Nondestructive thermographic detection of internal defects using pixel-pattern based laser excitation and photothermal super resolution reconstruction

Thermographic testing, as a non-destructive testing method, is a non-contact imaging technique for examining specimens by characterizing their thermal conduction behavior. One of the main limitations of thermography is the inherent limit to the resolution of the internal structures of any specimen due to the diffusive nature of heat conduction. Overcoming this limit is the goal of thermographic super resolution methods. In this work, we present a method using two-dimensional pixel pattern based active photothermal laser heating in conjunction with subsequent numerical reconstruction to achieve a high-resolution reconstruction of internal defect structures. Furthermore, a forward solution to the underlying inverse problem is proposed along with an appropriate heuristic to find the regularization parameters necessary for the inversion in a laboratory setting. This allows the generation of synthetic measurement data, opening the door for the application of machine learning based methods for future improvements towards full automation of the method. The proposed experimental approach is validated using the latest laser coupled DLP projector technology as a heat source. Finally, the proposed method is shown to outperform several established conventional thermographic testing techniques while improving the required measurement times by a factor of 8 compared to currently available photothermal super resolution techniques.

Thermographic detection of internal defects using 2D photothermal super resolution reconstruction with sequential laser heating

Thermographic photothermal super resolution reconstruction enables the resolution of internal defects/inhomogeneities below the classical limit which is governed by the diffusion properties of thermal wave propagation. Based on a combination of the application of special sampling strategies and a subsequent numerical optimization step in post-processing, thermographic super resolution has already proven to be superior to standard thermographic methods in the detection of one-dimensional defect/inhomogeneity structures. In our work, we report an extension of the capabilities of the method for efficient detection and resolution of defect cross sections with fully two-dimensional structured laser-based heating. The reconstruction is carried out using one of two different algorithms which are proposed within this work. Both algorithms utilize the combination of several coherent measurements using convex optimization and exploit the sparse nature of defects/inhomogeneities as is typical for most nondestructive testing scenarios. Finally, the performance of each algorithm is rated on reconstruction quality and algorithmic complexity. The presented experimental approach is based on repeated spatially structured heating by a high power laser. As a result, a two-dimensional sparse defect/inhomogeneity map can be obtained. In addition, the obtained results are compared with those of conventional thermographic inspection methods which make use of homogeneous illumination.