Commercial off-the-shelf rubber isolators often come with no additional information other than the static stiffness in three translational directions. Hydraulic testing machines can be used to obtain frequency dependent dynamic stiffnesses of rubber isolators in translational degrees of freedom (DoF). Alternatively, dynamic substructuring based methods can be used, which can additionally identify the dynamic stiffness in rotational DoF while requiring only standard vibration testing equipment.
Results of two substructuring methods will be compared to those from a hydraulic machine. Both of the presented methods use locally rigid fixtures, mounted to the bottom and top of the isolators. Frequency based substructuring (FBS) requires knowing the fixtures dynamics to decouple them. Inverse substructuring, also called in-situ decoupling, does not require knowing the fixtures dynamics, but is assuming negligible mass and a special stiffness matrix topology of the rubber isolator.
Both methods produce accurate results for translational DoF up to the kilo Hertz range, which is confirmed by comparison to measurements on the hydraulic machine. However, FBS does not rely on specific assumptions about the isolator, like inverse substructuring. The limits of inverse substructuring’s underlying assumptions are shown theoretically and in the measurements presented here. We propose two extensions to compensate for the assumptions and present their results.
Nevertheless, the rubber model obtained with the FBS decoupling can provide better results when used in an assembly. This is illustrated by testing the experimental rubber element models, obtained with either method, in a substructuring prediction of coupled frequency response functions (FRFs) and comparing that to reference measurements.