An ultrasonic guided wave based damage detection technique has been developed for structural health monitoring
(SHM) of composite structures submerged in water. Specially designed guided wave transducers are utilized to
selectively excite and receive guided waves with dominant shear horizontal particle displacements. It has been shown
that the SH type waves are insensitive to water loading conditions. With appropriate water sealing, the transducers can
be applied to composite structures submerged in water. The guided wave signals collected from an underwater
composite structure are almost identical to the signals that are obtained before the structure is submerged. Experiments
have been conducted to demonstrate the feasibility of damage detection in underwater composite structures. A thick
carbon/epoxy composite beam is used as the test sample. Excellent damage detection results were obtained for both dry
and underwater tests.
Mode and frequency control always plays an important role in ultrasonic guided wave applications. In this paper,
theoretical understanding of guided wave excitations of axisymmetric sources on plate structures is established. It is
shown that a wave number spectrum can be used to investigate the guided wave excitations of an axisymmetric source.
The wave number spectrum is calculated from a Hankel transform of the axial source loading profile. On the basis of
the theoretical understanding, phased annular array transducers are developed as a powerful tool for guided wave mode
and frequency control. By applying appropriate time delays to phase the multiple elements of an annular array
transducer, guided wave mode and frequency tuning can be achieved fully electronically. The phased annular array
transducers have been successfully used for various applications. Example applications presented in this paper include
phased annular arrays for guided wave beamforming and a novel ultrasonic vibration modal analysis technique for
damage detection.
A new ultrasonic guided wave modal analysis technique (UMAT) is being studied to bridge the gap between ultrasonic
guided wave methods and lower frequency vibration modal analysis methods for Nondestructive Evaluation (NDE) and
Structural Health Monitoring (SHM). The new technique provides improved defect detection sensitivity superior to
modal analysis alone, and, at the same time, reduces the number of inspection positions required by the guided wave
techniques for a complete coverage of the structures being inspected. Instead of focusing on the transient structural
response to a guided wave input, the proposed UMAT puts the emphasis on the long time structural response to a
specifically defined ultrasonic guided wave input. Since different guided wave modes and frequencies yield good
sensitivities to different kinds of defects, the specified guided wave input which is selected to target on a certain defect
type provides a special sensitivity to the defect type. By varying the input guided wave modes and frequencies, good
sensitivities to all different kinds of defects can be achieved. In UMAT, the defect information is extracted through
modal analyses on the long time structural responses to the controlled guided wave inputs. Thanks to the fact that the
long time structural responses result from multiple reflections and scatterings of the input guided wave energy, an
overall coverage of the structure can be reached from a very limited number of tests. UMAT is also capable of inspecting
odd shaped parts with different attachment considerations or boundary conditions and even hidden, coated, or insulated parts as long as a small section is accessible.
The ultrasonic guided wave phased array technique offers an efficient means to interrogate damages in plate-like
structures. When applying this technique to multilayer composite plates, however, the anisotropic behavior of the
composite materials leads to significant influences on the beam steering performances of the phased arrays. This paper
investigates the beam steering performances of guided wave phased arrays for multilayer composite plates in terms of
phased array directivity profiles under influences of anisotropy. Angular dependences of guided wave amplitudes and
phase variations in composite plates obtained through a Green's function based method are implemented into directivity
profile calculations to account for the influences of anisotropy in a quantitative way. Guided wave phased array
experiments are carried out to validate the directivity profile calculations.
Guided wave phased array systems have great potential in structural health monitoring (SHM), especially for aircraft
applications due to its capability of steering the emitted guided wave beam to inspect a large area with the sensor array at
just one accessible position. However, when the guided wave phased array is applied to composite panels of an aircraft
component, the anisotropic behavior of the composite materials leads to a significant influence on the beam steering
performance of the phased array. In this study, mode neighborhoods in dispersion curves where guided waves have
quasi-isotropic behavior (i.e. constant or similar phase velocities in different wave propagation directions) are explored
for unidirectional, cross-ply, and quasi-isotropic composite plates. It was demonstrated that the energy skewness of
guided waves was well suppressed in these mode neighborhoods. Furthermore, by utilizing a modified delay-and-sum
beam forming algorithm, the guided wave beam of a linear phased array can be steered quite well to the desired
directions in a composite plate.
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