In this paper, a method for remote measurement and shape reconstruction of fatigue cracks by using laser-line thermography (LLT) technique is developed. A new feature parameter derived from the LLT signals is proposed to evaluate the length and estimate the general inner profile of a surface-breaking fatigue crack. An inversion analysis scheme based on conjugate gradient optimization algorithm is then applied to reconstruct the detailed inner profile and dimension of the crack. The reconstruction results with both numerically simulated LLT signals and experimental signals proved the feasibility of proposed inversion scheme and remote LLT method. Based on methods of this paper, not only the size of cracks can be quantitatively evaluated, but the inner profile is also reconstructed for cracks in different shapes from the LLT signals.

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Thermal barrier coating (TBC) system is indispensable for the heavy duty gas turbines or the aero-engine blades working at high temperature. Since the interface crack is one of the crucial issues affecting the integrity of the TBC, it is necessary to inspect the debonding defects in TBC by using a non-destructive testing (NDT) technique. Laser infrared thermography (LIRT) is a high efficiency NDT tool for thin-wall structures such like the TBC, and the LIRT using a homogeneous laser beam is especially suitable for detection of the debonding defects. In this paper, the feasibility of the homogeneous LIRT is verified for NDT of the debonding detection at first through finite element numerical simulation. Second, an experimental system using the diffractive optical element and a plano-concave lens is setup to conduct LIRT measurement with a wide homogeneous square laser spot. By using TBC specimen with debonding defects fabricated using the salt dissolving method, the pulsed LIRT and lock-In LIRT are conducted to detect the artificial debonding defects in TBC test-pieces. It is proved that the pulsed LIRT is of higher efficiency and the measured thermal image can be improved through denoising processing by using algorithm such as TSR, PCA etc. It is also found that the TSR image processing method costs less and can give results of higher contrast. The experimental results show that the debonding defects of diameter larger than 3 mm can be efficiently and accurately detected with the LIRT technique.