Noncontact measurement of guided ultrasonic wave scattering for fatigue crack characterization

Abstract

ABSTRACT Fatigue cracks can develop in aerospace structures at locations of stress concentration such as fasteners. For the safe operation of the aircraft fatigue cracks need to be detected before reaching a critical length. Guided ultrasonic waves offer an efficient method for the detection and characterization of fatigue cracks in large aerospace structures. Noncontact excitation of guided waves was achieved using electromagnetic acoustic transducers (EMAT). The transducers were developed for the specific excitation of the A 0 Lamb mode. Based on the induced eddy currents in the plate a simple theoretical model was developed and reason ably good agreement with the measurements was achieved. However, the detection sensitivity for fatigue cracks depends on the location and orientation of the crack relative to the measurement locations. Crack-like defects have a directionality pattern of the scattered field depending on the angle of the incident wave relative to the defect orientation and on th e ratio of the characteristic defect size to wavelength. The detailed angular dependency of the guided wave field scattered at crack-like defects in plate structures has been measured using a noncontact laser interf erometer. Good agreement with 3D Finite Element simulation predictions was achieved for machined part-through and through-thickness notches. The amplitude of the scattered wave was quantified for a variation of angle of the incident wave relative to the defect orientation and the defect depth. These results provide the basis for the defect characterization in ae rospace structures using guided wave sensors. Keywords: Noncontact Measurement, Guided Ultrasonic Waves, Fatigue Crack Detection, Laser Interferometer 1. INTRODUCTION For aerospace structures the development of fatigue cracks at fastener holes due to stress concentration presents a common maintenance problem, necessitating structural health monitoring (SHM) [1]. The detection of fatigue cracks before they have reached a critical length is a safety requirement for aircraft, which have to be inspected regularly during their service life [2]. Ultrasonic bulk waves possess the necessary sensitivity for the detection and sizing of cracks [3], and sensors can be integrated into the fasteners [4]. An ultrasonic-based SHM method has been developed for the real time, in-situ monitoring of fatigue cracks at fastener holes using an angle beam through transmission technique [5]. However, bulk wave ultrasonic testing necessitates local contact measurements on the damaged area of the inspected structure [6]. Guided ultrasonic waves allow for the monitoring of hard to insp ect areas of large structures with limited access [7, 8], and have been successfully employed to monitor fatigue crack growth [1, 9]. Localized and distributed array systems using low-frequency guided ultrasonic waves have been developed fo r the detection of defects in plate structures [7, 10, 11]. The potential for noncontact measurement of the guided waves using a laser vibrometer has been demonstrated for fatigue crack detection in metallic structures [12, 13]. Both the excitation and measurement of guided ultrasonic waves can be performed using noncontact laser technology [14]. Noncontact excitation and reception of guided waves was achieve d using electromagnetic acoustic trans ducers (EMAT) [15]. The modelling and different applications for EMATs have been studied [16, 17]. However, the wavelength of the employed guided waves is usually significantly larger than in bulk wave ultrasonic testing, thus limiting the sensitivity for the detection of small defects [18]. The scattering of the A