The whole goal of a threaded fastener is to provide a clamp load between the mating parts and, just as importantly, maintain this clamp load for the life of the machine. This is accomplished by tightening the bolt until it is under a certain amount of tensile stress which directly relates to the amount that the bolt is stretched. The allowable stress level is a function of the grade of the bolt - a Grade 2 bolt cannot handle near the amount of stress as a Grade 8. When torquing a bolt you never want to exceed the yield stress since that would mean that the clamp load would not be maintained. At the same time you don't want to under-torque a bolt - for example, torque a Gr8 bolt to Gr2 specs - since you won't achieve enough bolt stretch to properly maintain the clamp load long-term. The bolt torque charts you typically see are figured at 75% of the yield stress.
Unfortunately the torque at which a fastener is tightened is a very imprecise way of determining exactly what kind of clamp load you'll end up with. But, it is by far the simplest method so that is what is commonly used in all but the most critical applications. The cleanliness of the threads, hardness of the bearing surface, type of finish, presence or lack of lubrication, and other factors all combine to alter the amount of torque that is actually turned into the desired result which is bolt stretch. The torque that isn't converted into bolt stretch is just eaten up overcoming friction.
There might be some confusion to as to what happens to a bolt that is holding two pieces together that are getting pulled apart. Until the two parts begin to separate the bolt won't even "feel" the force pulling them apart. For example, a 1/2-13 Gr8 bolt has a clamp load of 12750 lbs when torqued properly which means that the two pieces being bolted together have a force of 12750 lbs holding them together. If you apply a "pull apart" force of 10000 lbs to the two parts the bolt is still only seeing 12750 lbs of load but you're down to just 2750 lbs of clamping force between the two parts. Not until the "pull apart" force exceeds the bolt's clamp load and the two parts start moving apart will the tension of the bolt start to increase. But, it would be proper to say that the bolted joint had failed long before this occurs. The clamping force and resulting friction is what keeps the parts from moving sideways relative to each other and so a minimum load must be maintained. In most bolted joint applications the bolts should never be subjected to shear stress (a shear bolt being an obvious exception but this isn't really a "bolted joint") and if they do get loaded in this manner you'd say the joint has failed due to loose or not enough bolts.
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Today's Featured Article - An Old-Time Tractor Demonstration - by Kim Pratt. Sam was born in rural Kansas in 1926. His dad was a hard-working farmer and the children worked hard everyday to help ends meet. In the rural area he grew up in, the highlight of the week was Saturday when many people took a break from their work to go to town. It was on one such Saturday in the early 1940's when Sam was 16 years old that he ended up in Dennison, Kansas to watch a demonstration of a new tractor being put on by a local dealer. It was an Allis-Chalmers tractor dealership,
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