The propagation of spherically divergent stress pulses in linear viscoelastic solids

Abstract

M easurements of the change in shape of spherically divergent stress pulses in three viscoelastic olids, namely polyethylene, polymethylmethacrylate and polystyrene, have been made. The stress pulses were produced by the detonation of small explosive charges on blocks of these materials and the pulses were detected with condenser microphones. The stress amplitudes were sufficiently small for the theory of linear viscoelasticity to apply and the attenuation and dispersion of the pulses enabled values to be obtained for the real and imaginary parts of the bulk modulus. It was found that for both polyethylene and polymethylmethacrylate the ‘bulk loss’ was about one fifth of the mechanical loss in shear, while for polystyrene the bulk losses were too small for an accurate quantitative estimate to be made. It was shown, however, that for this material, also, the assumption that the bulk loss was a constant fraction of the loss in shear gave good agreement with the observed results. It is suggested that, since the bulk loss appears to follow the shear loss, the same microscopic dissipative processes may be responsible for both types of loss, since, on a molecular scale, density changes of long chain polymers must be accompanied by local shear.