To do a prestressed analysis, Include geometric nonlinearity must be selected in the
Frequency Domain study step. This is automatic when you add the
Frequency Domain, Prestressed study type.
Sometimes you want to study the time history over a period for the results of a frequency domain analysis. You can do that by adding a Frequency to Time FFT study step. The frequency response results are then viewed as terms in a Fourier series, which can be transformed to time domain. It is possible combine the results for several frequencies into a single time history, under the assumption that they are all multiples of the same fundamental frequency.