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Surface Crack Detection by A.C.F.M
FOR INSPECTION THROUGH COATINGS & HIGH TEMPERATURE OPERATION
ACFM (Alternating Current Field Measurement) is an electromagnetic
technique for the rapid detection and sizing of cracks in conductive
materials. It is non-contacting and the inspection can be carried
out through up to 5mm of non-conductive coating such as paint and
scale.
Advantages & Cost Savings
- Through coatings
- Minimum preparation/cleaning therefore no recoating
- Accurate sizing
- High temperature operation
- Remote operation.
- Hard copy via PC
Principles
The ACFM technique was developed from the AC potential drop (ACPD)
technique to combine the ability of ACPD to size without calibration
with the ability of eddy current techniques to work without electrical
contact. This is achieved by maintaining the uniform input field
(induced rather than injected) but measuring the magnetic fields
above the specimen surface instead of the surface voltages. The
presence of a defect in the specimen will influence the magnetic
fields in a predictable way.
Unlike eddy current techniques calibration defects are not required,
sizing is based on theoretical models. This removes a major source
of error, since artificial calibration defects are rarely representative
of service induced defects.
Site Implementation
The ACFM system operated by Torch Integrated Inspection Services
consists of a probe, a TSC U9 Crack Microgauge and a notebook computer
for the display, analysis and storage of data. The system can be
either battery or mains operated and a probe umbilical is available
for operation remote from the equipment in areas with difficult
access. The inspection is usually carried out using a two man team,
one trained and approved to view and interpret the results, the
other to manipulate the probe.
The ACFM probes are designed primarily to run along the line expected
to be followed by a crack, such as a weld toe. A standard ACFM probe
can inspect a band about l0 mm either side of the probe centre so
that complete welds up to 20 mm wide can be inspected in this way.
For wider welds, additional scans along the weld cap are required.
For inspection of an area where no preferred crack orientation exists,
two scans are made at right angles to each other. Typical scanning
speeds are in the range of 50 mm – 100 mm per second.
As the probe is scanned, the operator views the trace on the computer
screen. The presence of a defect is readily apparent. When a defect
is detected, measurements are made on the computer trace and the
software calculates the predicted crack dimensions based on these
measurements. Surface breaking cracks as small as l0 mm long x 1
mm deep, are readily detectable in steel welds through the coating.
Full details of the inspection, including defect traces, can be
stored and re-run on the computer for future reference, and as such,
the system is ideal as a ‘fingerprinting’ tool.
Applications
ACFM can be applied to the detection of cracks and other defects
in most conductive materials. Its main applications to date have
been for the inspection of welds without the requirements to remove
paint and scale, particularly off-shore, both sub-sea and topside.
It has also been used to inspect operating process plant at up to
500oC. Other uses have been in the nuclear, steel and railway industries.
