Electrical Non-Destructive Testing Methods. Electric Potential Method

Electrical control is a type of non-destructive testing (NDT) based on recording the parameters of an electric field by a measuring device interacting with the object under control (OU).

Electrical NDT methods are currently successfully used for solving tasks in defectoscopy, thickness measurement, and structural examination. The areas where electrical methods are most effectively used include:

• detection of delaminations in rolled sheet metal, defects in castings;
• poor-quality welds;
• defective joints;
• delaminations in bimetallic plates;
• cracks in metal products;
• sorting or identification of metal products.

Physical Principles of the Electric Potential Method

Among electrical NDT methods, the most common is the electric potential method (EPM), based on recording the distribution of potentials over the surface of the object under control (OU).
The EPM implementation scheme is shown in Figure 1.

An electric current (either direct or alternating) is applied from an external source to the area of the OU under investigation, with the current density distributed along the current lines (Figures 2, 3) between two current-carrying (current) electrodes placed at a distance from each other. As the current passes through the electrically conductive OU, it creates a potential drop on each section of its surface. The value of the potential difference U on the controlled area of the OU is measured using measuring (receiving) electrodes positioned at a fixed distance from each other.

The value of U indicates the geometric dimensions of the OU, the presence and location of surface defects, as well as the size parameters of these defects.

A surface defect, such as a crack, creates an additional obstacle to the current flow through the OU. Figure 2 schematically shows the distribution of equal current density lines - current isolines and equal potential lines - equipotentials using direct current. These lines are mutually orthogonal.

Comparing the arrangement of lines with no defect (Figure 2) and with a surface defect such as a crack (Figure 3) shows that a defect in a solid conductive medium oriented across the current isolines distorts both the isolines and equipotentials, which should cause a change in U between fixed points on the surface (between measuring electrodes). This indicates the fundamental possibility of defectoscopy of electrically conductive materials using the electric potential method.

It is important to note that the current density in the area measured by the receiving electrodes depends on: the distance between the current electrodes; the distance between the current and receiving electrodes; the ratio between the depth of the crack and the distance between the current electrodes; the electromagnetic properties of the metal (electrical conductivity and magnetic permeability); the thickness of the product.

Among the geometric parameters of the OU, the value of U is influenced by the thickness of the OU. This influence is most intense when the thickness of the OU is comparable to the depth of penetration of the electric field into the OU. This indicates the fundamental possibility of assessing the thickness of metal films, sheet material, and metal coatings using the electric potential method. Moreover, this control method is implemented with one-sided access to the OU.

However, from the perspective of defectoscopy tasks, the effect of thickness on the measured parameter U is a disturbance. One approach to reducing the influence of this disturbance on the control result is to use high-frequency alternating current, which actively exhibits the skin effect.

Skin effect (from English "skin") refers to the phenomenon where the electric field of high-frequency current covers only a surface segment of the conductor rather than the entire height of the cross-section between the electrodes.

As the frequency of the alternating current increases, the depth of penetration of the electric field into the OU decreases. The current contours thus concentrate in the surface layer of the OU at a certain depth, which reduces the influence of the OU's thickness on the measurement of U.

Theoretical Principles of EPM

In defectoscopy using the electric potential method, a common approach in NDT is used to account for the influence of uncontrolled factors on the measurement result of the informative parameter. In this case, such factors are the electrophysical parameters of the medium and the parameters of the electric current source, and the essence of the approach is to transition from absolute measurements to relative ones.

Along with measuring the potential difference between the receiving electrodes U_d at a site with a crack, U₀ is determined at a known defect-free area (Figure 4), and the quality of the OU is judged by the value of the relative potential difference U', determined by the expression:
U'=(U_d-U₀)/U₀

Defect depth h is judged by the value of the ratio U_d/U₀. This way, the influence of the material's specific electrical conductivity on the control result is accounted for. Therefore, the value of U_d/U₀ is mainly determined by the depth of the defect h and the relative position of the current and potential electrodes.

To ensure that the ratio U_d/U₀ = U' does not depend on the electrophysical properties of the controlled area during direct current measurements, it is sufficient for the specific electrical conductivities of the defected and defect-free areas to match. It should be noted that calibration dependencies U_d=U_d(h) are the same for all non-magnetic metals homogeneous in specific electrical conductivity.

Features of Using Direct and Alternating Current

The electric potential measurement method using direct current is internationally known as Direct Current Potential Drop (DCPD), while the use of alternating current is known as Alternating Current Potential Drop (ACPD).

The distribution of current density in the control area and, accordingly, the measured value of U will depend on the magnitude of the applied current, the electrophysical properties of the OU material (specific electrical conductivity), its geometric parameters, and the quality of the surface layer (presence and characteristics of local defects). In the case of using alternating current, the magnetic permeability of the material and the frequency of the applied current also affect the value of U.

When using alternating current, the distribution of current density due to the skin effect depends on the electrophysical properties of the metal. This necessitates the use of calibration characteristics obtained on samples with electrophysical properties matching those of the controlled object. This solution is a compromise, as metals with the same U₀ can have different ratios of electrical (specific electrical conductivity σ) and magnetic properties. This corresponds to different current density distributions, which affects the calibration characteristics. Thus, the calibration procedure using alternating current is quite ambiguous.

It should be noted that even complete identity between the metal of the controlled object and the calibration sample does not exclude the influence of magnetic property variations. This is because cracks typically result from mechanical stresses, which lead to significant changes in magnetic permeability in the defect zone. Methods for detecting areas with increased mechanical stresses based on changes in their magnetic state during metal deformation (Villard effect) are based on this principle.

Nevertheless, electric potential measurements using alternating current are widely used in practice, as the skin effect causes the current to concentrate in the surface layers, bypassing the crack along its surface. As a result, the thickness of the controlled object and edge effects have less influence on the recorded signal compared to using direct current, which spreads more evenly throughout the metal volume.

By concentrating the alternating current around the crack, the error related to its length is reduced. Applying the appropriate correction allows for no more than 10% additional measurement error due to variation in crack length across its entire range. Without correction, the measurement error related to crack length ℓ depends on the ratio ℓ/h and amounts to 45% when ℓ/h = 1.

The NPP "Mashproject" produces the portable electric potential crack gauge 281M using alternating current, sensors for it, and calibration samples simulating cracks of varying depths. Custom control and measurement tools can be manufactured according to the Customer's technical specifications.

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