###### Abstract  
An investigation of real corrosion cracks that can be partially conductive in electromagnetic non-destructive evaluation using sweep-frequency eddy-current frequency-response analysis is carried out in this study. A new approach incorporating innovative solutions is proposed. The goal was to increase the probability of detection of real corrosion cracks in contrast to the conventional sweep-frequency method that is based on single-point signal detection. The proposed procedure was tested on real material specimens where differential responses were gained from real corrosion cracks. Seven austenitic steel plate specimens having corrosion cracks were inspected. Eddy-current responses due to the material cracks were sensed, while a multi-point approach was used for this purpose. The presented unique results clearly showed that the detection ability of a fixed probe driven with sweep-frequency excitation signal could be increased when the multi-point detection is used, and the gained signals are further mathematically processed. non-destructive evaluation; sweep frequency eddy current testing; corrosion cracking; harmonic excitation +
Footnote †: journal: Applied  
In some cases, there is a need for thorough and specific control of conductive materials, and the conventional ECT method is insufficient or inappropriate. As an example, we can mention the investigation of the area of interest without the possibility of moving the probe over the material. Such circumstances lead to the search for other solutions and thus to modifications of the conventional ECT method. One such modification is the method of eddy currents by sweeping the frequency (Sweep Frequency Eddy Current Testing, SFECT). Behind the emergence of the SFECT method is the need for continuous and long-term monitoring of the material. This can be achieved by permanently attaching the sensor(s) to the site under investigation. The principle of frequency sweep is based on generating an excitation signal in each frequency interval in discrete steps. From the initial frequency to the final frequency of the interval, the excitation frequency is generated with a certain fixed step. The step can be linear or logarithmic. It consists in obtaining data from the response of eddy currents in a wide range of frequencies. With SFECT, the probe is fixed at a certain point of interest in the examined material, and it does not move. As a result, the surface of the entire material is not investigated but only in a certain place or given area of interest. This method can therefore be used especially for continuous and long-term monitoring of the product directly in operation Ref. [1]. [PERSON] et al. devoted themselves to the use of the eddy currents by frequency sweep to measure the coating thickness of Ni alloy on austenitic steel. The study was focused on the comparison of numerical simulations and experimental results of plated material and the effect of heat treatment on the material. Using numerical simulations, they developed a model that was used in the inverse analysis. The achieved results confirmed the possibility of using the SFECT method for non-destructive investigation of plating thickness, and the maximum error between the real and estimated value was 22 \(\upmu\)m, which represented an error of approximately 11% Ref. [2]. The study was followed up by the same team of authors in Ref. [3] with experimental verification. By comparing with the simulation, they found that the magnetic permeability of the coating was reduced because of the heat treatment. The maximum measurement error reached 33 \(\upmu\)m or 28 \(\upmu\)m, which represented an error of approximately 13%. Measuring the thickness of a metal pipe using the SFECT method was described by [PERSON] and [PERSON] in Ref. [4]. In the thesis, they proposed an inverse algorithm based on the Levenberg-Marquardt algorithm. They proved the effectiveness of the algorithm in an experiment on a stainless-steel cylindrical pipe.