Designing and evaluating transducers for narrowband ultrasonic spectroscopy

Abstract

Introduction of new composite materials in aerospace applications has created a demand for an efficient NDE technique. Ultrasound resonance inspection is especially suitable for the inspection of multilayered structures. In the narrowband ultrasonic spectroscopy (NBUS), which has been developed recently (1), the surface of an inspected structure is scanned with a resonant transducer whose frequency response is monitored in a narrow frequency band. The paper is concerned with optimizing the NBUS setup consisting of a piezoelectric transducer coupled to a multi-layered structure. Differences in the electrical impedance of a piezoelectric transducer caused by variations of parameters of the inspected structure are estimated using an equivalent circuit model and a finite element tool. Design guidelines for NBUS transducers are presented, including an explanation for why NBUS transducers can efficiently operate at lower frequencies than those adequate for pulse-echo ultrasonics od similar objects. I. INTRODUCTION In a broadband ultrasonic resonance spectroscopy (URS) test a frequency spectrum of the inspected object is acquired and an analysis is performed to evaluate material properties and/or to detect flaws. A broadband transducer, weakly cou- pled to the inspected structure acquires its frequency response in some frequency band where a number of resonances should occur. The resonance frequencies depend on a number of factors, such as, material composition, temperature and the presence of flaws in the structure. A transmitting transducer is excited by a swept frequency signal and the spectrum is acquired using a number of receiving transducers. Changes in the condition of the structure will influence the position of the resonances in the frequency response and a diagnosis can be