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both of the driving frequencies used.
 Although analysis did show some sensitivity to changes in temperature,
these effects were very minor and therefore well suited to further
processing.
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Acousto-Ultrasonics (AU)
 Technique that sends acoustic waves into the structure and
intercepts them when they emerge on the other side.
 Deviations from the expected wave pattern indicate the
presence of cracks or delamination.
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Eddy Current
 Eddy Current Testing Foil Sensors (ETFS) are suitable for use on metallic structures.
 Cracks and corrosion alter the electromagnetic field induced by the eddy current generated by the
sensor (flexible).
 Micro Eddy Current Sensor - Sandia Labs is developing a customized eddy current sensor
for crack detection in thick steel structure. The probe must be able to detect deep,
second-layer cracks as much as 0.5” below the surface.
 Impedance bridge and other differential circuits were explored to maximize the magnetic
flux density and corresponding eddy current strength.
 Successful crack detection was achieved with a dual coil configuration that combines a
pancake excitation inductor with a co-located pickup coil to produce a transducer that
requires very little drive current (75 mA) and operates in the desired 10 kHz range.
 Excellent crack detection was achieved even when inspecting through composite repair
doublers approaching 0.5” thickness.
 More sophisticated & rugged electronics package – including digital signal processing to
filter and detect phase shifts - to further improve probe sensitivity were being developed.
GE Inspection Technologies
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In-situ Sensors & Durability
 Advantage/disadvantage compared to established NDT techniques is that
future sensors remain permanently attached / embedded in place, required
to withstand many decades of aircraft service life
 mechanical structural stresses and ensure sensor performance & substrate bonding
 hot/cold and wet conditions
 Embedded sensors can integrate well with composite materials -
piezoelectric fibers or fiber-optic sensors can be fabricated with CFRP or
GFRP (mitigating risks of sensor debonding) however two difficulties arise
 if component is replaced due of wear or damage, the embedded sensor is also
 maintaining the sensor is difficult, virtually impossible to repair, and not replaceable
 Once installed, sensors provide a simple means of monitoring even areas
that are difficult or dangerous for inspectors to access such as fuel tanks,
wing spars, engine beams, etc. – areas that are difficult to detect
microscopic cracks or corrosion.
 Detection might be in online (measured continuously in flight) or offline
(data downloaded at next inspection or maintenance) modes.
 In-situ SHM sensors would be capable of spotting defects much faster,
leading to considerably shorter inspection times.
 Still, SHM should not entirely replace conventional NDT inspection practices
 Conventional maintenance checks are rarely limited to the precise area specified in
the maintenance manual - maintenance technicians typically take a careful look at
surrounding area outside the actual inspection range.
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Signal Analysis & Power Mgt
 Processor capabilities now are not limiting damage detection signal
processing, but weight / power are always concerns.
 Data must be managed in a robust system to ensure value and translated
into useful structural health and usage information. DO-178B provide some
structured development guidelines, which is recognized by FAA & EASA
 Many published works recognize patterns from known damage rather than
identify unknown damage. Parametric recognition systems are beginning to
appear (UK published policy in ’07 to guide development for future military
aircraft).
 Parallel processing appears to be mature for some limited applications such
as maneuver recognition in usage monitoring, but not prevalent in damage
detection.
 Energy harvesting methods have been studied that can power sensors
systems by converting structural stresses (strain energy harvesting) into
electrical power via piezoelectric transducers (Sandia Labs, Kansas State,
etc.)
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Directions
 Aircraft structural design optimization is the ultimate
benefit of SHM
 Structural maintenance inspections are a significant factor in operator’s
Direct Operating Costs.
 Minimize conventional NDI for periodic interval inspections for
　
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