Precise determination of the Poisson ratio in soft materials with 2D digital image correlation

Abstract

In this article, we demonstrate the application of digital image correlation (DIC) in evaluating the strains and Poisson ratio of a range of soft materials in terms of their spatial and temporal resolutions. Four samples of Polydimethylsiloxane (PDMS) were used as control substances and were measured to have Poisson ratios of 0.498, 0.503, 0.500, and 0.499, in agreement with the reported incompressible value of 0.50 [1]. Two carbon nanotube (CNT) elastomers of identical composition, but one of a homogeneous and the other of an inhomogeneous CNT distribution, were used to determine the spatial resolution with good results (Fig. 2 and Fig. 4). The relaxation of a 3D polydomain liquid crystal elastomer (3D-LCE), a cholesteric liquid crystal elastomer (CLCE), and a polyacrylamide gel (PAAm) in water, were used to determine the temporal resolution. A 10min video short at 25fps was used to evaluate the time dependence of the 3D-LCE over which time an increase in the Poisson ratio was observed. The 3D-LCE relaxes from its initial state at 0.42 to 0.50, converging towards incompressibility. The CLCE was found to have a similar initial value of 0.44 but converged to ∼ 0.60, a consequence of its anisotropic nature. PAAm gel relaxation in water was studied over a time period of 7 hours with digital images taken periodically every minute. Its Poisson ratio was found to decrease smoothly from 0.50 to 0.26, with an accompanying reduction in force. The equilibrium result compares well to the 0.25 value predicted by theories of the strain induced swelling of dilute gels. In summary, we find DIC to be a powerful and easy to implement method of accurately measuring local strains in a range of soft materials.

Effect of an osmotic stress on multicellular aggregates

“…A good agreement between the nominal and the measured shear moduli of polyacrylamide films is obtained for a Poisson coefficient of 0.43 ± 0.01. This value is consistent with the experimental measurements on polyacrylamide gels, whose Poisson coefficient ranges from 0.25 (theoretical value [25]) to 0.5 according to the method and the timescale of the measurements [26,27]. In conclusion, we observed that PA gels respond to the dextran pressure as they would do in case for a pure mechanical pressure.…”

Three-dimensional elastic constitutive relations of aligned carbon nanotube architectures

“…To map the evolution of strain in the tensile tests with a high degree of accuracy, DIC was performed using the Vic 2D software from Correlated Solutions, Inc. This method is well known for its high resolution, 37 and was recently used to determine the moduli (elastic and shear), 38 and Poisson ratios, 38,39 of low V f ( 1 vol. % CNTs) polymer composites.…”

Transversely isotropic viscoelastic materials: Contact mechanics and friction

“…The viscoelastic anisotropic rubber studied experimentally has a low anisotropy, therefore, in the numerical calculations, the Poisson's ratio was assumed to be the same in all directions and similar to isotropic rubbers (ν ¼ 0:45). However, the exact Poisson's ratio might be different at different directions (as mentioned in Section 2.1) which can be accurately measured by novel methods [41]. Not precise determination of the Poisson's ratio might result in a disagreement between the model and the experiments for a material with high anisotropy.…”