Sonic infrared imaging of fatigue cracks

We describe a novel hybrid ultrasonic/infrared nondestructive technique for detecting cracks. This technique uses a short pulse of low frequency ultrasound to cause the crack surfaces to rub or clap, thus inducing frictional heating. The heating is then observed through the use of an infrared (IR) video camera. In the case of a surface-breaking crack, the appearance of the image of the crack takes only milliseconds after the initiation of the ultrasonic pulse. Subsurface cracks become visible to the IR camera with time delays that are determined by diffusion of the heat from the crack to the surface of the sample. Using this method we have observed fatigue cracks as short as 20 μm in metal samples.