How Bolts Work
Steel, no matter what its composition, is elastic, and when a load is applied to it in tension, it stretches; like any other spring, the further it's stretched, the more it resists. Think of bolts as very stiff springs, and you'll have the gist of it. Picture two steel plates, held together by a nut and bolt. Turning the nut advances it along the threads of the bolt, which creates tension on the portion of the bolt that passes through the joint, stretching it slightly. The more torque we apply to the nut, the further the bolt stretches, and the harder it tries to return to its relaxed state. As the bolt stretches a compressive load is placed on the joint, binding it together. That compressive load is directly proportional to the distance the bolt is stretched, which itself is dependent on how tight we can make the nut. In a nutshell, that's how bolts work, and you can now understand why it's so important to properly torque fasteners. Unless a screw is placed under the proper tension, it won't be stretched far enough to secure the connection; in effect, it'll be like any other slack spring-too relaxed to do its job.