We address the characterization of defects that behave as heat sources in nondestructive thermographic techniques. First, we consider tilted heat sources of rectangular shape. We calculate the evolution of the surface temperature distribution generated in a short excitation. For the characterization, we make use of the thermogram obtained at the end of the excitation and the temperature evolution at the center of the early heated region. A sensitivity analysis indicates that the optimum excitation duration corresponds to a thermal diffusion length similar to the depth of the deepest end of the heat source. By fitting synthetic data with added noise, we analyze the influence of the signal to noise ratio and the inclination of the heat source on the fitted parameters. Inductive thermography experiments carried out on insulating samples with embedded Cu slabs confirm the ability of the method to characterize tilted heat sources and indicate that the penetration is the most elusive parameter. In the second part, we present a methodology to deal with horizontal heat sources of unknown geometry. We average the thermogram obtained at the end of the excitation in circumferences concentric with the center of the heated region. This averaged radial profile, together with the temperature evolution at the center of the heated region is fitted to a circular heat source model. Fittings of experimental data taken on samples with horizontal rectangular Cu slabs allow determining the area with accuracy better than 20% and the depth with 10%.
Scanning inductive thermography is a non-destructive inspection technique, which is suitable for detecting surface defects in long metallic work pieces. The work piece is moved below the inductor and the infrared (IR) camera, which is recording the surface temperature during the motion. To evaluate such measurements via phase image the recorded infrared image sequence must be reorganized according to the scanning speed. If the speed is not constant during the motion (e.g., due to manual scanning), visual fiducials (AprilTags) can be used in the camera’s field of view to register shifts between consecutive images. The main contribution of this work is image fusion, applied to scanning inductive thermography, combining the results of an infrared camera and a visual camera. An uncooled IR µ-bolometer camera with a thermal time constant of 8 ms is used for the infrared spectrum. Information of the motion speed during the scanning is acquired by the image registration, and it is used to deblur the IR image sequence before the evaluation to phase image via Fourier transform is performed. A second camera records the scanning process in the visual range. Using the AprilTags for registration, a panoramic view of the specimen is created. The results from both cameras are superimposed to improve the interpretation and localization of defects.
Inductive thermography is a well-established NDT method to detect surface cracks in metallic materials. The induced eddy current density decays exponentially below the surface, penetrating up to the so-called skin-depth. This depth depends on the excitation frequency and on material parameters, as the magnetic permeability. As a surface crack is an obstacle for the eddy current and for the heat flow, it becomes visible in the infrared images. It is investigated whether cracks ending below the surface, can be detected by inductive thermography. It is stated, that when the crack end is lying closer the surface than the half skin-depth, then it can be detected. This statement is investigated for ferro-magnetic and non-magnetic samples and for different excitation frequencies. The inspection is usually done in reflection mode, but for thin wall work-pieces the transmission mode provides a good detection possibility. Experimental results are compared with finite element simulation results.
Carbon fiber reinforced plastic (CFRP) specimens, charged with defined loads in impact tests, have been examined with flash and inductive thermography from the front and the rear side. In the case of inductive thermography, eddy currents are induced in electrically conductive materials, usually in metals. But it can be also excellently used for inspection of CFRP, as eddy current can be induced in the carbon fibers. The fiber’s orientation regarding the magnetic field of the induction coil also has an influence on the detection results. The sequence of the temperature images, recorded during and after the short inductive heating pulse, is evaluated with a Fourier Transform, and the obtained phase image is used for localizing the impact damages. The flash thermography tests in transmission and reflection mode were evaluated using PPT and TSR methods. The results of the flash and inductive inspection techniques are compared for samples with different degrees of damage, in order to learn more about the capability of induction thermography for detecting impact damages.
Subsurface defects can be well detected by flash thermography evaluating the temperature response at the sample surface. In many cases flat bottom holes or air inclusions are investigated as typical defects. In contrast, in the current paper the main emphasis is placed on metal inclusions hidden in an insulator material. As the thermal effusivity of the metal is significantly higher than of the base material, the temperature decreases quicker above such a defect. Thermal quadrupole calculations and finite element simulations have been used to investigate more closely these temperature signals. Additionally, 3D printed samples have been created, where in the plastic material different metal plates, as steel, aluminum and copper have been introduced. The measurement results on these samples show very good agreement with theoretically calculated curves.
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