In this study, we propose a new post processing algorithm, using a stable low-rank decomposed pseudo restored heat flux based on the ensemble variational Bayes tensor factorization (EVBTF-RPHF) algorithm for performing periodic square wave thermographic nondestructive testing (thermographic NDT). Previous studies have shown that both RPHF and EVBTF can separately improve the detectability of thermography by enhancing some defect features. However, both methods are limited by their particularly constraints: RPHF are heavily degraded by noises and missing data due to the assumptions under which the physical models are derived while efficiency of EVBT reduces when the lateral heat diffusion weights out. By embedding RPHF into the stable low-rank decomposition EVBTF, the proposed algorithm allows to improve the detectability of defects in thermographic NDT using a periodic heat flux with low-rank spatial distribution. The study verifies the capacity of the proposed method by theoretical analysis. Then, experiments were conducted on a carbon fiber composite panel with foreign inserts buried up to 5 mm deep. The sampled data are processed by the proposed method. The results are compared with existing methods such as phase-locked RPHF and EVBTF. The experimental results demonstrated that defects with normalized diameter-to-depth ratios as small as 0.9, barely detected with other available techniques, can reliably be detected by EVBTF-RPHF. The signal to noise ratio and the contrast are used as figure of merit to quantitatively compare the capacity of the proposed method with existing methods. However, the computation efficiency of the proposed algorithms needs further improvement.

The first practical attempts of sensing our world in the infrared spectrum were undertaken in the nineteenth century. One of the first implementations of the active thermal non-destructive testing (TNDT) was the inspection of Polaris rocket motor cases in 1965 but by the end of the 1970s applications of IR thermography inspired by successful developments of IR technology in military were still rather qualitative thus preventing the progressive competition of TNDT with other inspection techniques. A new level of TNDT was achieved after the wider use of elements of the heat conduction theory. Nowadays, IR thermographic diagnostics and TNDT represent mature high-technology fields which combine achievements in the understanding of heat conduction, material science, IR technology and computer data processing. The high interest to the IR/thermal inspection technique is related to its universal character, high testing productivity and in-service safety. The purpose of this review is to briefly review the basics of TNDT, which include elements of many disciplines of science and technology, such as heat conduction, IR engineering, optoelectronics, image processing and statistics.

The history of thermal nondestructive testing and infrared thermography is briefly reviewed. The state-of-the art of thermal testing with a focus on its applications to testing of composite materials