The operating point, or null point, is that point in the impedance plane at which the eddy current instrument is nulled.
For instrumentation with two dimensional displays, the operating point is usually the good or reference condition.
For instrumentation with one dimensional or meter displays, this would be unsatisfactory because any change in the
specimen would cause an unbalance of the bridge circuit with a subsequent indication displayed on the meter. It would
be impossible to discriminate against any variable affecting the impedance of the test coil. Consequently, the operating
point is displaced from the point representing null for a good condition.
Suppression techniques are used to eliminate or reduce instrument response to one or more inspection variables to
permit better identification of changes in the parameters of interest during eddy current inspection. When the display is
rotated as previously indicate, lift-off variations produce little or no signals in the vertical direction. Even though the
crack signal is predominately horizontal, it has a significant vertical component. This vertical component can be
amplified independently and monitored visually or electronically. A box gate is a rectangle whose position, height and
width can be adjusted to selectively monitor a portion of the impedance plane. In the example shown, defect
indications will enter the box gate over a fairly large area of lift-off conditions while the slight vertical component of
these lift-off responses remains outside.
Variability in permeability can make the eddy current inspection of ferromagnetic materials difficult. Permeability and
lift-off have approximately the same direction of impedance change in unmagnetized ferromagnetic materials; but there
can be very large variations in permeability that are very difficult to compensate for. Magnetic saturation can be
employed to overcome the difficulties presented by permeability effects. In this technique, the material is magnetically
saturated by a high DC magnetic field. This reduces the permeability to about 1 and makes it a constant. This results
in a relatively low conductivity material, essentially non-ferromagnetic, for eddy current inspection applications.
Each of the eddy current inspection variables (lift-off conductivity, thickness, permeability, and flaws) has a
characteristic effect on the net impedance of a coil. The display of the impedance curves caused by changes in the
inspection variables can be of great assistance in determining the cause of a change.
Another technique that is useful in separating signals of interest from other signals relies on an analysis of signals as a
function of time. Occasionally the suppression techniques discussed previously arent sufficient to separate small defect
signals from other non-essential signals. A straightforward procedure often used is to scan the part and display the
resultant signals as a function of time or position on the part. These signals can be considered a modulation of the eddy
current frequency and as such have frequency characteristics themselves. Often, the characteristics can be correlated to
the presence of defects.
Frequency response analysis is the most common form of modulation analysis. During eddy current inspection, the
impedance of the test coil remains constant provided there is no change in inspection conditions or material properties.
When variations in impedance do occur, the rates of change in the impedance and resultant eddy current signal are
proportional to the rates at which material properties are changing and the scanning speed. Consequently, a small
crack would provide a rapid change in impedance during scanning and a corresponding high frequency eddy current
signal. These signals can be viewed on a video display or a strip chart recorder as a function of time. Figure 4-32
illustrates the effect on amplitude as a result of encountering different kinds of material variations when scanning at a
constant speed. A fast signal change is often a good indicator of a small flaw or an abrupt change in material
characteristics; a slow signal change usually indicates a gradual change in dimensions, lift-off or some other property.