7-1. General. With magnetic metals such as steel, the reaction of the metal to the applied magnetic field is much more
dependent upon the magnetic permeability of the metal than on Its electrical conductivity. When magnetic properties are
predominant, the form of testing is more accurately termed magnetoinductive testing. Eddy current testing and
magnetoinductive testing are grouped together under the more general classification of electromagnetic testing.
7-2. Purpose. Electromagnetic inspections are used in aircraft maintenance to inspect jet engine-turbine shaft and
veins, wing skins, wheels, bolt holes, and spark plug bores for cracks, heat or frame damage. In aircraft manufacturing
plants, eddy current is used to inspect castings, stamping, machine parts, forgings, and extrusions.
7-3. Eddy Currents. Eddy currents are electrical currents Induced in a conductor of electricity by reaction with a
magnetic field. The eddy currents are circular In nature, and their paths are oriented perpendicular to the direction of the
applied magnetic field. In general, during eddy current testing, the varying magnetic field is generated by an alternating
electrical current flowing through a coil of wire positioned immediately adjacent to the conductor, around the conductor,
or within the conductor. A summary of the uses of eddy current inspection as related to the property or test conditions
measured is presented in table 7-1.
7-4. Field Application. Eddy current techniques are particularly well suited for detection of service-induced cracks in
the field. Service-induced cracks in aircraft structure are generally caused by fatigue or stess corrosion. Both types of
cracks initiate at the surface of a part. If this surface is accessible, eddy current inspection can be performed with a
minimum of part preparation and a high degree of sensitivity. Unlike penetrant inspection, eddy current inspection can
usually be performed without removing such surface coatings as primer, paint, and anodic films. Eddy current inspection
has greatest application for inspecting small localized areas where possible crack initiation is suspected rather than for
scanning broad areas of metal for randomly oriented cracks. In some instances, it is more economical to scan relatively
large areas with eddy current rather than strip surface coatings, inspect by another method, and then refinish.
7-5. Eddy Current Test Set Model ED-520. The ED-520 eddy current test instrument is a compact, lightweight,
battery-operated portable unit designed primarily for the detection of cracks and flaws. The ED-520 is an impedance
type instrument where variations in conductivity, permeability, or physical characteristics result in impedance changes in
the test coil attached to the bridge circuit. The impedance change causes the bridge to become unbalanced; the
unbalanced voltage is amplified and indicated by the panel meter. The instrument can be used with a wide variety of
probes and coils. The two-wire system is generally used with a single coil.
Operation. Front panel controls consist of a mode switch, lift-off frequency control, sensitivity and balance. The
mode switch has six positions: OFF, TEST, LO, MED, HI, and CHG. In the OFF position the meter terminals are short
circuited and the battery charger is shut off. In the LO, MED, and HI positions the instrument is turned on and the meter
is connected to the output of the bridge circuit. The sensitivity of the instrument is increased by a factor of approximately
2 with each change in position from LO to MED to HI. If ac power is applied to the unit while in any one of these
positions, a limited charging current is provided to the unit. When the unit is in the TEST position, the meter is
connected directly across the rechargeable batteries and indicates the state of charge of the batteries. When the
batteries are adequately charged, the meter will read above the red line on the face of the meter. In the CHG position,
maximum charging is applied to the batteries and all other circuits are shut off. The CHARGING light will be on
whenever the line voltage is being supplied to the battery charging circuits.
Testing Procedure. For testing, the operating frequency is set by the LIFT-OFF/FREQ. control. Defect
detection usually requires a procedure which will minimize the effects of lift-off on the eddy current response. This is
accomplished by establishing an operating frequency with the LIFT-OFF/FREQ. control which will provide the same
probe coil impedance with the probe in contact with bare metal of the part. Operating frequencies for lift-off
compensation are determined by adjusting the LIFT-OFF/FREQ. until the same meter reading is obtained with the probe
in each of these two positions. More than one frequency may provide lift-off compensation. However, the highest
frequency (lowest LIFTOFF/FREQ. setting) usually provides the best