Fatigue test: A paper clip bent back and forth seems to get weaker with each cycle. We call this process fatigue, from the French fatigué, which means tired. This is a low cycle fatigue test. Because the wire is so thin, relatively large strains are easily applied, and the clip breaks within a few tens of cycles. If you repeat the experiment, bending the clip by a smaller amount, the number of failed cycles increases.
Fatigue is a form of subcritical crack growth that provides a mechanism for crack growth until it is long enough for the applied load to cause catastrophic failure. As the board expands and contracts with temperature changes, a small potential crack in the board can propagate until it destroys the track and the board stops functioning. Fatigue failure of uncracked materials is usually attributed to crack initiation and subsequent growth under cyclic loading or strain. Due to the irreversibility of plasticity, smooth samples will develop surface cracks. For some materials, such as mild steel, there appears to be a lower stress limit (durability or fatigue limit) below which cracks will not form no matter how many cycles. This does not appear to be the case for other materials, such as aluminum. 7 or 10 8 loops.
Cracks often start with material defects that act as powerful stress concentrators and grow with each fatigue test cycle until the material fails.
