Ultrasound excited Lockin Thermography (ULT)

The detection of defects with optically excited thermography is only possible by comparing the observed with an faultless structure.
A method, that is selective to damaged areas would improve testing reliability. This is possible if the changed mechanical properties of defects are used to generate modulated heat. The physical mechanism of this is the increase of the mechanical loss angle (hysteresis) in the defect area. Therefore ultrasound is injected into the component, which is preferably damped in the defect area and is thereby generating heat. If the ultrasound amplitude is modulated with low frequencies, then the heat oscillates and the defect emits a thermal wave, which highlights the defect like a dark field method. Therefore, the lamp is replaced by an ultrasound converter, which is fixed at the test object.
Despite similar test configurations of both lockin methods, the images contain completely different information: With optical excitation a thermal wave is generated at the surface and reflected at boundaries within the component. The image therefore shows boundaries. Using ultrasound excitation the local hysteresis area is the contrast mechanism. This allows the distinction between different defects. A gaping crack is easier to detect with optical excitation than with ultrasound excitation, whereas it is vice versa when analysing closed cracks.