| Autor: |
Wadley, Haydn N. G., Lu, Yichi, Goldman, Jeffrey A. |
| Zdroj: |
Journal of Nondestructive Evaluation; March 1995, Vol. 14 Issue: 1 p31-38, 8p |
| Abstrakt: |
Single crystal a-Al2O3, (sapphire) fibers of ~ 100 µm diameter have recently emerged as candidates for stiffening and strengthening high temperature composites. The Young's modulus of these fibers depends upon their crystallographic orientation, ranging from a high of 461 GPa for thec-orientation to a low of ~373 GPa for orientations 45° off thec-axis. A deviation of the fibers' axial orientation from the c-axis and thus a reduction in the fibers' axial modulus can sometimes occur during the fiber growth process, and so a simple reliable method is needed to characterize the modulus and/or orientation of the fibers. A laser generated-piezoelectric transducer detected ultrasonic method has been used for this purpose. It has been found that a clear correlation exists between the velocity of the first arriving ultrasonic signal and the deviation of the fiber's axis from thec-axis. The measured velocity is found to be in reasonably good agreement with the calculated bar velocity,?b=vE/?, for the fiber, providing an estimate of the fiber's orientation dependent Young's modulus. The small differences between the measured and the calculated velocities are believed to be caused by a combination of measurement errors, uncertainties in the reference elastic compliance constants of a-Al2O3 and the presence of small volume fractions of pores and other (low modulus) aluminum oxide phases in the fibers. |
| Databáze: |
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