Forced Frequency Response Analysis of Multimaterial Systems

When finite element models are used to calculate frequency response, the value of structural damping has a large effect on the value of computed response at resonance. In order to properly understand the NVH characteristics of a system, as well as to achieve an accurate correlation of analytical data with modal tests, it is imperative that damping effects should be simulated as accurately as possible. This is especially true when the system contains more than one material, since different materials have varying damping properties. MSC NASTRAN is a commercially available modeling tool which offers two analysis methods for calculating the frequency response of a structure. One approach is a direct frequency response analysis, and the other is a modal frequency response analysis. The two approaches treat damping differently.

This paper evaluates the capabilities and limitations of the direct and modal frequency response solutions available in MSC NASTRAN, as applied to multi-material systems. Specific examples are presented to demonstrate the use to material dependent loss factors in the frequency response analyses of such systems. It is concluded that only with the direct frequency response analysis method can the effects of material damping in multi-material systems be properly simulated. Furthermore, the importance of accurate modeling of damping effects is emphasized when using CAE tools to either determine new design directions or perform A-to-B design comparisons. With inaccurate modeling of damping characteristics, one can draw incorrect conclusions from CAE studies, thereby either under or over designing the system.