Instruments implementing electropotential non-destructive testing methods

Electropotential Non-Destructive Testing Method (EPT) is a type of electrical testing based on registering the parameters of the electrical field of the tested object with a measuring instrument.

Technical means implementing EPT can be either specialized or universal, suitable for measuring both the thickness of the object under control (OUC) and the depth of cracks. Instruments include the following main elements:

• source of stabilized direct or alternating (harmonic or pulsed) current;
• amplifier;
• detector (for alternating current);
• interpolator (antilogarithm);
• indicator of inspection results.

The most important components of these devices are the current and potential (measuring or receiving) electrodes. They ensure electrical contact with the OUC when current is applied to its surface and measure the informative parameter. Electrodes come in various designs, allowing the selection of the most suitable option for a specific OUC and the task at hand.

The principle of EPT implementation and the positioning of electrodes at a defect site are illustrated in Figure 1.

Usually, electrodes are combined into probes (sensors) connected by cable to the electronic block of the device. Electrodes are made pointed for fixation on a small area and spring-loaded to create increased pressure in the contact zone.

The most suitable material for manufacturing electrodes is hardened steel, which provides high wear resistance necessary for maintaining sharp tips for a long time. There are mainly two types of sensors: four-electrode and three-electrode (see Fig. 2).

Four-electrode sensor includes two current and two potential electrodes. Structurally, these probes have different distances between the current electrodes. When measuring the thickness of OUC, probes with a distance between electrodes of more than 13 mm (for direct current) are generally used, while for crack depth measurement, sensors are chosen with the distance depending on the required measurement range.

Figure 2 shows four-electrode sensors of different designs for the electropotential crack gauge 281M:

• Sensor "1x4", where electrodes are arranged in a single row, designed for measuring crack depths from 0.5 to 30 mm;
• Sensor "2x2" has 2 rows of electrodes and is designed for measuring crack depths from 0.5 to 20 mm, for use in hard-to-reach areas and on surfaces with small distances between cracks.

Three-electrode sensor contains two potential electrodes located 2-5 mm apart and one current electrode. The second current electrode is made as an external element with a magnet for quick attachment to the required point on the OUC surface. This design allows placing the current electrode at a greater distance from the potential electrodes, ensuring that the condition "the crack depth must be less than the distance between the current electrodes" is met. This significantly reduces the non-linearity of the instrument's readings when measuring cracks of different depths.

Figure 2 shows the Sensor "3+1" with an external current electrode, which is used to determine crack depths from 30 mm to 100 mm and is also convenient for use in hard-to-reach places.

Special attention is paid to setting the distance between measuring electrodes when manufacturing probes, as it directly affects measurement accuracy.

Metrological support for electropotential instruments is based on the use of certified control samples with artificial defects of specified sizes (see Fig. 3) or plates of specified thickness.

Applications of EPT

The electropotential method (EPT) is used for inspecting parts and blanks made from virtually any conductive materials: cast irons, steels, non-ferrous alloys, and graphite.
EPM is widely applied in the following areas:

• measuring the wall thickness of various parts and pipes;
• measuring the thickness of metallic coatings on conductive and insulating substrates;
• testing the anisotropy of electrical and magnetic properties of OUC caused by mechanical stresses;
• testing delaminations in thick sheet metal during trimming of transverse and longitudinal edges;
• checking the quality of soldered joints.

The primary application of EPT is still the measurement of crack depth. It is practically the only method that allows simple and reliable measurement of surface crack depths in the range of 0.1 to 120 mm. It is especially effective in detecting fatigue cracks, which are most dangerous due to the risk of sudden failures and emergency situations.

In this regard, EPT is used to study the kinetics of fatigue crack development, as an important property of the method is not only the ability to detect the moment of crack initiation and measure its depth but also to study the crack front developing into the OUC.

The specificity of EPT makes it most effective when used in conjunction with other non-destructive testing methods that have low sensitivity thresholds but cannot accurately determine defect depth (such as magnetic powder, capillary, eddy current methods, etc.). The combined use of methods significantly increases the efficiency of OUC inspections and is widely used in the monitoring of pipelines operating under high pressure, transport, and energy equipment.

Limiting Factors

1. The accuracy of evaluating the thickness of OUC and crack depth largely depends on the homogeneity and isotropy of the electro-physical properties of the OUC material, primarily its specific electrical resistance. This factor determines the maximum allowable accuracy of measurement of the dimensional parameters of the OUC.
2. When measuring the depth of a surface defect, EPT is effective if the defect has a significant linear extent, which is characteristic of cracks. The length of the defect must be at least three times its depth. Therefore, while EPT effectively assesses crack depth, it is sometimes less suitable for determining the geometric parameters of other types of defects such as pits, cavities, volumetric inclusions, and pores.

NPP "Mashproject" produces a high-precision portable device for measuring crack depth - the electropotential crack gauge 281M, sensors for it, and control samples simulating cracks of various depths. Custom manufacturing of control and measuring tools according to the customer's technical requirements is available.

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