Photothermal radiometry and its multiplex version Lockin-thermography arebeing used since several years for remote non-destructive testing. They arebased on propagation and reflection of thermal waves which are launched fromthe surface into the inspected component by absorption of modulatedradiation. Phase angle images obtained by superposition of the initialthermal wave and its reflection display hidden structures down to a certaindepth underneath the surface.Elastic waves sent into the component propagate inside the sample until theyare converted into heat. A defect causes locally enhanced losses andconsequently selective heating up. Therefore amplitude modulation of theinjected elastic wave turns a defect into a thermal wave transmitter whosesignal is detected at the surface by lockin-thermography synchronised to thefrequency of amplitude modulation. This way ultrasound lockin thermography(ULT) allows for selective defect detection which enhances the probabilityof defect detection in the presence of complicated intact structures.Measurements were performed on various kinds of typical defects in aerospacestructures (both metal and non-metal). The obtained phase angle imagesreveal areas of hidden corrosion, cracks in rows of rivets, disbonds,impacts, and delaminations. In all these cases the intact structure wassuppressed since it heats up much less in the elastic wave field. We presentexamples which are relevant e.g. for maintenance and inspection of aircraft.
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