T.O. 33B-1-1
4-44
produce a variable amplitude direct current signal. This is the most common type of detection on meter types of eddy
current instruments.
4.4.3.19
Phase Detection.
If an eddy current system is to be used to detect a variety of flaw conditions, phase angle measurements may be needed
to determine the nature of the flaw condition. The information in the impedance diagram illustrates this fact. Decrease
of conductivity (i.e., cracks) and permeability changes could produce the same signal amplitude, and it would be
difficult to differentiate between cracks and normal permeability changes in a part. However, the phase angle of a
conductivity change is very different from a permeability change if the correct test frequency is chosen. Using phase
detection techniques, it becomes a simple matter to detect the difference between permeability variations and cracks.
This also applies to determining the depth of a flaw which is phase sensitive or separating lift-off effects from flaw
conditions. Phase sensitive detectors use a variety of techniques such as phase splitters, phase shifters, averaging, half-
wave and full-wave detection, sampling, and subtractive and additive techniques. The presentation of the impedance
plane on waveform display eddy current instruments utilize two phase sensitive detectors to provide horizontal and
vertical phase detection. This information is combined to produce a dot or point on the screen which represents the
relative phase and amplitude of an eddy current signal. Some types of meter instruments utilize an adjustable phase
control or phase gate to allow only signal detection at a particular phase angle of interest.
4.4.3.20
Multifrequency Technique.
A single test frequency can provide phase and amplitude information for one material condition. If a variety of
conditions are to be detected, there must be a frequency for each condition. Multifrequency eddy current systems, for
example, can be used to detect the cracks in the presence of geometric changes in a complex part. Each condition to be
suppressed must produce significant impedance changes for one frequency and less significant changes for the other
frequencies used in the inspection. An example would be using a dual frequency inspection for subsurface corrosion
while compensating for lift-off. A low frequency would be selected that would allow sufficient penetration to detect the
corrosion. Lift-off responses would also be present from this frequency. A higher frequency would then be used that
would respond to lift-off and not have sufficient penetration to respond to the corrosion. In this simple example, lift-off
could be compensated for. The analysis of these signals can become extremely complex. Presently most
multifrequency testing is limited to dual frequency testing and some three frequency testing. This technique is also used
in tube testing to discriminate between ID and OD defects in tubing.
4.4.3.21
Pulsed Eddy Current Techniques.
The pulsed eddy current technique is a non continuous wave test technique and also has multifrequency characteristics.
The width of the pulse establishes the lower frequency limit while the sharpness of the pulse corners establish the upper
frequency limit.
4.4.3.22
Eddy Current Instrument.
Table 4-5 lists current eddy current instruments and their applications, features and limitations. Figure 4-38 through
Figure 4-40 show examples of eddy current equipment.
