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

TL;DR

This paper advances thermographic super resolution techniques by enabling the detection of internal defects with 2D laser heating and convex optimization algorithms, surpassing traditional methods in resolution and efficiency.

Contribution

It introduces two novel algorithms for 2D defect reconstruction using structured laser heating and sparse optimization, extending super resolution thermography capabilities.

Findings

Enhanced defect detection resolution compared to standard thermography

Effective 2D defect mapping with structured laser heating

Algorithms demonstrate good reconstruction quality and manageable complexity

Abstract

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. Due to the sparse nature of the reconstructed defect/inhomogeneity map, this comparison is performed qualitatively.