Detecting incipient fatigue damage with scanning squid microscopy

A scanning microscope based on a high transition temperature Superconducting Quantum Interference Device (SQUID) was used to detect incipient fatigue failure in ferritic stainless steel. The results show the development of localized peaks in remanent magnetization prior to the formation of visible fatigue cracks, that is, in region 2 of fatigue damage. Because of the combined magnetic and spatial resolution of the SQUID microscope, these local changes can be detected well before there is any dramatic change in the overall magnetization of the specimen. An interesting secondary result is that, while the SQUID microscope also detects small cracks, until these have a significant crack opening (more than a few microns) the dominant contribution to the local magnetization peak is the damage associated with the crack (presumably, the damage that created the crack) rather than the crack itself.