Date of Award

May 2020

Degree Type

Thesis

Degree Name

Master of Science

Department

Engineering

First Advisor

Nathan Salowitz

Committee Members

Anoop K Dhingra, Rani El Hajjar

Abstract

Undetected damage in aircraft can lead to catastrophic failures and loss of life. Automated embedded damage detection systems can reduce manhours and downtime due to inspection. Ultrasonic inspection has become one of the most capable methods for thin plate-like structures, such as aircraft spars, stiffeners, and skins. Piezoelectric transducers can inspect structures by generating ultrasonic Lamb waves and sensing how they propagate. Many Lamb wave modes exist at a given frequency, which makes signal interpretation challenging. Selective actuation of a single mode simplifies the signal analysis. The A0 mode has the shortest wavelength, which increases sensitivity to small defects, and its group velocity is independent of composite structure layup. Recent research has found a shear-mode d15 piezoelectric transducer will selectively actuate the A0 mode when embedded at the neutral axis of a composite structure. Precise placement of the transducer at the neutral axis is difficult due to operator error, changing manufacturing environments, and design constraints. This work studies the relationship between off-axis d15 transducer placement and A0 mode selectivity through analytical, numerical, and experimental methods. A0 selectivity was found to be 12.7 dBV at the neutral axis but dropped 4.02 dBV when moved off-axis by 5.5% of the structure thickness, a drop of approximately 0.73 dBV/% of structure’s thickness from the neutral axis.

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