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Alternating Current Field Measurement (ACFM)
ACFM method and technology of its use were developed by TSC Inspection systems (TSC), founded in 1984 and specializing in the development and production of non-destructive testing systems, service software products, as well as providing consulting services for NDTinspectionand development. In addition to ACFM, the company has developed new NDT technologies: Alternating Current Potential Drop (ACPD), Alternating Current Field Measurement System (ACSM), which are now widely used in various industries around the world.
The aim of the method is to identify and measure the size of micro-cracks in metal structures. Typically, cracks occur in the weld areas and threaded connections. ACFMallows you to detect and measure any defects, if they locateon the material surface (this limitation is due to the surface principle of the electric current propagation in metals – the so-called “skin effect” and is common to all non-destructive testing methods, having an electrical nature). ACFM method allows inspecting through protective coatings, fouling or rust and does not require clearing to “bare metal”.
TheACFM probe induces an electric current into the inspected material and forms a magnetic field, the uniformity of which will be distorted at the defect sites. Micro sensors integrated into the probe monitor these distortions, and the received signals on the depth (B) and length (Bz) of the crack with the help of special algorithms give complete information about the magnitude and severity of the defect.
The main advantages of the method compared to other technologies are:
possibility of inspection without preliminary cleaning of the object surface;
easy operation and installation of the device;
itcan be used regardless of lighting or time of day; the ability to calculate the depth of the crack;
the drop in the voltage of the acting field as the probe is remoted from the object is very small, so deviations in the readings are minimized;
the current supplied is adjusted to the welding parameters, so the direction of the current does not change when the material conductivity changes, and therefore no signal is generated; this technology does not require calibration to measure the size of cracks.

The ACFMdevices are underwater (u21, U31) and surface versions. To increase the productivity of measurements, single sensors can be replaced with blocks of sensors, which allows you to reduce the time spent on inspection by times. The ACFM sensors have versions for various threaded connections, narrow joints and different configurations of the inspected structures.
It should also be noted that ACFM does not require the operator to have the level of knowledge of the NDT inspector. Analysis and processing of the results are performed remotely. For example, a diver only moves the probe (which can be done even by a remote-controlled underwater machine), and the inspector on the surface gets a picture of the defects in real time. Accordingly, the ACFM use becomes more cost-effective, since there is no need to involve “expensive” inspectors-divers.
Especially for high-rise applications,ACFMAMIGO, a lightweight and compact device was created. Its small size and robust design allows you to carry out measurements almost anywhere to be reached by the inspector or rigger.