Experimental Study of Dilatational vs Distortional Straining Action in Material-Damping Production

Abstract

Experimental test setups for measuring material damping in structural metals have invariably been limited to producing states of strain wherein the distortional component is dominant. In the case of flexural vibrations of plates and shells, however, this condition is often reversed, sometimes with the elastic energy associated with dilatational straining action exceeding the corresponding distortional component by an order of magnitude. It has, therefore, been conjectured that effects due to dilatational straining, small enough to be masked by experimental scatter in the aforementioned tests, might become significant under conditions of pronounced dilatational straining, viz., plate vibrations. For this purpose, a test machine was designed to measure material damping in thin-walled, cylindrical specimens subject to combined internal pressure and axial cyclic loading. Tests were carried out on a number of manganese-copper alloy specimens for which prior studies, already published, had established damping properties under uniaxial cyclic stress. The new tests extended the ratio of dilatational to distortional strain energies from a fraction up to a factor of 2, the latter limit being determined by possible onset of stress-history effects, which were avoided. The results display a small but definite influence of dilatational straining action in producing additional material damping to that arising from distortional straining action. As a consequence, a modified formula is suggested for the treatment of material damping in plate vibrations.