Fretting Induced Fracture of Coupling Driven Shafts

Failures of externally driven compressor crankshafts may sometimes be due to fretting induced fatigue cracks. Fretting is a degradation process which occurs between two faying surfaces where the relative motion between the components is very small. Discoloration, fine o~ide powder, and shallow surface pitting are the definitive signs of fretting and hence relative motion between components. Fracture analysis of a fretting induced fatigue failure will show one or more crack initiation sites originating from fretting pits. Simple, static stress analysis will often show that the gross operational stress cycles are far below the material fatigue limit; however, by involking accepted microscopic processes, local stresses in the pit surfaces may be in the low cycle fatigue range. Even so, fracture mechanics concepts would indicate that the small surface cracks should grow into the lower stressed bulk material of the shaft and become arrested. That this does not occur requires a third mechanism to continue to drive the crack. In many case histories of very short shaft life at low driving stress levels, this third mechanism must be