Methodology for analysis of vibration isolating systems

Abstract: Vibration causes problems in many kinds of machinery including fighter aircraft. It can, for example, cause structural fatigue, malfunction in equipment and injuries to humans. The sources of vibration are engines, flow disturbance and separation of external loads, to mention some. To isolate sensitive equipment from the source of vibration it is common to use vibration isolators made of rubber. This master’s thesis is performed at Saab Aeronautics in Linköping at the Department of Environmental Engineering & Thermal Analysis. The subject of the master’s thesis is to develop a methodology for the analysis of vibration isolating systems in aircraft. The work is performed by finite element analysis and laboratory tests using a shaker table. In the finite element analysis, part of the study, a method to condensate the structure to decrease the number of degrees of freedom is studied and used. Nonlinearities are investigated but only linear characteristics are implemented into the f inite element model. Rubber as a material has several non-linear characteristics that depend on the amplitude and frequency of the vibration. For example, the stiffness and damping are amplitude dependent. From laboratory results from tests on a shaker table, the stiffness and damping of the vibration isolators are extracted. To do this, the frequency transfer function is used by identifying eigenmodes and the eigenfrequency to calculate the stiffness and extract the damping by studying the shape of the peak. A finite element model with a condensed structure is used to find the eigenmodes and the eigenfrequencies. By exciting the finite element model the frequency response is also calculated. The tested vibration isolator shows large non-linearity for the stiffness and damping when varying the amplitude of a sinusoidal vibration. Using a condensed structure in finite element is a powerful tool to reduce computational time. If the solution of a simulation or calculation has to be very accurate or if the amplitudes of the vibration vary a lot, the non-linearities must be implemented into the simulation or calculation and accounted for.