Ultrasound lock-in thermography: feasibilities and limitations

Conventional thermal wave imaging methods based on optical excitation are sensitive to all thermal boundaries and structures. However, if ultrasound is used for energy deposition, areas of enhanced mechanical loss angle are heated up selectively. As defects are usually correlated with an enhanced loss angle, ultrasound lockin thermography (ULT) is a defect-selective NDE method. Thermal wave emission from defects is monitored that are activated by absorption of ultrasound. In this paper we investigate the applicability of ULT for maintenance and inspection. Examples are presented showing the detection of defects in thick composite materials, of vertical cracks in several materials, and of hidden corrosion. Limitations given by the use of ultrasound are also discussed.