Strength data for screws and bolts in the United States uses several terms that we might not be familiar with such as:

120,000 psi
Ultimate Tensile Strength
Proof Strength

is the bolt "strong enough". How do we use the strength data to answer our question? We need to know two things:

  1. the bolt's material strength
  2. how big is the bolt.
How Strong:

You might want to know how strong the bolt is before it starts to deform, or how strong the bolt is before it breaks. If you apply a stretching load to the bolt (tensile load), it starts to permanently deform at its Yield Strength and breaks at its Ultimate Tensile Strength (UTS) - at room temperature! If a steel alloy bolt is in shear, then you use 60% of tensile strength as your shear strength.

But often you are not given yield strength. Here is where Proof Strength comes in. Proof Strength is approximately 95% of Yield Strength so it is just a little less. Proof Strength is used to calculate torque values. So for our use, Proof Strength is a conservative Yield Strength.

How Big:

Our bolt we plan on using is an aircraft AN4 bolt (1/4-28). From charts you are told that the material strength is approximately 120KSI UTS. This means that if you have a 1 square inch piece of this bolt material it takes 120,000 pounds of tension to break it. So now its simple, just multiply 120,000 times the area of the bolt and you get the force in pounds to break the bolt. If our bolt were 1 inch square this is easy - 1 x 120,000 pounds = 120,000 pounds to break!

Not very helpful in that our bolt is not square, it is round, and it has threads cut into the shank further decreasing its diameter. So I'm going to simplify things a bit. The area of our AN4 bolt is 0.364 inch. you multiply 120,000 times .0364 = 4,368 pounds of tensile force required to break our AN4 bolt. But how did we get 0.0364? The Tensile Strength Chart from Mechanic's Toolbox Software.

Mechanic's Toolbox does the math so you get the answers quickly and easily.

Will our AN4 bolt hold up to 4,368 pounds?

No, probably not or maybe for a time. That is why engineers apply a "safety factor". Mechanic's call it a "fudge factor". There are several other considerations, a big one being the strength of our threads. Threads are loaded in shear. The shear strength of the nut threads and the bolt threads must be greater than the bolt's tensile strength. Also, proper amount of thread engagement - the nut must be fully threaded onto the bolt.

Often I hear talk about bolt strength or using a "Grade 8" bolt. Fine, but for tension loading, don't forget nut strength. The nut has to be strong enough in shear to support the load applied to the bolt. The bolt AND nut are a system. (Also, high-strength bolts need high-strength washers for support).

For a Time:

Strength and Endurance - our repair should work and it should last. There is another type of strength not mentioned yet that is very important in aircraft. Aircraft vibrate and are subject to cyclical loads. Buildings and structures do not suffer load reversals. That commercial grade rivet, bolt, screw may be strong when used in a building but quickly fail when installed in aircraft. To evaluate this problem we introduce another kind of strength called "fatigue strength".

Fatigue Strength:

The principle method in which fatigue strength is controlled by the mechanic is proper torque. Torque locks tension into the bolt. If the tension created by torque is greater than the tension created due to loading, then no stress cycles are felt by the bolt. Since fatigue failure results from many, many tension changes, our properly torqued bolt will not feel any of these tension changes and will not fail in fatigue.

Not quite true - but true enough for our discussion

"No stress cycles are felt by the bolt" - well not totally accurate. Some stress is felt and this depends upon elasticity of the joint. Ideally, our bolt should be more elastic (think of a bolt as a spring), and the joint material should not be elastic. Some bolts are designed to be elastic - they have a reduced shank diameter; Lycoming and Continental connecting rod bolts for example.

An interesting example of this is when a cylinder "breaks-off" of a Lycoming or Continental aircraft engine. It is usually the large studs that are broken while the skinny thru-bolts are intact. Why would the larger, stronger piece of steel break before the smaller, weaker piece? Remember elasticity. Our short studs stretch very little when torqued so easily loose their tension. The longer thru-bolts (think springs) stretch more for the same amount of torque. Now the larger studs are feeling the stress cycles and fail in fatigue while the longer studs, because they have stretched more, do not feel the stress cycles.

Stress Corrosion Cracking

Sometimes bolts break when they haven't been overloaded. Generally, the stronger the bolt material the more prone to this type of failure. Meaning stress corrosion cracking and hydrogen embrittlement cracking. Another general rule; the stronger the bolt, the more it needs to be protected from corrosion, pitting and nicks.

Fine Thread Vs Coarse Thread

Fine threads have a larger Tensile Stress Area than coarse threads.

Example 1/4-28 - 0.0364

1/4-20 - 0.0318

Everything else being equal - bolts with fine threads are stronger in tension than bolts with coarse threads.

NAS high strength bolt
NAS1587 washer
160,000 psi (160ksi) bolt and washer.
The high clamping force ("torque") used on high-strength bolts may crush a standard washer. High strength washers are often used.
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In the final analysis its joint strength that is important. If you don't torque (tighten) the bolt properly, the joint isn't going to last. Mechanic's Toolbox software includes torque software to assist you in torque calculations so you know the job is done right.
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