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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
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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.
2. Chose a propagation mode and frequency where dispersion (wave velocity is
a function of frequency and thickness of the plate) is kept to a minimum in
order to simplify signal analysis.
 Damage Localization in a Stiffened Composite Panel (D. Chetwynd, et.al.
University of Sheffield) work conducted as part of the Aircraft Reliability
Through Intelligent Materials Application (ARTIMA) EU project.
 Case study of damage detection in a curved carbon-fiber reinforced panel
with two omega stiffeners investigated using UT Lamb waves.
 Outlier statistical analysis was used as a way of pre-processing data prior
to damage classification. Multilayer perceptron neural networks were used
for classification and regression problems of damage detection.
 It was then investigated whether using wavelet analysis to perform prior
wavelet decompositions of experimental data could facilitate damage
classification.
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Fiber Bragg Gratings (FBG)
 Fiber-optic sensors with elastic properties similar to those of the tested material
 Can be used to monitor temperature, thermal and mechanical stress, damage caused by
collision or impact, and delamination.
 Fiber-optic sensors operate in similar manner as strain gauges.
 As the material under test expands due to the effect of temperature or mechanical forces,
properties of the sensor fibers vary in an easily measured way.
 In a strain gauge, the electrical resistance varies in proportion to it’s distension
 In fiber Bragg gratings, the characteristics of the reflected light change based on the position
of tiny mirrors that make up the Bragg grating and with which the optical fiber is doped using
a laser technique.
 Up to 25 measurement points can be integrated in a small single fiber
 To achieve the same number of measurement points using a strain gauge, it would be
necessary to lay 25 thick multi-wire cables – a hardware density that is already too high to be
of practical use in a test configuration.
 As a result, the network of measurement points used in conventional testing is
correspondingly widely spaced; FBGs would allow more closely spaced data
resolution.
 Quality of data would also be enhanced, using far less elaborate means
 Unlike more conventional types of sensors, fiber-optic
sensors are not subject to interference by EM fields,
so do not require elaborate shielding.
Cross-section of embedded fiber
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FBG Applications
 Betz, Staszewski, Thursby, & Culshaw; Structural Damage Identification
Using Multifunctional Bragg Grating Sensors: Damage Detection Results
and Analysis, 2006).
 Work attempted to determine sensitivity to temperature variation and,
most significantly, give an indication of damage detection sensitivity.
 Included the use of two driving frequencies at 260 kHz and 460 kHz to explore
the effect of frequency on damage detection.
 Also involved the use of several analytical tools and the use of both
piezoelectric and fiber optic Bragg Grating (FBG) sensors for the ultrasonics.
 Results suggest that analysis of amplitude and the propagation period
of the first two Lamb wave packets received gave the best damage
detection correlation with the ability to discriminate between damage
sizes of 0.8, 1.4 and 2 mm but for damage greater than 15 mm in size, a
saturation effect was observed.
 When using the analysis of amplitude and the propagation period of the
first two Lamb wave packets both the piezoelectric and FBG sensors
worked equally well, and it was possible to correlate damage size with
　
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