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ACFM
Introduction
What
is ACFM?
ACFM stands for Alternating
Current Field Measurement.
Background
The ACFM
technology was developed by TSC in the 1980's from the successful ACPD
(further details here) contacting
technique to provide a system for crack detection and sizing without
the need for any electrical contact.
The crack
sizing capability has resulted from the use of a uniform input field
which allowed theoretical studies at the NDE
Centre at University College London to predict crack depth
from a knowledge of the surrounding a.c. electromagnetic fields.
The technique was initially developed to allow
crack sizing underwater where the ACPD technique was hindered by
the need for good electrical contact. However, the other advantages
arising from non-contact and a uniform input current (ease of scanning,
little adverse effect from material property changes or probe lift-off)
meant that the technique was quickly applied to topside inspections
as well, particularly on painted or coated welded structures.
Operational
Principles
An ACFM
sensor probe is placed on the surface to be inspected and an alternating
current is induced into the surface. When no defects are present
the alternating current produces a uniform magnetic field above
the surface. Any defect present will perturb the current forcing
it to flow around and underneath the defect; this causes the magnetic
field to become non-uniform and sensors in the ACFM probe measure
these field variations.
Two components
of this magnetic field are measured - one provides information about
the depth or aspect ratio of the defect(s), the other provides information
on the positions of the ends of each defect. The two signals are
used together to confirm the presence of a defect and, together
with a sizing algorithm, measure its length and depth.
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