To identify deficiencies in the internal structure of structures and parts, non-destructive testing methods are used. They allow to fix defects of various kinds without intrusion into the basis of the material, determining their compliance with regulatory requirements. One of the most accurate methods of such an examination is eddy current control, in the process of application of which electrical equipment is used. This method has significant drawbacks, but in some areas it is used quite actively.
The principle of the method
The control is based on the analysis of the interaction parameters between the electromagnetic field of the flaw detector with its accessories and eddy currents, which are formed in the field of the object of study. As for the actuating element generating the active electromagnetic zone, it is usually represented by an inductive coil, which is also called a converter. The pulsed currents that excite eddy flows, and ultimately allow you to calculate the defective zone. The fact is that violations in the structure of the studied object will inevitably lead to uneven interaction of the material with the electromagnetic field over its entire area. It is the areas with current fluctuations that the eddy current method of non-destructive testing detects with the help of a special analysis. The principle of operation is quite complicated in comparison with alternative methods of magnetic control, but again it is also one of the most accurate ways to detect defects.
Features of eddy current troubleshooting
This method has two fundamental features. First of all, it is the possibility of eliminating direct contact with the target object. That is, it is not just about non-destructive testing, but about the technique of non-contact examination. This allows, for example, to diagnose structures and elements that are in motion. But contact methods of analysis are not excluded. For comparison, we can cite the method of magnetic powder analysis, which without fail requires the application of indicator material on the surface of the investigated object. The second feature that distinguishes eddy current control from the general group of fault detection methods is the possibility of additional analysis of the electrophysical properties of the material. But this functionality already depends on the specific model of the device used and the quality of the auxiliary equipment.
Eddy current flaw detectors
Devices can have different execution formats. Manual models, control stations, component and modular devices are widespread. They also differ in the ways of processing and presenting information: analog, digital and microprocessor-based modern monitoring devices can be distinguished. The internal filling is usually an electrotechnical base with the same coils, and the external organs are represented by sensitive elements of the analysis.
Eddy current control devices are also equipped with nozzles for convenient placement of the conversion device in front of the controlled surface. Although the devices provide for the possibility of contactless control, the position and direction of the sensitive element are of great importance in terms of obtaining a qualitative result. As for energy supply, the devices are powered by batteries or mains. In the first case, the devices enable autonomous diagnostics of structures and communications in remote areas.
Eddy Current Thickness Gauges
As already mentioned, troubleshooting is not the only task that the eddy current method of controlling materials can solve. The second most common function is the measurement of thickness. In this way, the parameters of plates, film products, pipe walls and other objects are evaluated. For this, special eddy current control devices with an optional thickness gauge are used. Often this is the main function that such devices solve. They are compact hand-held devices with a sensitive element, which also creates an electromagnetic field, but in the process of analysis does not record the presence of voids, but the thickness of the electrically conductive sheet.
Varieties of eddy current measurements
The main direction of the use of eddy current diagnostics is still troubleshooting, which also provides for various options for execution. In particular, absolute control allows you to measure deviations of the measured value from the previously set reference point, which is checked at the calibration stage of the device. Eddy current control also performs a comparative measurement, in which the difference between two quantities is calculated, one of which is taken as the reference.
A differential measurement is also used, which also calculates the difference between two fixed values in the control process. But in this case, the measurement path at a constant distance between the reference points remains the same. In each of the methods, eddy current control can work with both surface and internal characteristics of the material structure. However, in-depth analysis involves a small immersion of the order of 2-3 mm, which is the main disadvantage of this technology.
Preparing the device for work
The device is tuned to the workflow by calibrating and setting the optimal sensitivity indicator in relation to the characteristics of the target object. Depending on the functionality of the device, the user can adjust the parameters of the electromagnetic filters, the phase and gap size of the sensitive sensor. It is important to consider that the quality of the eddy current method of non-destructive testing is also influenced by external factors. To ensure convenient placement of the device, its installation, alignment and direction of the transducer, the workplace must be adequately prepared. Also, the examination site is thoroughly washed, cleaned of dirt and oil stains, after which it is dried and, if necessary, additionally treated with chemical solutions.
Technique for applying the method
At the first stage, the surface of the part is scanned by an electromagnetic signal over the entire target area, which can slightly capture an area that cannot be analyzed. Based on the characteristics of the received return signal, a conclusion is drawn about the physical condition of the object. The device using computer analysis provides data on the wear of the object, the presence of defects and damage. The specific set of parameters will depend on what task was set for eddy current control. GOST 15549-2009 for this method also determines the mandatory set of signal characteristics, including phase, amplitude and their combination in the previously indicated spectra. Further, the received information is entered in the technical control journal. Criteria for assessing identified defects are foreseen in the task for control and in the future allow you to decide on the admission of the product to the intended use.
Where is the eddy current monitoring method used?
This technology for the diagnosis of structures and parts for defects is widely used in construction and manufacturing. For example, multifunctional stations operate on conveyor lines producing metal products. The equipment automatically checks the details for compliance with regulatory parameters. In construction, the quality of metal structures, ceilings, racks, fasteners, welds, etc. is evaluated in this way. Eddy-current pipe monitoring is also widespread, which makes it possible to detect metal discontinuities and timely update problem areas of the communication network. In the household sphere, thickness gauges are often used, which determine the parameters of the same pipelines or casings of various equipment.
Conclusion
The eddy current method of troubleshooting can be called one of the most technologically advanced and modern. It allows you to work with parts of various shapes and purposes, fixing their physical parameters with high accuracy. But there are characteristics by which eddy current non-destructive testing is significantly inferior to alternative methods, including those from the group of magnetic methods. This applies not only to limitations in the depth of signal penetration. Electromagnetic currents can give information about the internal structure of a material only if it is conductive. The absence of this property makes the device useless in relation to such a product.