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Avoid metal fatigue

By unknown | Feb 11, 2009 | COMMENTS [ 0 ]

The way metal fatigue is caused on a car is rather different from the example of the coat hanger being bent back and forth. Instead of the metal being subjected to few, but large, cyclic movements, a component of a car has to withstand hundreds of thousands of stress cycles, each producing a minuscule deformation of the component.

But, the result is the same: eventually a fatigue fracture will start, unless the maximum stress level during these cycles remains below the so-called fatigue limit of the material, in which case it will withstand a practically infinite number of stress cycles.

For example, if the bolts clamping the flywheel to the crankshaft boss should become even slightly loose, the diminished friction can no longer do the job alone. The torsional stresses now lead to cyclic shear stress in the bolts each time the crankshaft speeds up . The metal fatigue created by such stress will finish off the best bolts in the world before long.

The home mechanic must be alert to the constant danger of metal fatigue. He should first try to prevent its initiation - don't give the first little crack a place to form. This can be done by avoiding all stress raisers on the surface of a component, because it will be at the point of maximum stress concentration where the fatigue crack will start. Any surface imperfection is a stress raiser, including fret marks on used bolts and pit marks from corrosion.

Also, there must be no sharp shoulders or corners, such as where the head of a bolt meets the shank. The thread runout region (where the threaded part of the shank meets the unthreaded part) is a known weak spot on a bolt and must not be placed at the interface between two clamped surfaces.

Even if the design of a component is as good as humanly possible, the manufacturing and installation is never perfect, and there is always the chance that metal fatigue will raise its ugly head in spite of precautions. Therefore it is essential that critical or highly stressed bolts (such as the U-bolts which featured in the tragic accident mentioned last week) be inspected regularly.

This is one of the reasons why critical bolts are often given a polished surface - it makes it easier to detect the start of a fatigue crack. It also removes surface imperfections which can act as stress raisers. That's the only good thing that can be said for metal fatigue: it will always give you warning. But, to pick that up before catastrophic failure occurs requires a strict routine of regular, thorough inspections.

The tightness to which a bolt or nut is installed is extremely important to provide the correct clamping force and prevent the bolt from loosening. We shall come back to this point next week. - Motoring Reporter


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