Phase angle thermography is basically spatially multiplexed photothermal radiometry. It provides images for remote detection and imaging of damage. For energy deposition one can use external heat sources ͑e.g., light or convective heating͒ or internal heat generation ͑e.g., electric current, microwaves, eddy current, or elastic wave͒. More complete information about defects is obtained by combining information from external and internal heating methods: Effects of intact thermal structure can be distinguished from defect specific thermal signatures. Our recent results relate to multifrequency excitation allowing for depth profiling from one lock-in-thermography measurement and, additionally, to modulated ultrasound frequency burst phase angle thermography where effects of standing elastic waves are eliminated. This way the applications for nondestructive evaluation become more reliable and faster.

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Elastic waves launched into a component by an ultrasonic transducer propagate inside the sample until they are converted into heat. A defect causes locally enhanced losses and consequently selective heating. Therefore amplitude modulation of the injected elastic wave turns a defect into a thermal wave transmitter whose signal is detected at the surface by lockin thermography that is synchronised to the frequency of amplitude modulation. This way ultrasound lockin thermography (ULT) allows for selective defect detection which enhances the probability of defect detection even in the presence of complicated intact structures. In this paper we report about investigations which are relevant e.g. for maintenance and inspection of aircraft and other safety-relevant areas. Using the phase angle images provided by ULT one can detect hidden corrosion, cracks in rows of rivets, disbonds, impacts, and delaminations.

Ultrasound excited thermography allows for defect selective imaging using thermal waves that are generated by elastic waves. The mechanism involved is local friction or hysteresis which turns a dynamically loaded defect into a heat source which is identified by a thermography system. If the excitation frequency matches to a resonance of the vibrating system, temperature patterns can occur that are caused by standing elastic waves. This undesirable patterns can affect the detection of damages in a negative way. We describe a technique how the defect detectability of ultrasound activated thermography can be improved. With the objective of a preferably diffuse distributed sonic field we applied frequency modulated ultrasound to the material. That way the standing waves can be eliminated or reduced and the detectability is improved.

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