Experimental Comparison of Lock-in and Pulsed Thermography for the Nondestructive Evaluation of Aerospace Materials

Infrared thermography is a nondestructive evaluation technique aimed to the detection of surface temperature variations related to the presence of subsurface defects. Several inspection techniques have been developed over the years with basically two approaches, lock-in thermography and pulsed thermography. These two techniques are based on the principle of heat diffusion and thermal wave reflection, but they differ in the way they are practically implemented. In lock-in thermography (LT), an amplitude modulated heat wave is applied to the inspected specimen in steady state. The presence of an anomaly is revealed by a phase shift between the thermal evolution for a non-defective region and a defective one. In pulsed thermography (PT), on the other hand, the inspected specimen is stimulated by a heat pulse of short duration and the surface thermal evolution is monitored in transient state. In this case, the thermal profiles of defective and non-defective regions on the surface will diverge at a given time, which is related to the defect depth. This paper presents some comparative results obtained with these two techniques for the case of aerospace materials. The advantages and limitations of each technique for both qualitative and quantitative analysis are discussed and illustrated with some examples.