Eddy current inspection is one of several NDT methods that use
the principal of “electromagnetism” as the basis for
conducting examinations. Several other methods such as Remote
Field Testing (RFT), Flux Leakage and Barkhausen Noise also use
this principle.
Eddy currents are created through a process called electromagnetic
induction. When alternating current is applied to the conductor,
such as copper wire, a magnetic field develops in and around the
conductor. This magnetic field expands as the alternating current
rises to maximum and collapses as the current is reduced to zero.
If another electrical conductor is brought into the close proximity
to this changing magnetic field, current will be induced in this
second conductor. Eddy currents are induced electrical currents
that flow in a circular path. They get their name from “eddies”
that are formed when a liquid or gas flows in a circular path
around obstacles when conditions are right.
One of the major advantages of eddy current as an
NDT tool is the variety of inspections and measurements that can
be performed. In the proper circumstances, eddy currents can be
used for:
Crack detection
Material thickness measurements
Coating thickness measurements
Conductivity measurements for:
Material identification
Heat damage detection
Case depth determination
Heat treatment monitoring
Some of the advantages of eddy current inspection
include:
Sensitive to small cracks and other defects
Detects surface and near surface defects
Inspection gives immediate results
Equipment is very portable
Method can be used for much more than flaw detection
Minimum part preparation is required
Test probe does not need to contact the part
Inspects complex shapes and sizes of conductive materials
Some of the limitations of eddy current inspection include:
Only conductive materials can be inspected
Surface must be accessible to the probe
Skill and training required is more extensive than other techniques
Surface finish and and roughness may interfere
Reference standards needed for setup
Depth of penetration is limited
Flaws such as delaminations that lie parallel to the probe
coil winding and probe scan direction are undetectable
