| The bolted joint is a very complicated beast. So many variables affect its behavior that its performance in service is difficult to predict or explain. These problems arise for several reasons. First of all, bolts and nuts work properly only if frictional forces resist relative motion between themand friction is an unreliable ally. Literally hundreds of factors can affect the friction restraint between bolt and nut, so the resulting forces are economically impossible to control or predict. This makes the behavior of the fastener difficult to predict. Worse, it introduces significant uncertainty when we tighten the fasteners with an economically attractive tool (like a torque wrench). We can't predict exactly how much preload we'll create in the bolts for a given torque, yet the magnitude of that preload will probably dominate joint behavior. |
| Second, joint behavior is strongly influenced by the elasticity of its partsthe nuts, bolts, and joint members. As a result, any attempt to explain this behavior must take elasticity into account. Gone are those happy, freshman days when we could treat everything as frictionless, rigid bodies. |
| The difficulties increase significantly if the joint contains a gasket, because we now must also deal with the behavior within the joint of a body which is neither rigid nor elastic, but is elasto-plastic. It is partially elastic, but the degree of elasticity (its resilience or stiffness) is not a constant but is a function of the compressive stresses which have been placed on the gasket during assembly and after it was put in service. Furthermore, thanks to its partially plastic nature, we must also take into account such things as permanent deformation and creep. |
| Gaskets also escalate our concern for another reason. Although failure of many non-gasketed joints (airplane or bridge structures, for example) can be catastrophic, the failure of the majority of non-gasketed joints is merely a nuisance. This is rarely true of the failurethrough leakageof gasketed joints. At best, this merely causes things like a large puddle on the floor or the temporary loss of horsepower in an engine; at worst, leakage can lead to pollution, fire, |
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