Spectrum-Based Method for Fatigue Damage under Excitation of Sinusoidal Sweeps for Automotive Systems

Vibration from a mechanical system not only produces unwanted noises annoying to people around, but also runs a risk of fatigue failure that would actually hinder its functionality. There are several forms of vibration depending on the sources of excitation forms. Mechanical systems with rotating components can be subjected to sinusoidal excitation due to the fact the center of mass is not perfectly aligned with the rotating axis. If the rotating speed is strictly ramping up or ramping down, this can create an excitation whose frequency is changing with time in a frequency range corresponding to the speeds swept. Compared with a single sinusoidal excitation, the issue with fatigue at swept sinusoidal excitation, is that as it sweeps through a wide frequency range, some swept frequencies will definitely coincide with the natural frequencies of the system. Certainly, the stress response exactly at the resonant frequency becomes the highest and could account for a lot of fatigue damage. However, the stress in the vicinity of that resonance frequency could also contribute to the accumulated damage, depending on how long the system is exposed to the vibration there. Hence, only using the stress magnitude at the resonance to evaluate the durability is not adequate. The purpose of this work is to present a spectrum-based approach to calculate the cumulative fatigue damage as the stress is oscillating around its means with gradually increased or decreased frequency. The fatigue prediction is based on the stress spectrum obtained from a steady-state solution to a linear vibration system at each individual frequency. The fatigue damage spectrum is derived from the stress spectrum, along with a sweeping mode for the excitation frequency. Two of the commonly-used sweep modes (i.e. logarithmic and linear sweeping modes) are studied for an automotive component and their difference in the effect on the damage results are discussed in detail, through an example of an automotive component. In this work, it will be demonstrated how to choose sine sweeping rate such that the steady-state solution is guaranteed and the premise of this spectral method for fatigue is still valid.