Development of Thermographic NDTTechnique Based on Lock-in Measurement of Thermal Wave Response for Structural Integrity Assessment of Aging Aircraft

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The present project developed a new quantitative nondestructive testing (NDT) technique for structural integrity assessment of aging aircrafts. The results obtained are summarized as follows.1. For detecting cracks, singular electrothermal field near crack tips under the application of periodically modulated electric current was measured using lock-in thermography. Experimental investigations were made on the resolution and the applicability in the identification of cracks embedded in steel and aluminum alloy plates. Significant singular electrothermal field was observed at the crack tip in die lock-in thermal image. Location and size of the crack were accurately determined, demonstrating the feasibility of the proposed technique.2. For material loss defects and delamination defects, a thermographic NDT technique based on the lock-in measurement of the surface temperature under periodical heating was developed. Experimental study was made on the applicability to the detection of flat b … More ottom hole defects and delamination defects. In-phase and out-of-phase temperature amplitude images were taken by the lock-in thermography, synchronized to the reference signal of the periodical heating. Further, another thermographic NDT technique based on the transient temperature under the pulse heating was developed. Temperature descent curve which reflects defect parameters such as size, shape and depth of defects was analyzed using Fourier series. Developed techniques were applied to the identification of material loss defects and delamination defects in aerospace structures. It was found that defect parameters can be accurately determined by the developed thermographic NDT techniques.3. Quantitative NDT technique for delamination defects in concrete structures was developed based on the phase delay measurement using a lock-in infrared thermography. Location and size of the defect can be estimated from the contrast change and depth can be estimated from the relationship between phase delay and heating period.4. Passive electric potential CT method was developed as a new monitoring technique. Electric potential distribution measurement on the surface of piezoelectric film glued on the structure was processed by inverse analysis. Less