A Comparative Study of Fatigue Damage from Simultaneous Swept Sine and Random Acceleration for Structures with Distinct Dynamic Response Characteristics

Engineers have been dealing with either random excitation or swept sinusoidal excitation quite often in the past, in order to estimate the fatigue damage in an automotive system. Efficient numerical methods in frequency domain for the fatigue due to either form of excitation (not both) have become more mature in the past decades. However, a greater demand for a fatigue estimation under a more complicated form of excitation has risen as electric vehicles are being developed in the automotive industry. In particular, delicate rotating components such as electric motors and gearboxes can be simultaneously subject to a random excitation due to any possible rough surfaces on the road as well as sinusoidal excitation due to its own rotation. Hence, this combined or mixed excitation, also known as swept sine-on-random (SSOR), has posed a challenge to the fatigue simulation community when using a frequency-domain method is desired.. The very challenge is due to the fact that when either random excitation or swept sine excitation individually acts on a system, the fundamental upon which the frequency-domain methods are based is totally different as the former is non-deterministic approach and the latter is deterministic approach. So far, very limited research work has been found in this area of fatigue when subjected to a combined excitation. Basically, the approach is based on the assumption that both excitation spectrum when combined by means of Fatigue Damage Spectrum (FDS) can be replaced by a single equivalent excitation such that it would lead to an equivalent damage to a set of single degree-of-freedom oscillators. In this paper, the authors will compare the numeric work based on those approaches with baseline results with high fidelity, for two specially designed components. While one design behaves like a narrowband vibrator, the other acts more like a wideband vibrator under typical random excitation. It will be very interesting to explore how the bandwidth of the dynamic response influences the accuracy of a solution.