Magnetic Flux leakage Testing

The Magnetic Flux Leakage (MFL) method induces a magnetic field within a ferromagnetic specimen, then seeks localized magnetic flux leakage fields along the surface. There are four steps in MFL:

  1. magnetize the test object in a direction such that the lines of magnetic flux are disturbed by discontinuities 
  2. scan the surface of the test object with a magnetic flux sensitive detector
  3. process the data to accentuate discontinuity signals
  4. present the test results clearly for interpretation
The method is based on the principle that magnetic flux is locally distorted by the presence of a discontinuity. This localized distortion causes some of the magnetic field exit, and then reenter, the test object at surface and near-subsurface discontinuities. The magnetic dipole caused by field distortion is called Magnetic Flux leakage field.
There are limits to deep- discontinuity sensitivity using this method. However, these limits are controllable to some extent by the intensity of the induced magnetic field; by features of the discontinuity, such as depth, size and shape; and by the relative orientations of the discontinuity and magnetic flux lines.

About detection of flux leakage fields, unlike the particles used in the closely related magnetic particle test method, MFL employs a Hall effect sensor to detect flux leakage fields. The core of a Hall effect sensor is its semiconductor crystal, which has a constant current applied to it. The hall sensor is a four-terminal solid-state device, which produces an output voltage proportional to the input current, magnetic flux density, ant the cosine for the angle between the flux lines and the face of the semiconductor crystal. For example, a 10º off-angle orientation results in-´-1.5% decrease in measured flux density.
Magnetic flux lines passing the hall probe induce an electrical voltage potential. The polarity of the voltage potential changes with the direction of the magnetic flux, with one side of the sensor crystal becoming the negative portion and the other side positive. Hall probes may be configured to sense magnetic flux density oriented parallel to the test object`s surface, using a transverse probe, or oriented perpendicular to the magnetic field with an axial probe.