt is stretched, it tries to spring back to its original shape. So as the bolt stretches, it adds even more tension to the load as it tries to regain its original shape. If you were to keep tightening, the bolt would eventually become deformed permanently. The force at which a bolt becomes permanently deformed is called a bolt’s yield strength.

Torque-Tension Testing Basics

A torque-tension test is the measurement of the input torque required for a bolted joint to achieve a specified tension. In other words, how tight do I need to tighten my bolt in order for the joint to reach a certain tension? A typical test set-up is shown in Figure 2. It consists of a test bolt, test washer, and test nut loosely fitted in a test fixture. The test fixture contains a load cell that can measure the amount of tension in the joint.

The nut is slowly tightened until a preset amount of tension is reached. As the joint is tightened, this action stretches the bolt, creating a clamp load on the joint (in this case the test fixture). The amount of torque needed to rotate the nut to the desired tension is measured. These tests are always run below the material’s yield strength, so no permanent deformation of the bolt occurs.

Friction

The most important factor affecting the relationship between torque and tension is friction. This makes sense: a nut with a smooth surface and lubrication will turn easier than one with a rough surface and no lubrication. There are several factors that can affect the amount of friction in a bolted joint. These include:

• ¹ó²¹²õ³Ù±ð²Ô±ð°ùÌýmaterial and grade – The type of material used and how hard it is will impact friction.
• Class of fit for mating threads – Thread systems with a tighter fit will have more friction than systems with a looser fit.
• Bearing surface properties and area – The amount of surface area and the roughness of the bearing surface will affect friction.
• Coatings – Different fastener coatings will have a large impact on friction.
• Lubricants – Different lubricants will also have a large impact on friction.

Coatings

Coating (sometimes referred to as finish or plating) on steel fasteners provides a number of desirable properties, including corrosion resistance. Different coatings also change the friction (lubricity) from that of a plain part. The coefficient of friction is a number that is calculated for each different coating. It can be calculated from torque-tension test results and part geometry. Coated test bolts are set up as in Figure 2. Torque measurements are made at a specified clamp load value, generally around 75% of the yield strength. This procedure is repeated a specified number of times, and the coefficient of friction is calculated from the results. The ability to determine a coefficient of friction for each coating is quite useful. This helps coating manufacturers develop and maintain products with consistent friction properties. End-user manufacturing companies also use the coefficient of friction to help determine installation torque values.

Where We Come In

ºÚÁÏ´óÊÂ¼Ç Company does not actually perform any torque-tension testing. However, we have been supplying test bolts,Ìýnuts,Ìýand washers for over 35 years. We manufacture all of our test bolts, and we also stock test washers and test nuts. Many large OEMs have their own specifications for torque-tension tests that must be performed by suppliers. We stock parts for General Motors,ÌýFord,ÌýChrysler, and John Deere tests, as well as parts for tests based on ISO 16047. If there’s anything we might be able to help you with, by all means,Ìýlet us know.

This article was meant as an introduction to the subject, so if you’d like more information, we recommend taking a look at the  or individual OEM standards.

RELATED: Need a specialty bolt, screw, or stud? ºÚÁÏ´óÊÂ¼Ç can help.

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