Improving the Accuracy of Dynamic Vibration Fatigue Simulation

Finite Element Analysis (FEA)-based structural simulations are typically used to assess the durability of automotive components. Many parts experience vibration in use, and resonance effects are directly linked to many structural problems. In this case, dynamics must be included in the structural analysis. Dynamic FEA can be more realistic than static analysis, but it requires knowledge of additional characteristics such as mass and damping. Damping is an important property when performing dynamic FEA, whether transient or steady state dynamics, as it governs the magnitude of the dynamic stress response and hence durability. Unfortunately the importance of damping is often overlooked; sometimes a default damping value is erroneously assumed for all modes. Errors in damping lead to errors in the stress response, which in turn lead to significant changes in the fatigue life estimates. For example: 3% versus 5% damping ratio can lead to fatigue life estimates varying over an order of magnitude. Therefore, accurate fatigue life predictions rely heavily on realistic damping ratios. Damping must be evaluated from physical tests, in which the component is excited and responses are measured. The modal parameters for the structure’s modes of vibration are then extracted using experimental modal analysis techniques. The purpose of this paper is first, to explain the critical role that damping plays in fatigue damage; and second, to recommend best practices for improving FEA-based stress and durability analysis. Methods for quickly and easily determining damping ratios will be introduced. An example of an exhaust system will be used to illustrate the importance of using experimental modal analysis to adjust the properties of a FE-based dynamic structural analysis in order to obtain fatigue life results that correlate with testing.