A numerical simulation of modal testing on a structure with viscous damping

Abstract

A computer software package has been developed for the numerical simulation of steady-state sinusoidal multiple-shaker dynamic modal testing of a discretized structural model. The analytical grid model studied was assumed to be linear and to have viscous damping. Two cases of damping were investigated: the first did not couple the undamped normal modes, the second did. A procedure known as Asher's Method was utilized. The method determines the relative force-amplitude distributions to tune, as completely as possible, individual modes of vibration. Numerically generated structural transfer function data was used to determine the natural frequencies and their respective tuning force-amplitude distributions. Each separated mode had its particular force-amplitude distribution applied in a narrow frequency band about the resonance in order to calculate the complex frequency response. The coincident (real) and quadrature (imaginary) parts of the complex response for each degree of freedom were used to evaluate the quality of modal tuning by indicating the effects of modal coupling or interference. The method was found to be capable of isolating and tuning individual modes of vibration when fewer shakers than the total number of degrees of freedom were applied. The method appears to be particularly useful when close frequency spacing makes it difficult to obtain accurate modal data.