Development of Lock-In Infrared Thermography Techniques for Quantitative Nondestructive Evaluations

In this paper, lock-in thermography techniques for quantitative nondestructive evaluations developed by the present authors are reviewed. Self-reference lock-in thermography was developed for remote nondestructive testing of fatigue cracks. This technique is based on the measurement of thermoelastic temperature change due to stress change. Cracks can be identified from significant temperature change observed at crack tips due to the stress singularity. For accurate measurement of the thermoelastic temperature change under random loading, a self-reference lock-in data processing technique was developed, in which a reference signal was constructed by using the temperature data simultaneously taken at a remote area. Thermoelastic temperature change in a region of interest was correlated with that at the area for reference signal construction. It enabled us to measure the relative stress distribution under random loading without using any external loading signal. The self-reference lock-in thermography was applied for fatigue crack identification in welded steel plate specimens and actual steel structures. It was found that significant temperature change was observed at the crack tip in the self-reference lock-in thermal image, demonstrating the feasibility of the proposed technique. Lock-in thermography technique was also applied to quantitative nondestructive evaluation of material loss defects. Transient temperature data under pulse or step heating were measured by infrared thermography. Temperature data were processed by the lock-in analysis scheme based on the Fourier series expansion, in which Fourier coefficients synchronizing with sine and cosine waves were correlated with defect parameters. Experimental investigations were conducted using steel samples with artificial material loss defects. It was found that the defect parameters can be quantitatively determined from the Fourier coefficients, demonstrating the feasibility of the proposed technique.