Magnetic shape anisotropy effect on sensing performance and directional sensitivity in magnetostrictive nickel patch transducer

This article presents the magnetic shape anisotropy effect on sensing performance and directional sensitivity in a directional magnetostrictive patch transducer using a uniaxial comb-shaped nickel patch, for the use of ultrasonic guided Lamb wave technology in metallic planar structure. The uniaxial comb patch was modified from a disc-shaped patch made of polycrystalline nickel, so that its comb fingers were aligned to a specific radial direction to promote the magnetic shape anisotropy in the nickel patch. Previously, it was found that a magnetostrictive patch transducer using a nickel disc patch appeared to have omnidirectional sensitivity for sensing guided Lamb waves in an aluminum plate due to the isotropic magnetostrictive nature of polycrystalline nickel. In this work, we initially investigated a theoretical approach to determine demagnetization factor change caused by the geometrical shape modification of a nickel patch, which strongly correlated with a specific magnetization characteristic of the patch. Then, experimental guided Lamb wave analysis was followed to demonstrate the directional guided Lamb wave sensitivity of magnetostrictive patch transducers using nickel patches with three different geometrical configurations. We verified from the experimental guided Lamb wave testing that the proposed magnetostrictive patch transducer using the uniaxial comb-shaped nickel patch exhibited excellent directional sensitivity along the comb fingers’ orientation in the aluminum plate.