Detection and Characterization of Defects in Isotropic and Anisotropic Structures Using Lockin Thermography

Christiane Maierhofer,Philipp Myrach,Mathias Röllig,Rainer Krankenhagen,Henrik Steinfurth

doi:10.3390/jimaging1010220

Christiane Maierhofer, Philipp Myrach + Show 3 more

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https://doi.org/10.3390/jimaging1010220

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Abstract

Lockin thermography is a well-suited method for the characterization of structures made of both metal and fiber reinforced plastic. In most cases, only phase images are analyzed, although the amplitude images might contain useful information as well. Thus, systematic studies of lockin thermography are presented, assessing amplitude and phase images for the detection and quantification of defects in isotropic (steel) and anisotropic (carbon fiber reinforced plastic) materials. Characterized defects are flat bottom holes with different diameters and various remaining wall thicknesses as well as crossed notches at different depths. The excitation frequency was varied while keeping the number of analyzed excitation periods nearly constant for each material. The data analysis was focused on the detectability of the defects both in the amplitude and phase images, including the determination of the signal-to-noise ratio and of the spatial resolution. As a result, the limits of defect detectability and spatial resolution are given for each material.

Highlights

Among other established techniques for Non-Destructive Testing (NDT), active thermography is a relatively new technology for NDT of metallic as well as non-metallic structures
The investigations show that the detectability of defects in amplitude and phase images depends on Remaining Wall Thickness (RWT) of Flat Bottom Holes (FBH) and notches and lateral size of the defects as well as on the thermal diffusivity of the surrounding material and its anisotropy
It was expected that the phase images will show an information depth which is about a factor of two larger than that of the amplitude images, this was not revealed by the experimental data

Summary

Introduction

Among other established techniques for Non-Destructive Testing (NDT), active thermography is a relatively new technology for NDT of metallic as well as non-metallic structures. The main advantage of active thermography over other NDT methods is its comparably fast and contactless data acquisition, giving direct images of the structure under investigation. Only a one-sided optical access to the surface below which defects are expected is required. Active thermography is based on the generation of a non-stationary heat flux inside the structure, created by optical, infrared, electromagnetic, mechanical, or other excitation sources. The choice of an appropriate excitation source depends on the particular NDT setup and the object under investigation. Using an infrared (IR) camera, defects or inhomogeneities having different thermal properties than the sound material can be detected from temperature differences at the surface. From the temporal evolution of these temperature differences, information about defect properties, such as material and geometry, can be obtained

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