Structural health monitoring (SHM) of aerospace components is a rapidly emerging
field due in part to commercial and military transport vehicles remaining in operation
beyond their designed life cycles. Damage detection strategies are sought
that provide real-time information of the structure's integrity. One approach that
has shown promise to accurately identify and quantify structural defects is based on
guided ultrasonic wave (GUW) inspections, where low amplitude attenuation properties
allow for long range and large specimen evaluation. One drawback to GUWs
is that they exhibit a complex multi-modal response, such that each frequency corresponds
to at least two excited modes, and thus intelligent signal processing is required
for even the simplest of structures. In addition, GUWs are dispersive, whereby the
wave velocity is a function of frequency, and the shape of the wave packet changes
over the spatial domain, requiring sophisticated detection algorithms. Moreover, existing
damage quantification measures are typically formulated as a comparison of the
damaged to undamaged response, which has proven to be highly sensitive to changes
in environment, and therefore often unreliable.
As a response to these challenges inherent to GUW inspections, this research develops
techniques to locate and estimate the severity of the damage. Specifically, a
phase gradient based localization algorithm is introduced to identify the defect position
independent of excitation frequency and damage size. Mode separation through
the filtering technique is central in isolating and extracting single mode components,
such as reflected, converted, and transmitted modes that may arise from the incident
wave impacting a damage. Spatially-integrated single and multiple component mode coefficients are also formulated with the intent to better characterize wave reflections
and conversions and to increase the signal to noise ratios. The techniques are
applied to damaged isotropic finite element plate models and experimental data obtained
from Scanning Laser Doppler Vibrometry tests. Numerical and experimental
parametric studies are conducted, and the current strengths and weaknesses of the
proposed approaches are discussed. In particular, limitations to the damage profiling
characterization are shown for low ultrasonic frequency regimes, whereas the multiple
component mode conversion coefficients provide excellent noise mitigation. Multiple
component estimation relies on an experimental technique developed for the estimation
of Lamb wave polarization using a 1D Laser Vibrometer. Lastly, suggestions are
made to apply the techniques to more structurally complex geometries.
| Identifer | oai:union.ndltd.org:GATECH/oai:smartech.gatech.edu:1853/34723 |
| Date | 11 June 2010 |
| Creators | Ayers, James Thomas |
| Publisher | Georgia Institute of Technology |
| Source Sets | Georgia Tech Electronic Thesis and Dissertation Archive |
| Detected Language | English |
| Type | Dissertation |
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