Effect of Suspension Position on Apparent Values of Internal Friction Determined by Forster's Method

Abstract

Values of internal friction determined by Forster's method may be in error by an order of magnitude if the effect of suspension damping is not eliminated. This damping may be reduced by placing the suspension near the nodes of the vibrating specimen, but the suspension must be sufficiently far from the nodes to provide adequate drive and pickup. The effect of suspension damping on measured values of internal friction as a function of suspension position was calculated. The calculated results were compared with experimental results and found to describe the variation correctly. The equation expressing this variation can be used to correct for suspension damping and to obtain the value of the internal friction of the specimen itself. Values as low as 5×10⁻⁶ have been determined by this method.