Abstract
We combine three different approaches to greatly enhance the defect reconstruction ability of active thermographic testing. As experimental approach, laser-based structured illumination is performed in a stepwise manner. As an intermediate signal processing step, the virtual wave concept is used in order to effectively convert the notoriously difficult to solve diffusion-based inverse problem into a somewhat milder wave-based inverse problem. As a final step, a compressed-sensing-based optimization procedure is applied which efficiently solves the inverse problem by making advantage of the joint sparsity of multiple blind measurements. To evaluate our proposed processing technique, we investigate an additively manufactured stainless steel sample with eight internal defects. The concerted super resolution approach is compared to conventional thermographic reconstruction techniques and shows an at least four times better spatial resolution.
Highlights
I N RECENT years, active thermographic testing (TT) has gained more and more attention in industry, especially as a contactless, simple, and relatively inexpensive nondestructive testing method
[26] showcases limitations in spatial resolution already at aspect ratio (AR) = 1:1, with a sample made of stainless steel, which did not encourage us to make use of lock-in thermography
It is highly recommended to use this method in fields of nondestructive testing due to the given defect sparsity
Summary
I N RECENT years, active thermographic testing (TT) has gained more and more attention in industry, especially as a contactless, simple, and relatively inexpensive nondestructive testing method. In TT, a heat flow in the specimen is forced by an external heat source. Light sources such as highpower lasers or flash lamps are employed to make use of the photothermal effect [1], [2]. The resulting heat flow in the material is altered at defect regions, e.g., given by cavities in the material, which is indicated as a temperature change in the measured thermal image sequence by an infrared (IR) camera. The diffusive behavior of heat impedes a precise reconstruction of subsurface defects in a material, especially in metals such as stainless steel. Since stainless steel is a widely used material in the pharmaceutical industry, medical technology, and vehicle construction, due to its corrosion resistance, an improvement of the insufficient conventional thermographic techniques is necessary
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