Stress pulses in curved mechanical waveguides

Abstract

A wide-band, short duration, pulse technique has been used to investigate the nature of stress wave propagation in straight rods and helical springs. The stress pulses were initiated by means of electromechanical transducers and the distortion of the pulses, due to dispersion, was investigated by recording the surface strain at various points along the waveguides using small ceramic strain gauges. Two types of generating transducer were used, one employing the electrostrietive properties of PT-5 ceramic, supplied by Brush Clevite Company, and the other employing the magnetostrictive properties of nickel wire.The latter was used for all the quantitative experimental results obtained. The surface strain gauges were made from the P3T-5 ceramic. The signal from the strain gauge was amplified and displayed on an oscilloscope screen from which direct measurements were taken. The oscilloscope traces were also photographed. Some of the mathematical theories used to describe the propagation of flexural waves along a straight rod have been outlined; these include the exact and three approximate theories, namely an elementary theory, an improved theory due to Rayleigh and the more accurate Timoshenko theory. Approximate theories for helical springs and plane rings, which are comparable with the Timoshenko theory for a straight rod, are now available and these theories were investigated in this work. The accuracy of the approximate theories has been assessed by obtaining the corresponding dispersion curves and then comparing the theoretical predictions, according to Kelvin's method of stationary phase, with the experimental results. The comparison indicates that the approximate theories investigated give a good indication of the dispersion characteristics for the lowest longitudinal, torsional and flexural modes in a helical spring over the frequency range considered.