Ultrasonic guided waves in composite plates: a study of interface bond condition and material properties determination with broadband focused air-coupled ultrasounds
This work is focused on two important aspects of the study of the composite materials: characterization of interface bonding in layered composites and evaluation of elastic material properties in air-coupled experiments.;The characterization of interface condition in a layered composite is critical to understand the behavior of the material under various stress situations. A closed disbond, alternatively known as zero-volume disbond or " kissing-disbond" (KSD) is difficult to detection by conventional normal-incidence ultrasonic nondestructive evaluation (NDE) methods. Such undetected defect may be dangerous because it can produce a significant decrease in structural integrity or lead to a catastrophic failure under certain loading conditions. We have successfully demonstrated a new technique for KSD detection in carried-to-completion experiments involving various artificially created defects.;The dependence of the modes of the propagating wave in a plate on the elastic material properties has motivated us to develop a method suitable for rapid non-contact reconstruction of the guided waves spectra. To overcome the weak and inefficient generation of ultrasonic waves in air of conventional transducers and to achieve the desired wide angular spread of the acoustic beam in air, we employed capacitive foil transducers and designed focusing devices. Measurements of the acoustic pressure profile of the transducers have been performed in both planar and focused configurations and compared with our computationally efficient received voltage model.;We exploited our custom broadband focused air-coupled system and a synthetic aperture scan technique to produce, in one line scan with only one incident angle, an almost complete spectrum of the Rayleigh-Lamb waves of various engineering materials subject to a 700-kHz frequency and 16° angular bandwidth limitation of the transducers. A pulse-compression method along with a fast data acquisition and processing capability allowed us to perform rapid reconstruction and evaluation of the guided waves spectra. Tests of our method on numerous materials showed excellent agreement with theoretical predictions.*;*This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Adobe Acrobat.