Fatigue Failure and Heavy Hex Bolts

Fatigue failure is a very real threat in many different applications. Heavy hex bolts need to be designed to withstand great amounts of both static and dynamic loads. But most experts will agree that bolt strength has as much to do with proper installation procedures as actual product design.
Fatigue failure is more likely when the heavy hex bolt is not tightened to the proper level. In fact when dynamic loads are present, such as in an engine, a bolt tightened to a low level of tension will fail after only a few thousand cycles. Whereas the same bolt tightened closer to the proper levels will endure millions of cycles before failure. And when that bolt is tightened to the recommended levels testing shows that fatigue failure may occur after countless cycles or not at all.

Proper Levels of Bolt Tension
You need to provide the assemblers with the level of cyclical stress applied to the bolt. This number will give the assembler a target. All heavy hex bolts are to be tightened to an equal level of tension and may even be tightened beyond that level.
Just as in many other cases of load and stress on bolts, the level of tension applied to it determines the actual strength of the fastener. A rule of thumb – ensure the tension level goes beyond the maximum amount of dynamic load. Estimates are fine, although known dynamic loads are best.
Bolts of Today
The fasteners manufactured and sold today offer much higher levels of strength than those seen in the past. The materials and methods of manufacturing used provide high amounts of residual tension that allow the above guidelines to work perfectly in any given application.
When the bolt tension levels exceed the external load in the joint you will have achieved a reliable and strong joint that will last. Even the strongest heavy hex bolts will succumb to fatigue failure when they are not tightened to the proper tension level, and are exposed to a higher level of external load and pressure. Do the calculations and be sure that your application is stable and secure with heavy hex bolts tightened to the proper levels.