Evaluation of the effects of cross-linking and swelling on the mechanical behaviors of hydrogels using the digital image correlation method.

Experimental evaluation and modeling are important steps in the investigation of the mechanical behaviors of hydrogels in the small- to large-strain range. In this study, the effects of cross-linking and swelling on the true stress-strain response of a specific type of hydrogel (polyacrylamide) were evaluated using a uniaxial tensile test. The development of true strain on the surface of the hydrogel was measured using the digital image correlation method. The specimens with higher cross-link density exhibited a higher initial elastic modulus and earlier orientation hardening. The initial elastic modulus was reduced by the swelling, whereas the orientation hardening occurred in an earlier strain range in the swollen hydrogel. The mechanical responses of the as-prepared and swollen hydrogels with different cross-linker contents were fitted using a non-Gaussian statistical model. The conventional model underestimated the decrease in the elasticity owing to the swelling effect and overestimated the increase in the stress in the large-strain range. The mechanical model was suitably modified to yield an accurate reproduction of the mechanical responses. The proposed model, which was characterized by five material parameters, was found to reproduce the characteristics of the mechanical responses of the as-prepared and swollen hydrogels with different cross-linker contents.

Experimental modeling of nonuniform deformation in finite volume evaluation region of heterogeneous material.

The objective of the present study is to establish the experimental modeling process of the nonuniform deformation behavior of heterogeneous materials. For this purpose, the constant stress moment, which is the work conjugate quantity of the constant strain gradient for the finite volume evaluation region, is introduced. The proposed stress moment can be evaluated from the stress field. The extended constitutive equation that relates the strain, stress, strain gradient, and stress moment is then formulated to predict the nonuniform deformation behavior of heterogeneous materials. In order to confirm that the proposed method is appropriate to represent the nonuniform deformation, finite element method (FEM) simulations of bending of macroscopically and microscopically heterogeneous materials were performed. The proposed method could predict the bending deformation of macroscopically heterogeneous material as precisely as the homogeneous case because the distribution of the heterogeneity is introduced in the extended constitutive equation. A bending simulation of a laminated cantilever was then performed using the extended constitutive equation for the microscopically heterogeneous material. The proposed method was capable of representing the analytically verified size-dependent bending deformation of the laminated cantilever.