Comparison in the transient regime of four lock-in thermography algorithms by means of synthetic and experimental data on CFRP

Lock-in thermography is considered a well-established technique for investigating fiber-reinforced polymer composite (FRP) materials. These materials are characterized by a low thermal diffusivity and for this motivation, lock-in tests on composite materials generally are time consuming and then, for some applications, not suitable for industrial in-situ inspections. A way for reducing the testing time is to carry out the lock-in tests in the transient regime, although the most well-known lock-in algorithms were originally developed for the post-processing analysis in the steady-state regime. In this regard, the aim of this paper is to present a comprehensive comparison, in the transient regime, of four well-known lock-in thermography algorithms: the Signal Reconstruction (SR), Fast Fourier Transform (FFT), Dual Integration (DI), and Four Points method (4PM). In particular, to evaluate and compare how the transient regime can affect the accuracy and precision of the algorithms in determining the phase contrast of defects, several experimental tests were carried out on a carbon fibre-reinforced polymer (CFRP) test piece having different artificial defects by modulating the sine excitation periods by means of halogen lamps. Furthermore, synthetic data were used for simulating and investigating the four lock-in algorithms in a steady-state regime, as well. In addition, in order to evaluate the robustness of each algorithm, some offline processing analyses were carried out on the experimental data to evaluate the performance of each algorithm as a function of two user-controllable parameters, the number of frames and the number of cycles, and one not directly controlled by the operators, the dropped frame. For each analysis, qualitative and quantitative results in terms of phase and normalized phase contrast are presented and discussed.