Inductive thermography is a non-destructive inspection technique. The sample is heated with a short heating pulse and an IR camera records the surface temperature, which is then evaluated to a phase image by Fourier transform. The technique can be well applied for detecting cracks in metals. Additionally, it has also the advantage of providing information about the depth of the crack. Larger contrast is an indication of deeper cracks, while small contrasts refer to shallow cracks. Therefore, the phase contrast can be used to make an estimation of the considered crack. In order to investigate these capabilities, short cracks (length =0.3-3mm) were created in Inconel 718 welded samples by a Varestraint test machine. The samples were then inspected with inductive thermography, computer tomography (CT) and by fluorescent penetrant test (FPT). The crack lengths obtained by all the three methods are compared. The dependency of the phase contrast on the crack depth and length is then analyzed in comparison to the CT results. Finally, additional finite element simulations were carried out and compared to the experimental results.
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.
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