Techniques are described for the fatigue testing of components, such as turbine buckets, used in large steam turbines and generators. Fatigue of these large structures, which have relatively high unit damping, is obtained through the use of resonant-vibration systems. An electromagnetic drive serves as the source of excitation. The drive system is designed to produce the maximum force from a given-size magnet. Control of the amplitudes of vibration, for very long periods, is obtained through the use of an automatic amplitude control, the stability of which maintains the amplitude with a variation of less than 2 percent.