On the Effect of Modal Coupling on the Measurement of Damping : Measurement from Magnitude, Real or Imaginary Part of FRF

Abstract

Damping is usually inferred from the response parameters measured in vibration tests, because it can hardly be measured directly. The accuracy of the inference is dependent on the mathematical model representing the behavior of the damping material and the structural dynamics. The simplest mathematical model is a single-degree-of-freedom system with viscous damping. This model is probably a close approximation to actual vibratory system with light damping. As the amount of damping is increased, however, the accuracy of the inference based on the single-degree-of-freedom system becomes poor. One of the reason is considered that the so-called "half-power bandwidth" of the resonant response peak is influenced by the another resonant peaks (called modal coupling), since most vibratory systems have many resonant response peaks in the frequency range. So in this paper, we will discuss the effect of modal coupling on the measurement of damping. Three methods of determining the damping are compared with each other: determination from the magnitude, real or imaginary part of the frequency response function. The theoretical results are compared with the experimental ones.