Extensive studies have been conducted to examine the use of piezoelectric transducers and other ultrasonic devices for structural health monitoring applications due to their high electroacoustic efficiency and simple operation principles. Most studies use guided Lamb wave inspection methods to detect, locate, and characterize damage in relatively simple plate-like structures. These studies have proven that this type of technique is useful for structural health monitoring in the simple structures, but little work has shown feasibility when using more realistic test articles. In our study, we present a sparse array technique using multiple piezoelectric transducers mounted to an external composite panel on a legacy F/A-18 wing section to investigate its capabilities for more realistic structural health monitoring applications. The panel is secured to the wing by a series of bolts surrounding the panel, and as the main method of simulating damage to the wing, these bolts are loosened by a specified torque in several different cases. The guided Lamb wave pitch-catch inspection method is used to identify such simulated damage by comparing baseline and damaged trials using differential analysis and a spatial mapping approach. This approach requires an estimate of the Lamb wave mode group velocities in the composite panel along with the geometry of the transducer array and the transducers location metrics. Despite the complexities of using a realistic test article, the sparse piezoelectric array combined with the Lamb wave inspection method shows promise in its ability to accurately detect, locate, and characterize the simulated damage to the airframe.
This paper presents the development of an ultrasonic guided Lamb wave (GLW)-based magnetostrictive phased array sensor (MPAS) using a circular comb-shaped nickel disc patch with 1” in diameter. And its damage detection capability to identify loosened joint bolts is experimentally demonstrated. The compact sized MPAS was comprised of the nickel disc patch and a detachable magnetic circuit device. The disc patch was machined with 24 comb fingers along its radial direction and the magnetic circuit device contained 6 sensing coils and cylindrical biasing magnets. The individual sensing coils appear to have distinct directional sensing preferences designated by the normal direction of coil winding. The directional sensing feature of the developed MPAS is offered by the combined effect of the magnetic shape anisotropy of comb finger formation in the nickel patch and the sensing directionality of the coil sensor. The MPAS detects the strain-induced magnetic property change on the nickel comb patch due to the mechanical interaction between the patch and GLWs. Although the MPAS holds only the 6 physical coil sensors, the array sensor enables to acquire additional GLW signal data from different sensing sections within the nickel patch, by simply altering the rotational orientation of the magnetic circuit device. Such signal data additions allow to provide a higher resolution damage detection scheme for the advanced phased array signal processing technique. The MPAS apparatus and its damage detection capability were experimentally validated by GLW inspection testing with a thin aluminum plate installed with numerous joint bolts.
This paper presents the development of a Galfenol-based directional magnetostrictive patch transducer (MPT) and the
results of experimental investigations with the developed MPT for guided Lamb wave (GLW) applications. We used
three magnetostrictive materials such as nickel, polycrystalline Galfenol, and single-crystal-like Galfenol (i.e. a highly
textured Galfenol) with a 1-inch-diameter thin disc form to design the proposed MPT used to evaluate the directionality
for sensing the GLW propagation in thin aluminum plates. Prior to the GLW inspection using the MPTs, the
magnetostriction of the magnetostrictive patches was experimentally measured by a strain gauge, and electron
backscatter diffraction (EBSD) patterns were captured and analyzed to explore grain configurations and crystal
orientations of the patches. The material level analysis validated that the highly textured Galfenol behaves like a single
crystal with large magnetostriction along a <100> orientation. Therefore, the developed MPT using the single-crystallike
Galfenol was expected to exhibit high directional sensitivity of the GLWs traveling in the metallic plate,
corresponding to the preferred orientation of the Galfenol patch. The experimental study of the MPT-based GLW
approach demonstrated that the MPT using the single-crystal-like Galfenol exhibits excellent sensitivity to the incoming
GLWs along the <100> preferred direction, while the nickel-based MPT has omnidirectional GLW sensitivity in the
plate specimen. In addition, the MPT using the polycrystalline Galfenol showed two preferred directions for sensing the
GLWs, corresponding to <110> orientations.
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