Sensitivity And Reliability Of Crack Detection.
In establishing eddy current procedures for crack detection, the following factors must be considered:
Location and size of cracks to be detected.
Type of material to be inspected.
Accessibility of inspection area.
Test system capabilities.
Service-induced cracks in aircraft structures are generally caused by fatigue or stress corrosion. Both types of cracks
initiate at the surface of a part. If this surface is accessible, either by direct surface contact or by penetration of the eddy
current field through the material, 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 film.
The material from which the inspection part is fabricated is of primary importance in determining the possibility of
eddy current inspection and the limitations involved. The conductivity and magnetic permeability influence frequency
requirements, instrument choice, signal-to-noise ratio, and resulting sensitivity and reliability of inspection. For
example, aluminum, a material of intermediate conductivity, can be inspected for discontinuities up to 1/8" below the
surface with modern instruments capable of operation at 1 KHz frequency. Titanium, a lower conductivity metal,
would require higher frequencies in the range of 1 MHz to obtain optimum sensitivity. If surface cracking is to be
detected in ferromagnetic material, a high frequency can be utilized to limit penetration and thus eliminate
permeability problems (i.e. noise) common to ferromagnetic material testing.
Most of the eddy current equipment presently available for use in the field is of the small, portable, battery powered
type. This instrumentation permits operation in relatively tight quarters. However, eddy current inspection is only
feasible for surface or near surface conditions because of its limited depth of penetration. For this reason, direct access
to the surface to be inspected is usually preferred. Inspection from the opposite surface is limited to materials with low
conductivity and magnetic permeability of 1. Sufficient freedom of movement must be available in the area to be
inspected to allow positioning and movement of the probe to detect or measure the specified variable. The inspection
area must be visible to enable the inspector to determine the position of the probe. Alternatively, a special probe, a
fixture, or a guide can be used to position and hold probes in the required location. The extent of disassembly required
for inspection should be defined in applicable written procedures.
The test system for crack detection includes the probe (or probes), the eddy current instrument, any additional recording
or measuring instruments, and calibration standards. For most crack detection applications, general purpose probes
manufactured for the specific test instrument should be employed. A wide variety of eddy current units are fabricated
for general purpose or specific applications. General purpose instruments are used for flaw detection. For some field
applications, a small, lightweight, battery-powered meter output instrument is desirable. More critical inspections may
require detailed phase and amplitude information which is most easily obtained with the scope type of eddy current
instrument. Also the storage scope-type of instrument and magnetic tape recording systems are very useful for
automated, high scan speed testing.
The primary consideration in selecting an eddy current probe is the type of inspection being performed. To detect small
cracks, a probe coil of small diameter with a ferrite core is desirable to concentrate the induced field into a small