Measurement of vibrational energy and point mobility of a beam subjected to moment excitation using a finite difference approximation

Moment excitation is often neglected in structural vibration analysis due to difficulties in measuring the applied moment and the resulting wave motion in the structure. Further, it is often assumed that moment induced vibrational energy is only significant in the high frequency region. However, recent studies have shown that moment excitation should be included in vibrational analysis at all frequencies when the source location is in close proximity to a structural discontinuity. In this paper a novel method is presented to measure the point mobility and resulting vibrational energy of a beam subjected to moment excitation. The proposed method utilises a finite difference approximation to calculate the rotational motion of the beam at the point of excitation. Moment excitation is induced by a specially designed impact rig which applies two equal and opposite forces on two moment arms that are perpendicularly attached to the beam. It is shown that, using the newly developed technique, the measured point mobility follows the trend of the equivalent theoretical structure. The technique also showed good agreement over a wide frequency range between the measured input energy and measured transmitted flexural wave energy along the beam.