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.
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