Ultrasound lock-in thermography - a defect-selective NDT method for the inspection of aerospace components

Photothermal radiometry and its multiplex version, lock-in thermography hav e been used for several years for remote non-destructive testing. They are based on propagation and reflection of thermal waves tr which are launched from? the surface into the inspected component by absorption of modulated r adiation. Phase angle images obtained by superposition of the initial therm al wave and its reflection display hidden structures down to a certain dept h underneath the surface. Elastic waves sent into the component propagate inside the sample until the y are converted into heat. A defect causes locally enhanced losses and cons equently selective heating lip. Therefore, amplitude modulation of the inje cted elastic wave turns a defect into a thermal wave transmitter whose sign al is detected at the surface by lock-in thermography?; synchronised to the frequency of amplitude modulation. In this way. ultrasound lock-in thermog raphy (ULT) allows for selective defect detection which enhances the probab ility? of defect detection in the presence of complicated intact structures . Measurements were performed on various kinds of typical defects ill aerospa ce structures (both metal anti non-metal). The obtained phase angle images reveal areas of hidden corrosion, cracks in rows of rivets, disbonds. impac ts, and delaminations. In all these cases the intact structure was suppress ed since it heats lip much less in the elastic,wave field. We present examp les tl which are relevant, for example for maintenance and inspection of ai rcraft.