Fracture characterization of hyperelastic polyacrylamide hydrogels

Hydrogels are three dimensional macromolecular networks, characterized by soft-wet structure. The increased demand of tough polymer materials for engineering or biomedical applications has led to develop robust hydrogels. Consequently, an increased interest in the understanding of their mechanical properties was recorded. This work aims at investigating the fracture behaviour of polyacrylamide hydrogels with various crosslink densities by several experimental methodologies. Polyacrylamide hydrogels were chosen for being highly stretchable, very soft, and non dissipative, i.e. they behave as ideal hyperelastic materials. Several experimental methodologies based on the J integral parameter were applied to evaluate their fracture resistance. Established as well as innovative methods were applied in quasi-static conditions: • a single specimen method based on the evaluation of the strain energy density introduced by Rivlin and Thomas [1]; • a multi-specimen methodology, based on the interruption of fracture tests at different levels of crack advancement; • an innovative approach based on the energy separation principle [2], which in principle allows to identify the point of fracture initiation without any visual observation of the test. Highly stretchable hydrogels deform with a severe blunting of the crack tip, and the processes occurring in the fracture process zone are poorly understood. Therefore, large strains finite element analyses were carried out to simulate the local strain distribution in the region ahead of the notches. Critical local strains obtained by numerical analyses have been compared to resistance parameters obtained by global approaches for hydrogels with different structural characteristics. Outcomes allowed to achieve a better understanding of the fracture behaviour of such nonlinear elastic materials.