a Fiber Optic-Based Thermal Acousto-Photonic System for Noncontacting Nondestructive Evaluation

The Thermal Acousto-Photonic (TAP) Nondestructive Evaluation (NDE) system is an optically based ultrasonic testing technique which extends the potential applications of ultrasonic flaw detection beyond the current capabilities of traditional piezoelectric transducer based technology. Laser based excitation and optical interferometric detection of acoustic waves are conducted through flexible noncontacting fiber optic probes, and a combination of these two methods is used to locate and characterize surface flaws in structural components. TAP has several advantages over conventional transducers; it is noncontacting, more sensitive, and permits truer pointwise sensing. These attributes increase the potential for in-situ applications and allows ultrasonic testing to be conducted in remote and inaccessible locations on structural components. This dissertation discusses the initial outcome of what is potentially a long term research project to explore the synthesis of photonics and ultrasonics to develop a reliable and rugged system for NDE. Based on a review of the considerable research done in the area of optical ultrasound, it is shown that, at present, TAP is the only all fiber optic system for laser based NDE. First, the excitation leg of the system is discussed. A high energy laser pulse is guided through a fiber optic to an appropriate location on the surface of the test specimen. The focused light creates a localized thermal shock which, in turn, generates a Rayleigh acoustic wave. When the wave interacts with a flaw in its path, the signal is modified by the defect in a way which can be correlated to the dimensions of the flaw. The modified Rayleigh wave is detected by an optical fiber interferometer (OFI) and analyzed using computational methods. The technique of time domain reflectometry and ultrasonic spectroscopy, are applied to determine the location and the salient features of the flaw, respectively. Finally, laser excitation, interferometric detection and signal processing are combined and tests are conducted to verify the capabilities of the all fiber optic NDE system. A list of recommendations and directions for future research brings the dissertation to its conclusion.