Implementation of Active Infrared NDT Techniques Using Long Square Heating Pulses

The present work describes the implementation of active IR Thermography techniques for the NDT of thick polymer and glass-fibre reinforced polymer (GRP) composite panels. A low cost Thermal NDT set-up is proposed, comprising a single-detector IR camera with low thermal resolution and low frame rate, and common low-power halogen lamps as external heat source devices. The use of halogen lamps in particular requires several seconds of switch-on time in order to deliver meaningful and effective heat quantities. The influence of such long heat deposition intervals is investigated on the possibility to implement Transient Thermography and Lock-In Thermography techniques. Regarding the Transient Thermography approach this is carried out as in traditional Pulsed Thermography, and the Thermal Contrast is in particular analyzed assessing the influence of the heat pulse duration and non-uniformity of heat deposition. Regarding the Lock-In approach a technique is adopted which modulates heat deposition into trains of square waves. A lock-in signal processing in the frequency domain is then performed to obtain Phase Contrast parameters.