Cyclic tests of viscoelastic highly elastic materials according to the ‘Nested cycles’ program

Abstract

Experimental studies of the viscoelastic properties of elastomeric nanocomposites based on styrene-butadiene rubber and binary filler (carbon black and detonation nanodiamonds) were carried out. In order to most effectively identify the features of the viscoelastic behavior of these materials (which depend not only on the deformation but also on its rate), we used a special test program with ‘nested cycles’ developed at the Institute of Continuous Media Mechanics of the Ural Branch of the Russian Academy of Sciences. During testing, the sample is subjected to uniaxial cyclic loading with the amplitude and rate of deformation decreasing at each step. At the beginning and end of the cycle (i.e., at minimum and maximum cyclic deformations), the grips of the tensile testing machine are stopped for a fixed period of time to measure relaxation dependences. Such an approach allows obtaining the maximum amount of information about both viscous and elastic properties of the tested material in one experiment. The mass content of nanodiamond particles in all samples was the same (7 parts by mass), the proportion of carbon black varied (23, 33 and 43 parts by mass). The equilibrium stress-strain curves (which are very important for theorists for constructing the governing rheological equations) were calculated based on experimental relaxation dependences (stress versus time). An analysis of stress-strain loading curves made it possible to obtain concentration dependences of mechanical characteristics such as maximum and minimum cyclic stresses, initial, minimum and maximum moduli for the first loading curve (elastic properties), hysteresis of cyclic losses (viscous properties at different loading rates). The Mullins softening effect (a drop in elastomer stiffness after the first loading cycle) was also studied.