A flexible eddy current probe can detect the defects in components with a complex structure due to its bendability and adaptation to the surface of the tested specimen for nondestructive testing. However, the traditional, flexible eddy current probe with circular and rectangular coils may neglect small cracks along certain special orientations. In this paper, the authors introduce the theory of fractal geometry to guide the design of flexible probes. An annular fractal curve is first proposed, and its system of iterative functions and geometric characteristics are then studied. Following this, six flexible differential excitation-based eddy current probes are designed based on the annular fractal curve. Simulations and experiments are used to compare the performance of the probes to identify the optimum one for inspecting defects. The results show that the M6K3 probe delivers the best performance in terms of the identification of cracks with lengths shorter than half the length of the coil of the probe.

This paper presents a new method to evaluate the distribution of eddy current density excited by four different planar eddy current probe inducers. The proposed method uses the information entropy to analyse eddy current distribution in different directions in the whole, or in part, of surface. Four finite-element models show the eddy current distribution and allow the entropy calculation. Finally, the paper highlights strengths and weaknesses of the studied current probe inducers.

Improving the performance of flexible eddy current (EC) probes (FECPs) is a current research hotspot. Thus, a novel FECP, fractal EC sensor technology is proposed. This FECP can enhance the consistency of the probe for detecting short cracks in different orientations by changing the coil structure of the probe, thereby reducing the probability of missing the particular direction cracks. However, due to the limit of the wires’ layout space, an issue remains: the fractal probe’s signal output is weak. To break through this limitation, this study proposes a method to achieve the passive amplification of the output signal of FECPs. A layer of homemade flexible sheet doped with pure iron particles is covered on a flexible Koch differential pick-up fractal EC probe. First, the manufacturing details of the flexible sheet and probe are provided; then, the influence of the probes with and without sheet on the amplification of the crack detection signals is compared via finite element analysis and experiment. The results show that the probe with the sheet can realize the passive amplification to the crack signals in C-scan experimental results of 28 kinds of cracks, and under the excitation frequencies of 50, 100, and 1000 kHz, the signal amplitudes output from the probe with sheet increase by at least 12.8%, 27.6%, and 40.4%, respectively.

A new rotational field planar eddy current probe is proposed. The probe is combined with two orthogonal driver traces and a pickup coil that includes two-circular sector windings with series connection. Rotational eddy currents are induced by driver traces of the same amplitude and frequency, but fed with 90° phase different alternating exciting currents. An experimental demonstration using prototypes of the probes and artificial defects showed that the probe with a two-circular sector pickup coil is more sensitive for detecting the short defects than the probe with a circular pickup coil.