The effects of magnetization level on flux leakage fields for metal loss defects are summarized in this link. The results are for specific material properties and were calculated using finite-element modeling.

The above plots represent the tangential component of the magnetic field at the centerline of a metal-loss region. The metal-loss region is 50 percent deep, 1.5 inches long, and 2.0 inches wide. The pipe is 0.300 inch thick. Four separate flux leakage fields are shown: the field at 6, 12, 16, and 18 kilogauss. The 6 kilogauss level corresponds to a low magnetization level; the others correspond to medium magnetization levels.

There are two parts to the measured signals: (1) a base signal amplitude and (2) the flux leakage signal. The base signal amplitude is the signal that would be present even in the absence of metal loss. The base signal amplitude is 320 gauss for an applied flux density of 18 kilogauss, and it is around 60 gauss for an applied flux density of 6 kilogauss.

The total flux leakage signal is the signal that is present near a metal-loss region. The flux leakage signal is 585 gauss for an applied flux density of 18 kilogauss, and it is 80 gauss for an applied flux density of 6 gauss.

The increase in flux leakage due to the defect is the difference between the base signal and the total flux leakage signal. For the 18 kilogauss case, the increase is 265 gauss(585 minus 320), which is about 80 percent of the base signal. For the 6 kilogauss case, the increase is 20 gauss, which is about 30 percent of the base signal.

The effect of magnetization level can be seen by examining the ratio of the increase in leakage signal to the base signal amplitude. For this example, the largest ratio occurs at a high applied flux density of approximately 18 kilogauss. As the applied field increases or decreases from this level, the ratio begins to decrease. Also, below a flux density of about 12 kilogauss, the ratio reaches a constant, low value. At extremely high and generally impractical magnetization levels, the ratio of the leakage signal to the base signal eventually decreases to zero.

Many inspection vendors strive to achieve base flux density levels of 20 or more kilogauss for corrosion detection and characterization, which corresponds to a magnetic field strength of 80 to 160 Oersted. For mechanical damage tools, the target range is lower, typically in the range of 16 to 18 kilogauss or 50 to 70 Oersted. At these levels, the ratio of leakage signal to base signal is in the acceptable range.

In summary, flux leakage depends strongly on the applied magnetization level. Applied field levels above saturation produce strong flux leakage fields, which enhance the potential for metal-loss detection. Significant differences in magnetization level (due to changes in wall thickness or permeability) will change the measured flux leakage signals.

For metal-loss detection and characterization, high magnetization levels are generally preferred because they provide strong leakage fields relative to the base signals. For mechanical-damage, medium fields are prefered. Because magnetization level affects both detection and characterization, variables that affect magnetization also affect detection and characterization.