28 Electromagnetic Testing (ET)

Electromagnetic examination (ET), also known as eddy current testing, uses electromagnetic energy to detect surface and internal quality of welds. ET can be used to detect porosity, slag inclusions, internal cracks, external cracks, and lack of fusion on ferrous and nonferrous metals. Electromagnetic examination procedures must be standardized, often using full-scale or mock-up calibration standards with simulated discontinuities.

ET is a mix of magnetic testing and digital examination. The magnetic flow of the electrons are digitally graphed which is then viewed on a computer screen and interpreted.

To inspect longitudinal weld quality in welded pipe or tubing, an energizing coil and a detector coil are required. Examination is performed by passing the pipe or tubing longitudinally through the primary energizing coil, causing the probe-type detector coil to move across the longitudinal weld from end to end. The primary coil is energized with an alternating frequency that is suitable for the part being inspected and induces eddy currents into the part.

To achieve electromagnetic induction, the electric coil may be an encircling coil or an inside coil. An encircling coil is wound so that the test specimen passes through the center of the coil, causing the eddy currents to flow around the rod or tube being tested. Inside coils pass through the inside of tubing and eddy currents flow around the tubing.