AEROSPACE NDT USING PIEZOCERAMIC AIR-COUPLED TRANSDUCERS D. K. Hsu 1 , V. Kommareddy 2 , D. J. Barnard 1 , J. J. Peters 1 . and V. Dayal 3 1 Center for NDE, Iowa State University, Ames, Iowa, USA 2 GE Global Research Center, Bangalore, India 3 Department of Aerospace Engineering, Iowa State University, Ames, Iowa, USA Abstract: In the nondestructive inspection of aerospace materials and structures, water-coupled ultrasonic inspection is often not desirable due to contamination and property alteration concerns. Examples include composite and foam structures in space applications and honeycomb sandwiches that cannot tolerate water ingression. For practical and operational reasons, water-coupled ultrasound is also not preferred for on-aircraft inspection in maintenance hangars. Non-contact, air-coupled ultrasonic transducers are therefore an attractive alternative for such applications. This paper describes our experience of using piezoceramic air-coupled transducers for flaw detection and repair evaluation of composite parts. The paper first discusses the basic principles of air-coupled ultrasonic measurement and laboratory tests of various materials and structures, followed by a description of a portable, manual, air-coupled ultrasonic scan system that allowed the use of air-coupled ultrasound in the field inspection of aircraft components. Introduction: The goal of this work is to first gain experience of air-coupled ultrasonic testing (AC-UT) in the laboratory by applying the technique to a variety of composite and metallic structures, and then to develop a portable scanning system using commercially available components so that air-coupled ultrasonic testing can be applied in the field on aircraft structures. With this goal in mind, commercially available piezoelectric air-coupled ultrasonic transducers and the associated pulser and receiver [1] were acquired for conducting the laboratory experiments and for incorporating into a portable scanning system. The considerations used for making the choice were mainly the penetrating power of the system and the compactness and robustness for field applications. The laboratory investigation for AC-UT consisted of transducer field profile mapping for planar and focused probes, quantitative measurement of the transmission insertion loss of air-coupled ultrasound in various solids, and imaging of defects and flaws in composite and metallic structures using a conventional laboratory immersion ultrasonic scanning system with the immersion tank removed. The effort of developing a portable system for field use of AC-UT has concentrated on the adaptation of a commercially available magnetically coupled wireless position and orientation tracker [2] as a position encoding device, the fabrication of a light weight yoke fixture for AC-UT transducers, and the development of a data acquisition and analysis software. The fieldable AC-UT system has been completed and initial on-aircraft tests have produced good results. NDE Using Air-Coupled Ultrasound: Ultrasonic testing (UT) is one of the mature modalities of nondestructive inspection for aerospace structures. Contact mode UT typically uses oil or gel as a couplant and is applied manually using portable flaw detecting instrument. Water-coupled ultrasonic inspection are widely used by OEM in manufacturing where large aerospace parts are scanned in automated squirter systems [3]. Ultrasonic inspection by immersion or squirter systems cannot be conveniently applied in the field, although closed-cycle, water- coupled systems have been developed for field use [4]. Air-coupled ultrasonic inspection has the distinct advantage of being couplant-free, but also suffers from several significant disadvantages. The primary limitation of air-coupled ultrasound are the large reflection loss at the air-solid interface [5] and the large attenuation of high frequency ultrasound in air [6]. The latter has limited the application of AC-UT to frequencies mainly below 1 MHz or so. In the NDE of aerospace structures, where internal flaws and defects are to be detected and imaged, the current state of AC-UT technology is mainly on transmission mode inspection that requires two-sided access. The piezoelectric air-coupled ultrasonic transducers used in this work are planar or focused narrow-band probes with center frequencies of 120 kHz or 400 kHz; purchased from QMI, Inc. [1] These transducers are driven by their matching electronics, the SONDA 007CX pulser and receiver system. The dimensions of the piezoceramic elements of the QMI air-coupled transducers range from 3/4" to 1" diameter and the focal length is typically 1" to 2". Figure 2 shows a photo of the transducers and a representative field profile of a 400 kHz, 1" diameter transducer with a focal length of 2 inches. The focal spot size (FWHM) of this transducer is about 1/10