Vibrotermography and its use for Detecting cracks

This paper deals with ultrasonic infrared nondestructive technique for detecting cracks in materials. This technique uses a short pulse of low frequency ultrasound to cause the crack surfaces to rub or clap, thus inducing frictional heating. This heating is then observed through the use of an infrared 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 infrared 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 can observe fatigue cracks as short as 20 micrometers in metal samples.