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The non-linear elastic behavior of a series of pitch-based graphite fibers was studied using the method of laser generated ultrasound. A Q-switched Nd-YAG laser was used to produce pulses which generated stress waves in the fibers. The velocity of the acoustic wave was determined from the time of flight of the wave at the point of impact to a piezoelectric transducer positioned near one end of each specimen. The effective Young's modulus of the fibers was determined for a wide range of static tensile stresses and varying temperatures. X-ray diffraction measurements were also made on these fibers, to determine such factors as crystallite size, orientation, and modulus. From these data, theoretical calculations of fiber modulus were performed, and the results compared to the ultrasonic modulus measurements. These results are useful for characterizing the elastic behavior of composites in which the fibers are used.
graphite, crystal, modulus, X-ray diffraction, uniform stress, crystallite
Materials Engineer, NASA Goddard Space Flight Center, Greenbelt, MD