By far, the majority of mechanical failures happen from fatigue. (If someone asks you why something failed, just tell them fatigue. You’ll be right 90% of the time and that’s a great average in any activity.) But saying that fatigue caused a failure is like saying the hill caused a car’s brakes to fail. The value of failure analysis is that you can use it to look at the broken parts, determine the type and magnitude of the forces involved, then do something to prevent their recurrence. The appearance of the fracture face, the shape of the progression marks, the location, shape and size of the instantaneous zone (fast failure zone), and the direction of the failure propagation really tell how the fracture occurred.