The process of progressive localized permanent structural change occurring in a material subjected to conditions which produce fluctuating stresses that may culminate in cracks or complete failure after a sufficient number of fluctuations. Fatigue is often identified within the following classes: High Cycle Fatigue - Loading is primarily elastic and material failure occurs in excess of 10,000 stress cycles. Low Cycle Fatigue - Loading is mostly elastic and material failure occurs in 1,000 to 10,000 stress cycles. Fatigue Ultra-Low Cycle Fatigue - Loading is plastic and material failure typically occurs in less than 1,000 stress cycles.