Mechanical properties measurement of solids at infralow frequencies


Background. The most qualitative indicators of solid state mechanical properties are complex Young's or shear modulus - E*, G*, Poisson module- μ and mechanical losses tangent - tgδ. There are several different methods for the study of mechanical properties using DMA, one of which is a method of torsional vibrations. This method has been known for over 100 years, but even today attracts developers of equipment for rheological studies and to measure the complex shear modulus and internal friction.
Methods. We described the most advanced devices of this type, called as inverse torsion pendulum, in which the inertial part is suspended on thin torsion over the studied sample and balanced by counterweight. The electromagnetic system, consisting of permanent magnets and two Helmholtz coil located diametrically opposite to the axis of the pendulum are used to create a torsion moment. The deformation, that occurs in the sample, is determined by the photoelectric sensor that consists of a semiconductor laser and a differential photodiode.
Results. An automated measurement setup, based on the free or forced oscillations of torsion pendulum, allows at temperature range from 77 to 600 K, mechanical stress - from 0 to 107 Pa amplitudes of relative deformation - from 10-6 to 10-1 carry out the following types of measurements:
- in static load mode - study the strain ε (at σ = const), or mechanical stress σ (at ε = const) relaxation, getting thermomechanical curves ε(T) of solids at σ = const,
- in free damped vibrations mode - determination of temperature and frequency dependence of internal friction Q-1 = tgδ and shear modulus in the frequency range 1 - 20 Hz,
- using forced oscillation at infralow frequencies (10-4 - 10-1 Hz) - recording mechanical hysteresis loops with further definition of the quasi-static shear modulus,
- combined measurement modes - i.e. mechanical hysteresis loop recording or free damped oscillations observation at statically loaded sample.
LabVIEW graphical programming environment was chosen for the automating the measuring process of the temperature and frequency dependence of internal friction and shear modulus.
Conclusion. The developed measurement devices and automation software has been successfully applied in experiments to investigate the mechanical properties of chalcogenide glassy materials both in static and low and infralow frequencies, but can be used to study the characteristics of a wide range of materials (from polymers to high-strength ceramics ) and measuring attenuation coefficients and elastic moduli, studying structural and phase transformations, clarification the mechanisms of deformation and relaxation under external loads

Keywords: shear modulus, internal friction, reverse torsion pendulum, relative deformation