FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

- 16 -
REDUCING UNWANTED EFFECTS ~ RECENT DEVELOPMENTS IN EDDY CURRENT
EQUIPMENT FEATURES
A Cati G. Jut let
Hocking Et cet ion les Inc.
Haio Iiiit'iumeufi Div-Lii.cn
Temp te H(££.i, MP
1. ABSTRACT
It has long been established that the distribution of eddy currents is
affected by many variables, some of which are desirable and some are not.
This paper will seek to illustrate by reference to recent applications how
eddy current equipment features have been developed to overcome some of these
problem effects.
2. PRESENTATION
It has long been established that the distribution of eddy currents is
affected by many variables, some of which are desirable and some of which are
not. Within the time available today, it would be impossible to try to
enumerate all of the problems and all of the solutions. So I will
concentrate on four particular applications—two involving probes and two
involving instrumentation. I will illustrate how their designs have been
developed in order to overcome the associated problem effects.
A recent task was the requirement to detect a fatigue crack at the root of
small titanium helicopter engine rotor blades (Figure 1). In this example,
the distance of the crack from the fir tree root was unknown, but expected to
be less than 2 mm and could occur anywhere between the leading and trailing
blade edges.
Using conventional high frequency pencil probes the unwanted effects in this
application were:
1. Zero changes due to the proximity of edges and corners
2. Variable surface geometry
3. Difficulty in identification of indications
The design, manufacture and use of shielded probes has been described at
conferences and in technical publications previously, and so I do not propose
to dwell on that aspect. Nevertheless, it should be remembered that a
shielded probe has a smaller magnetic field than a normal unshielded pencil
probe, and that the field is further reduced by incrasing the frequency.
For this application, we chose to use an instrument that had three switch
selectable test frequencies of 500 kHz, 2 MHz and 6 MHz. Using this unit we
were able to construct some curves which allowed us to examine the effect of
shielding at each of the three frequencies.