(33b) A Combined Torsional-Axial DMA Platform for Determination of Viscoelastic Poisson’s Ratio

The lateral contraction of a material when stressing the material in axial direction is described by the Poisson’s ratio [1]. In case of viscoelastic materials, like polymers, this parameter is a function of time (frequency) and temperature and important for e.g. structural mechanics simulations [1,2]. Several methods are described to determine the viscoelastic Poisson’s ratio. The methods can be classified in direct methods which directly measure the change of the dimensions of the specimen and indirect methods from which the measurement of two moduli like shear modulus and Young’s modulus seems to be the most effective [3].

A new measuring device concept is introduced which combines an electronically commutated (EC) motor as rotational top drive and a moving magnet linear drive, as bottom drive to enable rheological measurements and dynamic mechanical analysis (DMA) with one single device. Such a device concept enables one to do combined torsional-axial measurements on one device. In this contribution work with both cylindrical and rectangular specimens is demonstrated in order to determine the viscoelastic Poisson’s ratio of different solid polymers. Using a linear and a rotational measuring drive in one instrument enables the determination of complex Young’s modulus |E*| as well as the complex shear modulus |G*| with a single sample in a continuous measurement run. Consecutive frequency sweeps at room temperature in both, torsion and tension deformation modes were performed to obtain the viscoelastic Poisson’s ratio, following the protocol proposed by Tschoegl et al. [2]. The suitability of the method and usage of cylindrical or rectangular specimens for Poisson ratio determination is further examined and discussed by comparing the experimental results on polymers with literature data.

[1] Pandini, S., Pegoretti, A.: Time and temperature effects on Poisson’s ratio of poly(butylene terephthalate). Polymer Letters 5 (2011) 685-697.

[2] Tschoegl, N.W. et al.: Poisson’s Ratio in linear viscoelasticity – a critical review. Mechanics of Time-Dependent Materials 6 (2002) 3-51.

[3] Pritz, T.: Measurement methods of complex Poisson’s ratio of viscoelastic materials. Applied Acoustics 60 (2000) 279-292.