What is the difference between eddy current testing and magnetic flux leakage testing?

What is the difference between eddy current testing and magnetic flux leakage testing?

Eddy current testing and magnetic flux leakage testing are both commonly used non-destructive testing methods for seamless steel pipes. Both mainly detect surface and near-surface defects of steel pipes. Both have their own focuses and limitations. Choose according to the situation.

Flaw detection principle

1. Eddy current testing Eddy current testing is a non-destructive testing method based on electromagnetic induction. This method has obvious effect on point defects. It excites the probe coil with a sine wave current. When the probe approaches the surface of the seamless steel pipe, the alternating magnetic field around the coil generates an induced electromotive force on the surface of the steel pipe, thereby generating an induced current, that is, an eddy current. The eddy currents create an induced collision field that acts on the coil. Thereby changing the electrical parameters of the coil. The loss resistance of the eddy current channel, as well as the anti-collision generated by the eddy current, are reflected to the probe coil, changing the current size and phase of the coil, that is, changing the impedance of the coil. The probe moves relatively on the surface of the seamless steel pipe. When a defect is encountered, the eddy current magnetic field reacts differently to the coil, causing the coil impedance to change. Therefore, surface defects of the seamless steel pipe can be identified by measuring its changes.

2. The basic principle of seamless steel pipe magnetic flux leakage detection is based on the high magnetic permeability of ferromagnetic materials. After the ferromagnetic seamless steel pipe is magnetized, a leakage magnetic field is formed due to defects on the inner and outer surfaces. By detecting the change signal of the leakage magnetic field, the presence of defects can be detected, and the magnetic permeability of the defects can be detected. Seamless steel pipes are much smaller than steel pipes. When the seamless steel pipe enters the steel pipe inspection line in a streamlined shape, it will be magnetized by the transverse and longitudinal magnetic fields generated by the DC coil as it passes quickly. If the seamless steel pipe has no defects, most of the magnetic field lines pass through the seamless steel pipe, which means that the magnetic field lines are evenly distributed; if the seamless steel pipe is defective, the magnetic field lines are bent and some of the magnetic field lines leak from the surface of the seamless steel pipe.

the difference:

1. Eddy current flaw detection
1) The mechanical transmission of flaw detection equipment is relatively simple
2) As long as the seamless steel pipe is conductive and the outer diameter and wall thickness meet the conditions, the eddy current method can be used for testing. Widely used in surface defect detection of carbon steel, alloy steel, stainless steel and other seamless steel pipes.
3) Generally, only single-sided surface defects of seamless pipes (inner surface or outer surface) can be detected

2. Magnetic flux leakage detection
1) The mechanical transmission device of flaw detection equipment is relatively complex
2) Only applicable to the detection of ferromagnetic seamless steel pipes. It can detect inner and outer surface defects of seamless steel pipes, and is suitable for the detection of large-diameter and thick-walled seamless pipes.
3) It can simultaneously detect internal and external surface defects of seamless pipes and has certain sensitivity to internal defects.

Article source: https://article-realm.com/article/Business/51683-What-is-the-difference-between-eddy-current-testing-and-magnetic-flux-leakage-testing.html