In eddy current testing, the
use of reference standards in setting up the equipment is particularly
important since signals are affected by many different variables
and slight changes in equipment setup can drastically alter the
appearance of a signal. As with most other NDT methods, the most
useful information is obtained when comparing the results from
an unknown object to results from a similar object with well characterized
features and defects. In almost all cases, eddy current inspection
procedures require the equipment to be configured using reference
For crack detection, corrosion
thinning and other material damage, reference standards are used
to setup the equipment to produce a recognizable signal or set
of signals from a defect or set of defects. In many cases, the
appearance of a test signal can be related to the appearance of
a signal from a known defect on the reference standard to estimate
the size of a defect in the test component. Signals that vary
significantly from the responses produced by the reference standard
must be further investigated to the determine the source of the
The reference standard should be of the same material
as the test article. If this is not possible or practical, it
should be of material that has the same electrical conductivity
and magnetic permeability. Component features (material thickness,
geometry, etc.) should be the same in the reference standard
as those in the test region of interest. If the reference standard
is the type with intentional defects, these defects should be
as representative of actual defects in the test component as possible.
The closer the reference standard is to the actual test component,
the better. However, since cracks and corrosion damage are often
difficult and costly to produce, artificial defects are commonly
used. Narrow notches produced with electron discharge machining
(EDM) and saw cuts are commonly used to represent cracks, and
drilled holes are often used to simulate corrosion pitting.
Common eddy current reference
- Conductivity standards.
- Flat plate discontinuity standards.
- Flat plate metal thinning standards (step or tapered wedges).
- Tube discontinuity standards.
- Tube metal thinning standards.
- Hole (with and without fastener) discontinuity standards.