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Fatigue crack identification using near-tip singular temperature field measured by lock-in thermography

Applicability of the newly developed two different lock-in thermographic NDT techniques is discussed. One of the proposed techniques is based on the lock-in measurement of the singular temperature field, which appears near crack tips under the application of periodically modulated electric current. Experimental study is made on the resolution and the applicability in the detection of through-thickness cracks embedded in steel and aluminum alloy plate samples. Modulated electric current is applied to the cracked sample by an induction coil. Temperature amplitude and phase delay thermal images synchronized to the reference current modulation signal are taken by the lock-in thermography. Significant temperature rise related to singular temperature field is observed at the crack tips in the lock-in thermal images. It is found that the cracks are sensitively detected by the lock-in thermography technique combined with near-tip singular temperature field measurement. The other technique is based on the lock-in measurement of the surface temperature under the application of periodical xenon light heating. Experimental study is made on the applicability to the detection of flat objective body. In-phase and out-of-phase temperature amplitude images are taken by the lock-in thermography, synchronized to the reference signal of the electric shutter operation. It is found that the location and size of the defects can be identified by the localized contrast change in the out-of- phase images. Further, the depths of the defects can be identified from the heat penetration depth, which is changed by the frequency of thermal wave stimulation.