Eddy currents operation parameters optimization

1. Introduction

As inspection conditions become more challenging, there is a need for increased detection sensitivity. In the case of the eddy current technique, there are basically two ways to modify the inspection conditions, namely: changing the physical configurations of the coil or changing the operating parameters of the system. In the first case, the aim is to increase the sensitivity of detection through changes in the morphology of the sensors. This can occur by varying the geometry, wire diameter, number of turns and number of coils used and how they behave with each other [1–4]. Another way is to use different types of electromagnetic sensors together to help in the characterization of defects [5]. The second option, is to evaluate the behavior of eddy currents according to the change in operating parameters in the same coil. Among the possibilities, the one that stands out most is the excitation frequency, as this directly affects the currents induced in the material to be inspected. This is because, due to the skin effect, the depth of penetration of eddy currents in the material is inversely proportional to the frequency [6,7]. Added to this, the higher the frequency, the more cohesive the currents are [8]. The current way to define the frequency used in the tests is by calculating the standard depth of penetration or visual analysis of the signal in the equipment according to the standards. However, the choice of frequency through this methodology may not represent the optimal configuration and in more challenging cases it may lead to failure to detect the defect.

Thus, the present work aims to develop a methodology for optimizing the operating parameters of the eddy current technique. This will be carried out looking for the greatest differentiation between a signal of interest and an unwanted signal, to reach the configuration with the highest detection sensitivity. Precisely, the parameters reached will be responsible for generating a signal as required by the previously mentioned standards, in such a way that the obtained configuration can be used directly in the inspection equipment.

2. Methodology

3. Results

For this analysis, the ability to detect the presence of fatigue cracks at the root of welded joints of clad steels will be considered. This inspection condition is encountered by autonomous tools, such as inspection PIG’s, when inspecting oil rigid pipelines.

First, to carry out the inspections, a differential probe formed by two coils in orthogonal configuration was used. This type of probe has the least influence on lift-off variations and the best performance for inspection of complex geometries. Together, in order to simulate the inspection condition, a clad steel specimen containing an EDM notch at the root of the weld joint measuring 10 mm × 0.25 mm × 2 mm (length, width and depth) was used, Fig. 1. Thus, in the case in question, the region of interest is formed by the weld plus notch joint and the “non-interest” region would be the weld without notch.

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Fig. 1.

CLAD sample.

Aiming to compare the results achieved with the parameters obtained by the methodology, the choice of operating parameters was previously carried out through the trial and error methodology [11]. Method similar to that presented by the ASME standard for detecting surface notches [10]. Subsequently, the frequency optimization procedure was carried out, Fig. 2(a), varying from 20 kHz to 700 kHz in 10 kHz steps. Finally, the optimization of rotation and vertical gain for the selected frequency, Figs 2(b). The parameters obtained by both procedures are shown in Table 1.

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Fig. 2.

Optimization of parameters for notch inspection in clad steel.

In possession of the parameters, a scan of the entire specimen was performed in both configurations and a C-scan graphs of the impedance amplitude was made. Where the color bar represents the amplitude of the signal at the X and Y position on the sample. As can be seen in Fig. 3, Despite the amplitude reduction throughout the inspection, the result obtained by the proposed procedure only shows the detection of the notch with a good signal-to-noise ratio Fig. 3(b) while the other shows the overlap of this signal with the weld signal Fig. 3(a).

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Fig. 3.

Comparison between the results obtained through the try and error methodology and those obtained through optimization.

However, the objective of the analysis is to enable the detection of fatigue cracking under such conditions. Therefore, using a specimen containing the defect in question and using the optimized parameters, a scan of the entire surface of the sample was performed. In Fig. 4 we can see that the parameters obtained through a sample with an EDM notch could be used for an inspection of a real case. Allowing clear detection of the defect despite it being close to a weld root.

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Fig. 4.

Inspection of a fatigue crack in clad steel simulating real inspection conditions encountered by autonomous tools.

Although the standards present methodologies and recommendations on how inspections should be performed and how data should be displayed on the equipment screen, these are very specific for the type of test to be performed. However, many of these standards only inform how the data must be presented without a defined procedure for defining the operating parameters. Added to this, the development of new materials and applications brings with it the need to carry out inspections in order to guarantee quality and safety. Therefore, due to the innovative and challenging character of such technologies, they also do not have a defined way to obtain the parameters.

Therefore, the present work proposed the development of a methodology for defining the operating parameters. This took as a principle the fact that the parameters can be divided into two distinct groups, physical and post-processing parameters. In this way, the frequency optimization was carried out first through successive inspections. Although this step seems costly due to the need to carry out inspections for each analyzed frequency, aiming at the construction of the analysis graph, there is the possibility of simplification. For example, performing inspections with larger frequency steps and, after identifying the region of maximum amplitude, refining the analysis in smaller frequency steps in the region or using multi-frequency inspections to obtain a group of amplitude values in each inspection.

The optimization capacity of the fatigue crack detection signal in clad steels was studied, comparing the result with those obtained by the parameter determination methodology through the visual analysis of the signal on the equipment screen (try and error). The achieved results show that the procedure allowed the detection of only the notch. Applying the parameters obtained in a inspection of a real fatigue crack, it was observed that the detection of the defect was similar to that obtained in the specimen with an EDM notch.

Therefore, the procedure proved to be effective for obtaining the operating parameters. This being applicable not only for the presented tests, but to differentiate any two signs of interest.