Fatigue is failure under repeated or otherwise varying load which never reaches a level sufficient to cause failure in a single application. Component seems to lose strength after multiple load applications, appears to get tired, hence the name “fatigue”.     Failure caused by fatigue can be minor or catastrophic. The consequences of failure are often very costly and it is estimated that 80-90% of all structural failures are caused by fatigue. Catastrophic failure by fatigue often occurs without warning at low stress levels (see Figure 1, 2 and 3).   A paper clip that is straightened and then bent back and forth in the same spot about five times will fail and break into two pieces. That is an example of a low cycle…