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and not detected by the NDT carried out, was found to correlate with the damage. It
was located ~ 20 cm away from the true site of the damage and identified the need
to use an alternative technique for damage location in realistic structures.
 Approach requires the characterization of an installation by inserting 300 kHz
signals into the structure in known locations during the system’s installation, and
then using signature recognition and a 3D model of the structure in the analysis,
which has improved the positional accuracy to within 2 cm.
AE Applications
29
AE Application on MLG Fitting
 For the Tornado GR4 retraction jack fitting, the only technically viable damage
detection method to provide an on-line condition monitoring capability was
considered to be acoustic emission.
 With CVM, flanges on the bushings may preclude damage detection and the
movement of the retraction jack was likely to damage the sensors.
 Given the limitations of CVM for wide area unpredictable damage mechanisms
 fatigue damage may occur in virtually any location of the retraction jack fitting and take a
number of directions.
 Over a 10 year period the labor savings of introducing an AE on-line condition
monitoring capability for the Tornado GR4 retraction jack fitting are estimated
to be approximately £800k in NPV terms.
 estimated to increase aircraft availability by 61 aircraft days per year across the fleet.
 When acquisition and design incorporation costs of an AE system are taken into
account to provide an on-line condition monitoring it is unlikely that the system
would be cost effective in terms of labor savings alone.
 However, the increased availability may yield sufficient
savings to make the system viable and was recommended
to be further investigated.
T. Ewbank, Cranfield University,
Application Of Condition Based Maintenance On Aerospace Structures
30
PZT Network System using Lamb Waves
 Acellent Technologies uses built-in network of piezoelectric
transducers embedded in a thin dielectric carrier film.
 system includes the PZT network connected to portable, diagnostic
hardware and software.
 Performs in-situ monitoring, data collection, signal processing,
and real-time data interpretation to produce a two-dimensional
image of the structure being interrogated.
 Software controls the actuators to generate pre-selected
diagnostic signals and transmit them to neighboring sensors.
 wave types including 3, 5, and 10-peak narrow band frequency
waveforms, chirp, random, and user defined excitations
 Software links each sensor with its neighbors to form a web, or
network, covering the area of interest and collects responses from
each of the sensor sets as each PZT is activated.
 Changes in Lamb waves generated within the structure are used
with triangulation methods to detect the presence of anomalies
and to determine size & location.
31
PZT tests on Boron Epoxy patch
 Similar to conventional UT, PZT data analysis can include one or
more of the following measurements:
 Time of wave transit (or delay), path length, frequency, phase, amplitude
and angle of wave deflection (reflection & refraction)
 A series of excitation frequencies were used to optimize detection:
50 kHz, 200 kHz, 350 kHz, and 500 kHz.
 Results revealed that disbond flaws were most strongly detected with 50
kHz, while the crack growth was monitored best with the highest 500 kHz
excitation
 Signal attenuation, corresponding to disbonds between the patch
and metal skin were apparent
 Both flaws from one complete disbonded due to a Teflon insert, and a weak
bond produced by a mold release agent
D. Roach, Sandia Labs, HEALTH MONITORING OF AIRCRAFT
STRUCTURES USING DISTRIBUTED SENSOR SYSTEMS
32
Lamb Wave Applications
 A wingbox was tested (Grondel, Assaad, Delebarre, & Moulin, 2004) with
delamination of the plate sections of the composite structure and disbonds,
with stringers being readily detected using amplitude analysis.
 However, one key conclusion drawn was the need to identify the Lamb
wave propagation modes possible in the structure at the frequency used
 In this case there were 4 modes at the 400 kHz transducer frequency with
wavelengths ranging from 3.75 mm to 15 mm. Higher frequency modes
were very sensitive to damage, whereas the modes with the longer
wavelengths were relatively insensitive.
1. Use a frequency region where only fundamental propagation modes exist.
　
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