Non-Destructive Infrared Lock-in Thermal Tests: Update on the Current Defect Detectability

Non-destructive testing (NDT) is one of the best alternatives to perform inspections and maintenance operations in aerospace and aeronautics industries. In Lock-in Thermal Tests (LTT) the stimulation is modulated in a sinusoidal wave using mechanical loads, ultrasounds, microwaves or, as in this work, visible light through halogen lamps. This work assesses the influence of the parameters of LTT, such as defect geometry, cycle period, and number of cycles, interpolation method, and the type of image to identify the sensitivity of the LTT (parameter c). Several samples were manufactured with precise notches to simulate defects (slots). And performed several LTT in a controlled environment and with a custom jig to secure the samples. The performed tests permitted the analysis of various results for numerous types of controlled situations and defects, such as the slot width, depth, and cycle period. This work compared the number of cycles used during the test (1–15), the interpolation method (Harmonic or DFT) and the type of analysis (phase or amplitude). The cycle period indirectly defines the amount of energy applied during the test; therefore, it was expected to have a great impact in the results. Shorter cycles produced lower thermal differences, while longer cycles resulted in blurred images. The type of image was also found to be one of the most important setting, with the phase delay analysis presenting a higher differentiation of defects and its boundaries. The results from the variation of the number of cycles revealed these should be kept between three and nine. Additionally, the optical stimulation may also be a decisive setting, depending the defect geometry. As a major conclusion, the current LTT can detect defects with a width to depth ratio of 1.25, far less than 2.0, as is stated by the current literature.