A monitoring system for contact fatigue crack testing

A new monitoring system is developed to detect crack initiation and propagation in the surface layer during contact fatigue running. The magnetic head of a commercial tape recorder is used as a probe. A combination of leakage and eddy current methods is ussed for the contact roller specimen on the JPM-1 machine. Experimental results show that this system is very sensitive to artificial defects and can distinguish defects on the surface from those beneath the surface. The defect signals agree well with their positions. Testing of an ion-nitrided specimen shows that a tiny subsurface crack (∼ 1 μm), which can only be examined under a microscope with 200 × magnification, is observable with this system.