See also: Basic Theory of Velocity Effects

Flux leakage fields are influenced by velocity. This effect is in addition to the affect of velocity on the applied field. Velocity:

• reduces the leakage field amplitude
• changes the leading and trailing edges of the leakage field
• shifts the leakage field toward the leading pole piece
• decreases the base signal amplitude.

These effects vary with defect geometry, magnetization level, and pole spacing.

The effects of velocity depend on defect depth for metal-loss defects, as shown above. The leading edge of the signal for the 80 percent defect is slightly reduced by velocity, but there is almost no effect on the trailing edge. As depth decreases, though, velocity becomes more and more important. For the 20 percent defect, velocity significantly affects the leakage field: the leakage field shifts to the left and the shape changes. So, the effects of velocity depend on the metal-loss depth. In general, leakage fields from shallow regions are more strongly affected than those from deep regions.

The effects of velocity depend on magnetization level, as shown above. These results correspond to a 50 percent deep, 2-inch long metal-loss defect. The top set of curves corresponds to a high applied magnetization level; the bottom corresponds to a low applied level. At a low magnetization level, the leakage fields are significantly lower in amplitude for both the static and dynamic cases.

The effects of velocity on the leakage fields at a low magnetization level are similar to those discussed above for a shallow defect: the fields shift to the left and change shape. At higher magnetization levels, the effects are less. Because higher magnetization levels show smaller velocity effects, they generally produce leakage signals that are easier to analyze.