Localizing surface cracks with inductive thermographical inspection: from measurement to image processing

Induction Thermography for Surface Crack Detection and Depth Determination

“…The induction coil is a so-called Helmholtz coil consisting of two circular windings placed from each other at a distance (see Figure 7). This coil geometry has the advantage that the magnetic field in the region between the windings is approximately homogenous [10], and the induced eddy current flows roughly parallel to the coil circles. The workpiece to be inspected is placed in this middle part.…”

“…It is important to note that the simulations and the measurements were set up in a way such that the induced eddy current flows perpendicular to the crack direction. Additional investigations were carried out on how the signal is influenced if this angle is less than 90° [10,23]. A possible setup was also presented using two Helmholtz coils with different excitation frequencies, whereby the detection becomes independent of the crack orientation.…”

“…Three curves were calculated for ferro-magnetic steel (dash-dot lines) and the other three were for non-magnetic steel (solid lines). To be able to compare different materials, the abscissa is normalized to (10) at which time the heat flows into a distance of d due to diffusion. In Figure 23a, it can be observed that…”

“…In these simplified models, the cracks were modeled either as a long notch or as a short slot going through the whole sample thickness [7,8]. Experimental results for surface crack detection with induction thermography have been reported by several groups in different ferro-magnetic samples, for example, in forged parts [9,10] and in rail surfaces [11,12]. As these head checks lay typically inclined to the surface, further numerical models have been set up for angular defects [12][13][14].…”

Investigation into eddy current pulsed thermography for rolling contact fatigue detection and characterization

“…To resolve or alleviate these difficulties, generation of a uniform magnetic field (UMF) using Helmholtz coils [24] is a potential solution to these issues in ECPT. Helmholtz coils provide a flexible inductive heating solution independent of the geometry of the sample in ECPT applications, such as detection of failure in bonded wires in insulated-gate bipolar transistors [25] or identification of natural cracks in a casting sample [26].…”