The usual purpose of a bolted joint is to clamp two or more parts together. The clamping force is achieved by applying torque to the bolt head and the nut. The mechanical advantage of the wrench and threads allows one to actually stretch the section of the bolt between the head and the nut (an area known as the grip), creating tension in the bolt. This tension is known as pretension because it exists before any other forces are applied to the joint.

The pretension is transmitted to the mating parts through the head, nut, and any washers that may be present. It squeezes the mating parts together, and, if the joint is designed, assembled, and maintained properly, prevents the mating parts from separating or sliding under normal loads.

The pretension in a bolt is often quite high: 70% of the bolt's tensile strength is not uncommon. At first glance it may seem that this leaves very little strength left to carry the externally applied loads, but it turns out that bolted joints are pretty clever when it comes to carrying loads, and their capacity is greater than you might expect.

Consider the joint shown below: Two D-shaped yokes clamped together with a single bolt. First the bolt is tightened. This draws the faying surfaces (the portions of the mating parts that come in contact) together and applies a compressive force to the mating parts. The joint is in equilibrium, with the compressive force across the faying surfaces equal in magnitude to the pretension in the bolt.

As the external tensile load is applied to the joint, the joint responds by distributing the load. As expected, part of the load goes into the bolt, increasing its tension. But part of the load also goes into the mating parts, decreasing the compression at the faying surfaces. In most joints, this decrease in compression absorbs most of the applied load, shielding the bolt from large increases in tension. Exactly how much is shielded depends on the geometry and material makeup of the joint, but it is not unusual for as much as 90% of the applied load to be taken by the faying surfaces, leaving only 10% to be borne by the bolt.

Of course, the compression at the faying surfaces can only decrease so far. Once the compression has been reduced to zero, the faying surfaces lose contact with each other and the bolt then carries all the applied load. A graph of the bolt tension vs. the applied load shows this change in behavior as a kink at the point at which the faying surfaces part (sometimes referred to as the decompression point.)

Parting of the faying surfaces is generally considered a bad thing, especially in joints that are subjected to fluctuating loads. If the applied load cycles back and forth between levels below the decompression point, the cyclic stress range in the bolt is relatively low. If, however, the applied load cycles to levels beyond the decompression point, the cyclic stress range in the bolt jumps dramatically and can lead to an early fatigue failure. The graph below shows how a lowered pretension can create this effect.

For this reason, fatigue failures in bolts are often associated with low pretension.