O. O. Balogun, R. D. Huber, D. J. Chinn, University of California, Livermore, CA; J. B. Spicer, The Johns Hopkins University, Whiting School of Engineering, Baltimore, MD
Microstructural variations in materials lead to measurable changes in ultrasonic wave velocity and attenuation. Consequently, ultrasound is sensitive to the presence of microstructural defects including dislocation networks, microcracks and voids and can be used to assess the damage state in a material. Furthermore, it is known that effects associated with material microstructures can lead to the nonlinear elastic behavior leading to harmonic generation of ultrasound. These effects are explored in this work for characterization of microscale defects and mesoscale structure in metals and metal alloys. To perform this type of characterization, a laser-based ultrasonic system has been used in which ultrasonic wave generation is achieved using a 900 picosecond pulse duration Nd:YAG laser and ultrasonic wave amplitudes are measured using a path-stabilized Michelson interferometer. Micron scale spatial resolution is achieved with this system by monitoring the variation in the amplitude and attenuation of ultrasound at frequencies up to 1 GHz. Experimental results obtained in commercially pure polycrystalline materials without engineered defects indicate that ultrasonic properties over mesoscale dimensions are strongly influenced by local variation in material microstructure.
Summary: Microstructural variations in materials lead to measurable changes in ultrasonic wave velocity and attenuation. Consequently, ultrasound is sensitive to the presence of microstructural defects including dislocation networks, microcracks and voids and can be used to assess the damage state in a material. Furthermore, it is known that effects associated with material microstructures can lead to the nonlinear elastic behavior leading to harmonic generation of ultrasound. These effects are explored in this work for characterization of microscale defects and mesoscale structure in metals and metal alloys. To perform this type of characterization, a laser-based ultrasonic system has been used in which ultrasonic wave generation is achieved using a 900 picosecond pulse duration Nd:YAG laser and ultrasonic wave amplitudes are measured using a path-stabilized Michelson interferometer. Micron scale spatial resolution is achieved with this system by monitoring the variation in the amplitude and attenuation of ultrasound at frequencies up to 1 GHz. Experimental results obtained in commercially pure polycrystalline materials without engineered defects indicate that ultrasonic properties over mesoscale dimensions are strongly influenced by local variation in material microstructure.
