3ACFM

ACFM - Surface ECT

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Below
is a discussion of
ACFM & Eddy Current Test
techniques
which can be utilized
for the inspection of
surface breaking cracks in
Ferrous & Non-Ferrous
Materials

Crack Depth Measurement
Using ACFM

AC field measurement (ACFM) is a re-
cently developed electromagnetic tech-
nique which offers the capability of de-
tection and sizing of surface-breaking
cracks without the need for calibration
or cleaning to bare metal. ACFM is in
effect a natural extension of Eddy Cur-
rent inspection techniques with the
uniform injected current replaced by a
uniform field induced by a driver coil
and the contact electrodes replaced by
a set of orthogonal pick-up coils. The
measurements are performed by scan-
ning the probe along the crack face.

Eddy-Current Crack Depth
Measurement

The detection of surface and slightly
sub-surface cracks by the Eddy Current
Technique has been successively uti-
lized by many different industries (aero-
space, automotive, etc.) for over 20
years. The ability of detecting surface
breaking cracks basically involves pas-
sing a coil(s) of correct design and orien-
tation over a defect where the operating
characteristics change and are presen-
ted to the equipment operator.

The Eddy Current response is based on
the inter-action of the parent material
and the electrical condition of the probe
as it passes material with no defect or in
the presence of a crack/corrosion.

As the inspection probe encounters a
defect the probe delivers a response, a
signal which contains both amplitude
and phase information which can be
correlated to the defect in question.
Interpretations of these components
provides the inspector with information
of the defect type, orientation, extent,
and depth.

absence of any fractographic or de-
tailed fracture mechanics information.
Such problems are less severe in the
case of quadrant or corner cracks,
where the crack intersects two sur-
faces and abnormal crack geometry
can be more easily ruled out.

In principle it is possible to estimate
crack depth by comparing the eddy-
current signal from an unknown crack
against data from a calibration crack
of the same surface length in the same
component, presuming the depth of
the calibration crack is already known
and   that materials factors such as
crack closure, crack branching and
crack-face contact are equivalent. In
practice, such a library of calibration
cracks is rarely available, however,
defect response and signal presen-
tation can be utilized in conjunction to
give fairly accurate   crack   configu-
ration, depth and   extension   infor-
mation.

Current methods for practical crack-
sizing using ACFM or Eddy Current
techniques are based either on the use
of calibration cracks or on the estima-
tion of crack depth from measurements
of the surface crack length assuming
that the crack has a known aspect ratio.
Both approaches have limitations.

A major drawback in using the surface
crack length (whether determined from
the eddy-current probe response or di-
rectly from in-situ metallography) to infer
crack depth is that the assumed length-
to-depth ratio may be incorrect for the
particular crack under investigation. For
example, the crack depth will be over-
estimated if the crack has grown through
coalescence of shallow cracks having
multiple origins rather than through the
growth of a single crack, or may be un-
derestimated if the crack has initiated
from an unexpected sub-surface defect.
A further question is what length-to-
depth ratio should be assumed in the