When it comes to tightening bolted joints, many people will use torque as the primary way of determining whether or not the fastener has done its job. The question, "How tight is that assembly?" is often answered by, "Well, I tightened it to 50 foot-pounds." The problem is that 50 foot-pounds only tells you how much energy it took to turn the fastener, and very little about how tight the joint has become.
Tightening a bolted joint is an energy transfer process where the energy required to turn the fastener, in the form of torque, is transferred into stretching the bolt and producing clamp load. To keep an assembly together we need to stretch the fastener and produce enough clamp load that the assembly will be able to withstand the working load.
Even though we can easily measure the amount of torque we're applying, the amount of friction present in the joint will determine how much torque energy actually goes into producing clamp load. In most typical bolted joints, as much as 80 to 90 percent of the torque energy goes into overcoming friction in the joint. Knowing the state of friction is why fastener manufacturers and affiliated coatings and plating companies are required to conduct batch testing for coefficient of friction before they can ship their product.
Standardized test specifications will often start by measuring how much torque it takes to reach a specified clamp load using an accepted set of test equipment. Even though we aren't determining the actual friction coefficients, the relationship between torque and clamp load will give us a reasonable estimation of friction. Some specialized sets of test equipment can provide the additional measurements that can determine the actual friction coefficients.