Amplitude Dependence of Internal Friction and Shear Modulus of Boron Fibers

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).