Objective  Laser long pulse thermography technology has distinct advantage in quantitative detection of the width of surface microscopic cracks, which is of great importance to the service safety of aluminum alloy structure. However, the heat source excited by long pulse laser decays rapidly in the vicinity of the crack when it expands, which limits the accuracy of the crack width. Besides, crack width detection based on temperature magnitude would be affected obviously even if with slight measurement error. While temperature rise distribution of the crack along the direction of the long pulse laser under the quasi-steady-state temperature field could reflect the width of the surface crack precisely. Therefore, it is necessary to establish the temperature rise characteristics for width detection of microscopic surface crack of aluminum alloy structure. For this purpose, the heat conduction law of surface crack of aluminum alloy under the quasi-steady-state temperature field excited by long pulse laser is proposed, and a systematic experiment is designed in this study.

  Methods  It is found that the temperature rise is minimum at the center of the crack, and the amplitude of temperature rise reduces with the increase of the crack width based on systematical analysis of the quasi-steady-state temperature field distribution excited by long pulse laser at the surface crack. After that, a laser thermal imaging test platform is built, and temperature rise distribution of cracks with different width is collected. The contour distribution of the highest temperature rise at the center of the laser line is extracted (Fig.8), and the weight vector of the crack surface temperature rise distribution is calculated by using the principal component analysis method (Fig. 10-11). The normalized Euclidean norm of the temperature rise curve is extracted based on the temperature rise change at the crack within 4 s after the long pulse laser excitation (Fig.13).

  Results and Discussions   Both parameters decrease rapidly with the increase of the crack width, γ is constructed based on the contour distribution of the quasi-steady-state distribution of temperature (Fig.12), δ is defined based on the Euclidean norm of the temperature rise phase excited by long pulse laser (Fig.14). After that, composite parameter Λ, combined γ with δ, is constructed to eliminate the influence of laser power. The results show that, when the crack width is smaller than 700 μm, parameter Λ shows good linearity to the crack width, and the width pathology effect could be reduced effectively. The detection of crack width would be affected by the resolution of the thermal imaging camera. In this study, a single pixel of the thermal imaging camera is about 256 μm. So when the crack width is smaller than 300 μm, the temperature of the crack collected by the camera is not only from the crack, but also from the edge of the crack, which would affect the accuracy of the width calculation. And the accuracy would be improved by the improvement of the thermal imaging camera resolution.

  Conclusion  In this study, the thermal response of the crack width on the surface of aluminium alloy excited by long pulse laser is analysed based on the heat conduction law. Temperature rise distribution and temperature rise variation of cracks with different width under long pulse laser excitation are investigated. According to the characteristic parameters of temperature rise distribution and temperature rise variation of crack of different widths, a composite index of crack width independent of laser power was established by using ratio definition method. The results show that the composite index of spatial characteristic parameters can greatly improve the reliability and accuracy of crack width detection, and can achieve quantitative detection of cracks of 50-700 μm, which can lay a solid foundation for the application of laser infrared thermography in aerospace crack quantitative detection.