Influence of the modal damping on the estimated fatigue life

Abstract

The durability assessment of a component or structure is often assessed using Finite Element Analysis (FEA)-based structural simulations. Fatigue is a progressive failure mode that is related to the stress cycle’s range in a power law such that the fatigue damage increases exponentially as the stress range increases.When the frequency content of the loading gets closer to the natural frequencies of the structure, resonance effects are activated. In this case, the dynamic properties of the structure must be included in the analysis in order to better account for the increased stress response. A dynamic FEA produces therefore more realistic results compared to a static analysis, but requires knowledge of additional characteristics such as mass and damping. In a dynamic FEA, whether transient or steady state frequency response, the damping property governs indeed the magnitude of the dynamic stress response and hence the durability of the component.Unfortunately a default damping value is sometimes erroneously assumed for all modes leading to errors in the stress response, which in turn leads to significant errors in the fatigue life estimates. The modal parameters – including damping ratios - for the structure’s modes of vibration can be extracted using experimental modal analysis techniques.The purpose of this article is first, to explain the critical role that damping plays in fatigue damage; and second, to recommend best practices for determining modal damping experimentally.A simple example of an automotive component will be used to illustrate the importance of using experimental modal analysis to adjust the properties of a FE-based dynamic structural analysis. An experimental modal test followed by a modal analysis will be described. The mode shapes obtained experimentally and analytically will be compared to validate the mass, stiffness and boundary conditions used in the modelling. Then, it will be shown that using realistic values for the damping ratios will help to obtain stress results that correlate with the measured stress.Finally, this example will illustrate the very high sensitivity of the fatigue life estimate to the damping ratios.