Axial damping in metal-matrix composites. I: A new technique for measuring phase difference to 10⁻⁴ radians

Abstract

A majority of the current experimental techniques for measuring damping employ either flexural or torsional vibrations.In either case the strain field is nonhomogeneous. If the material damping is linear, i.e., strain independent, then the measured quantity equals the intrinsic material damping. if, on the other hand, the material damping is nonlinear, i.e., strain dependent, then the measured quantity also reflects the nonhomogeneity of the strain field and, therefore, is not equal to the intrinsic material damping. In this work we describe a new experimental technique in which the foregoing problem is circumvented by employing a homogeneous strain field, namely, uniform uniaxial tension. Damping is viewed as the phase angle by which the stress leads the strain. The finiteduration, time-harmonic stress and strain signals are transformed to the frequency domain via the use of Fourier transforms. It is shown that if one confines attention to the immediate neighborhood of the excitation frequency then, for all practical purposes, the phase difference between the two sinusoids is equal to the phase difference between their Fourier transforms. We will demonstrate that this phase difference can be measured to an accuracy of 2π/2¹⁶ or 9.587×10⁻⁵ radians.