Amplitude Dependence of Internal Friction and Shear Modulus of Boron Fibers

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Abstract
The internal friction and shear modulus of vapor‐deposited boron fibers were measured at room temperature in an inverted torsion pendulum at about 5 Hz. The amplitude dependence was determined from −7G to 2×10−3G, where G is the shear modulus. Two regions were observed: a relatively amplitude‐independent range to about 10−4G and strong amplitude dependence for higher oscillatory stresses. The internal friction and modulus were found to be independent of the number of stress‐strain cycles for amplitudes −4G; irreversible changes started within a few cycles at or above 10−3G. The changes are qualitatively similar to the depinning of dislocations in metals. Absolute values of the shear modulus were determined at a strain amplitude of 2×10−6 and typically were 1.4×1012 dyn/cm2 (2×107 psi).