Working with long duration transient events in a finite element world can be extremely computationally expensive. If those events are very long, like the wheel hub forces over the lifespan of a vehicle, then it is impossible to simulate. One technique to overcome this limitation is to use something called Random Loading, or Random Analysis.
If a time signal can be considered properly random then it can be transformed from the time to the frequency domain and is known as a Power Spectral Density plot, or PSD. A quick check of randomness is that any section of a time history transformed in this way should give the same outcome as the whole signal. These PSD’s are best thought of as a statistical representation of the amount of energy in the signal as a function of frequency.