Laser ultrasonic techniques (LUTs) perform an inspection based on raster scanning pattern to obtain three-dimensional (3D) ultrasonic signals for damage detection in mechanical structures. Even though the raster scan-based in LUTs provides full-field ultrasonic data with high spatial resolution, the scan process consumes substantial time and generates redundant ultrasonic data in many applications. In this paper, statistical damage detection based on the full-field covariance of circumferential scanning is proposed to accelerate the damage detection process using LUTs. A laser ultrasonic interrogation method based on a Q-switched laser scanning system was used to interrogate 3D ultrasonic signals in a 6-mm aluminum plate with four square through-thickness at four different depths. The circumferential scans at a given radius were obtained from the 3D ultrasonic wavefield and represented in a two-dimensional (2D) matrix, angle-time (θ-t) domain. The proposed method was tested at three different circumferences where the defects were located right on, outside, and inside the area of the scan circumference. The covariance matrix, Cθ, of the vector variables in θ-direction was calculated and represented as a covariance image. The covariance image of Cθ demonstrated the ability to detect the defects at these three different circumferences. Hence, the covariance map of an ultrasound circumference can facilitate the existing LUTs to determine the damage existence instead operate in raster scanning mode.
In this paper, a full-field ultrasonic guided wave method is proposed to inspect a composite sandwich specimen made for an aircraft engine nacelle. The back skin/core interface of the specimen is built with two fabricated disbond defects (diameters of 12.7 mm and 25.4 mm) by removing areas of the adhesive used to bond the back skin to the core. A laser ultrasonic interrogation system (LUIS) incorporated with a disbond detection algorithm is developed. The system consists of a 1-kHz laser ultrasonic scanning system and a single fixed ultrasonic sensor to interrogate ultrasonic guided waves in the sandwich specimen. The interest area of 400 mm × 400 mm is scanned at a 0.5 mm scan interval. The corresponding full-field ultrasonic data is obtained and generated in the three-dimensional (3-D) space-time domain. Then, the 3-D full-field ultrasonic data is Fourier transformed and the ultrasonic frequency spectra are analyzed to determine the dominant frequency that is sensitive to the disbond defects. Continuous wavelet transform (CWT) based on fast Fourier transform (FFT) is implemented as a single-frequency bandpass filter to filter the full-field ultrasonic data in the 3-D space-time domain at the selected dominant frequency. The LUIS has shown the ability to detect the disbond with diameters of 11 mm and 23 mm which match to the pre-determined disbond sizes well. For future research, a robust signal processing algorithm and a model-based matched filter will be investigated to make the detection process autonomous and improve detectability
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.