Test methodExtended capabilities of an inverted torsion pendulum

The complex elastic modulus, or its real component and the internal friction, are the mechanical variables that characterize the dynamic behaviour of linear viscoelastic materials. Free or forced oscillations produced by inverted torsion pendulums or rheometers are usually used to measure these variables. Linearity is checked sometimes by recording the stress vs. strain curve, otherwise by measuring whether the internal friction or any component of the dynamic modulus is independent of the strain amplitude.This paper demonstrates that the linear dynamic response of viscoelastic materials cannot be assured just by looking at the stress-strain amplitude detected by forced oscillation torsion devices. Furthermore, it is shown that internal friction measured at the resonance frequency of the experimental device provides an extremely accurate tool to determine the presence of nonlinear mechanical effects. This issue is shown for polycarbonate samples submitted to forced oscillations in a torsion pendulum working at resonance.