by Gary L. Burkhardt and lay L. Fisher, SeD
A
eam of Southwest Research
Institute en gineers is developing
more effective methods of detecting cracks in airplanes that can lead to
deadly crashes - an increasingly urgent
conce rn as the service life of aging aircraft is further ex tend ed .
Several types of U.s. Air Force aircraft already have exceeded their
designed 20- to 30-year "[etimes; the Air
Force plans to extend some airframe
service lifetimes up to 80 years. For
example, the 1950s-vintage B-52 bomber
and KC-135 tanker will operate until 2030
and 2040, respective ly. The nation's commercial aircraft fleet also is aging as airlines cu rb capita l equ ipment costs by
extending the service lives of their
Dr. Jay Fisher, left, is a program manager in SwRI's
Applied Physics Division. Fisher is responsible for
program development and project management for
theoretical investigation and applied development of
advanced electromagnetic nondestructive evaluation (NDE) techniques. Gary Burkhardt, right, is a
staff scientist in the Applied Physics Division. His
expertise includes the development of electromagnetic techniques and sensors for NDE of components and structures. Burkhardt holds five patents
and has authored more than 60 publications in the
NDE field. Fisher and Burkhardt previously developed inspection techniques for aircraft structures,
and have evaluated reliability of other techniques.
These evaluations fed them to understand shortcomings in present inspection technology and the need
for improved sensors.
6
Technology Today . Fall 2001
aircraft. Aging can result i.n widespread
fatigu e cracking of the aircraft's structure.
Cracks emanating from fastener holes in
the outer skin of the wing an d fuselage
constitute a major safety issue. The in-air
skin failure 13 years ago of an Aloha
Airlines jet was the result of skin failure
An "arrow" plot of eddy-current density illustrates current paths produced by
the meander coil on an inspection surface with a long flaw. The arrows indicate
current density according to the color bar scale at the top of the iffustration
produced by cracks that formed from
many small rivet holes. Aloha Airlines
Flight 243 was en route to Honolulu on
April 28, 1988, at 24,000 feet when a large
section of the fuselage was ripped from
the Boeing 737 airliner. Miraculously, the
airplane landed safely, and the incident
resulted in only one fatality. The National
Transportation Safety Board accident
report noted that a passenger boarding the
airplane noticed a long crack in a row of
rivets above the cabin door but did not tell
the flight crew before takeoff.
The eddy-current testing (ECT)
method of nondestructive evaluation
(NDE) can detect tiny cracks, which must
be repaired before they become large
enough to threaten the safety of an aircraft. The most common technique
involves systematically moving a probe
around each fastener. This process is time
consuming because of the large number of
fasteners involved. For example, the fuselage of a C-5 cargo plane has more than
500,000 fasteners.
Although some new ECT technologies are in use, some are slow and difficult
to set up and calibrate. One ECT technique
can locate small cracks but cannot reliably
detect cracks that connect two fastener
holes. This shortcoming is of major
concern because these connecting cracks
can lead to the kind of catastrophic damage
that caused the Aloha Airlines incident.
SwRI's internal research and development program has funded work by engineers in the Institute's Applied Physics
Division w ho proposed development of a
more effective EeT
method for detecting
these dangerous cracks.
Background
Component
Detected by
Sensors
Eer systems use
alternating magnetic fields
to induce electrical current
flow into a test specimen.
This is usually accomplished using single or
multiple electromagnetic
coils, with the coil axes
perpendicular to the surface under inspection, or a
sheet conductor that
induces current flow over a
large area. SwRI
researchers believed that a
meander coil system could
detect cracks of all sizes consistently over a relatively
large area at one time.
A meander coil consists of a strip of
conducting metal that repeatedly doubles back on itself, resulting in a series of
parallel metallic pathways. An electrically activated meander coil generates
eddy currents in the inspected part, and
the magnetic fields from these currents
I
Sensors
Coil
The current flow (/) direction shown by the arrows alternates in
opposite directions for each adjacent conductor line in the meander coil. The meander excitation coil generates eddy currents in
the part under inspection, and flaws are detected using sensors
arranged so as to be sensitive to magnetic fields (8) parallel to
the inspection surface.
Technology Today. Fall 2001
7
A laboratory breadboard of a small-scale
array probe is shown on an aircraft wing
skin specimen containing fasteners. This
SwRI-developed probe could dramatically
cut the time required to inspect the many
thousands of fasteners ' holes in aircraft.
Modeling
As a first step, the research team
developed a three-dimensional electromagnetic model to optimize the meander
coil design. The software model calculated
the edd y-curren t distribution in the
inspected material and the associated magnetic fiel d d istribution above its surface.
The team used the resulting information to
determine the orientation and loca tion of
the receiver sensors and to design the
meander coil. The modeling showed that a
m eand er coil device would produce
detectable flaw responses for short and
long cracks. This d evice would thus be
superior to certain conventional sensors
that have d ifficulty detecting long cracks.
are detected by giant magnetoresistive
(GMR) sensors, solid-state devices that
change resistance in response to a m agnetic field. These GMR sensors are
arranged to sense the fields parallel to
the inspection surface. Where no crack
exists, there is no field component in this
direction, and no direct coupling occurs
between the main field and the sensor. If a
crack does exist, a magnetic field is produced in this parallel direction. This parallel field is generated in the inspected
component by eddy cu rrents, whose paths
are d isturbed by the crack.
The team built a prototype probe
with a single GMR sensor based on
promising modeling results. The m eand er
coil was fabricated with conductors
spaced close together on printed circuit
board material. A GMR sensor was placed
against th e meander coil with the sensor's
active area positioned between two conductors. Engineers bonded these components to form a probe package and placed
B
InspectIon
Surface
The non-conducting boundary of the flaw causes the
electrical currents to bend and flow parallel to the flaw.
This perturbed current generates magnetic field (B) in a
direction parallel to both the inspection surface and the
unperturbed current lines.
End View
Current
Lines
8
Technology Today. Fall 2001
D009432
II
The bottom of the small-scale array probe shows
the sensors positioned in holes cut between the
meander coil traces.
a bar magnet on the package to provide
the magnetic bias field required by the
GMR senso r.
Engineers used the probe to systematically scan several aluminum test panels. The panels contained fastener holes
with no cracks as well as fla wed holes
with induced notches to simulate cracks.
The flaws that connected fastener holes
showed up well, as did those that were
not connecting. The probe was also
scanned over a plastic shim fastened to
the panel's surface to simulate paint that
frequently separates a sensor from the
metal part under inspection. These separations can result in high electromagnetic
noi se and reduced crack detection capability in conventional EeT devices, but the
meander probe sustained almost no negative effect from this "lift-off" condition.
Small-scale array probe
The research team then prepared a
probe with several GMR sensors to
demonstrate the feasibility of an array
probe and to gain experience with fabricating an array. Engineers employed the
same coil configuration as with the singleprobe sensor, but they incorporated fom
GMR sensors in die form (a bare device
with no package).
The team evaluated the array probe
for sensitivity and spatial resolution. The
probe was scanned over the same panel
maanc;l4.IxI, 0 degree rotalioo
"
used in the sin gler-------- probe tests. The
20
probe readily
8.6-mmFlaw
detected connecting and nonconnecting flaws, even
th ough the resolution was reduced
because the 1.2mm spacing
between the array
sensors is g reater
60
than the O.5-nun
spacing between
20
3D
40
50
60
Index Increment (mm)
scans used for the
single sensor. In a
Color images are generated from meander probe signals as the probe is
test with an
scanned over a test specimen. The top image shows signals from fastener
un£lawed hole and
holes with a connecting flaw (top signal) and no flaws (bottom signal). The
holes with 3.2- and
bottom image shows signals from fastener holes with short flaws . Also
1.6-mm flaws, the
shown is an area where the probe was lifted off of the specimen surface
probe detected the
with very little response generated by the lift-off change.
3.2-mm flaw. To
detect the 1.6-mm
on-board electronics. Externally funded
flaw clearly, the sensor spacing would
need to be reduced to approximately
development may lead to a chip contain0.75 mm.
ing sensors and integrated electronics .•:.
"
Conclusions
SwRI staff demonstrated the feasibility of meander probe technology. The
Instih.tte is seeking external funding to
develop the sensor probe further. Future
development could involve an array
w ith more elements and containing
Technology Today . Fall 2001
Comments about this article? Con.tact
Burkhardt at (210) 522-2705 or
gb urkhardt@swri. org.
Acknowledgement
The authors would like to acknowledge Staff
Technician David Jones for his contributions
to experimental lab testing of the probe.
9