An excellent illustration of metal fatigue can be found in doing something that most of us have done on occasion: bending a piece of wire back and forth repeatedly until it breaks.
If we experiment with a wire coat hanger, we will discover some interesting things. Firstly, the further you bend it each time, the fewer repetitions are needed before it breaks.
Secondly, the speed with which you bend it doesn't matter, provided it's slow enough not to produce significant heat. Neither does it matter how long you rest between cycles.
For instance, If you bend it back and forth through 90 degrees once a week, it will break at exactly the same number of cycles as when you do it once a minute. If the temperature goes up, perhaps from rapid bending cycles, the fatigue life will be shorter. In other words, the wire will withstand fewer cycles before breaking.
What happens in a metal to cause it to become "tired" and start cracking after being subjected to enough stress cycles? Very superficially, we can say that a tiny crack will first appear at a point on the surface of the object which could be a bolt, shaft or rod, where the stress concentration is at a maximum. Any sharp corner, ridge, nick, shoulder, groove, even a scratch mark on the surface, will serve as a stress raiser. It will be at such a point that the initial little crack will appear. From there it will spread wider and deeper into the material. Eventually the component will be weakened to such an extent that the nextsufficiently large load imposed on it produces a sudden and catastrophic failure of the remaining metal. The failure surfaces will show the typical footprint of a fatigue failure: a smooth portion with telltale concentric "beach marks" showing how the crack has spread in steps, and beyond this a rough surface where the final, catastrophic failure took place.
Metal fatigue has been the cause of some tragic air disasters, the best known probably being the crashes of De Havilland Comet aircraft in 1954.
As in aviation, so also in the world of cars and trucks, has metal fatigue left a trail of destruction. Sometimes it's merely inconvenient or fatal and costly as illustrated by a recent case involving the Road Accident Fund, which arose from an accident outside Pretoria on a stretch of road used by a lot of tipper trucks.
A man was driving his bakkie along this road, his grandson sitting beside him. A tipper truck was coming in the opposite direction and when it was close to them, half a leaf spring broke loose from the right rear spring pack, fell to the ground, and from there shot forward into the bakkie. This lethal projectile sliced through the windscreen of the bakkie, brushed the driver's arm, then struck the head of his grandson, inflicting fatal injuries.
Operators of tipper trucks will tell you that the springs on their trucks lead a hard life. They can and do break if maintenance is neglected.