Large deformation and fracture behaviour of gels

When gels are used in practice, their large-deformation and fracture characteristics are mostly far more relevant than small-deformation characteristics. In this paper fracture behaviour is discussed of various types of gels, viz. polymer and particle gels, the latter with fairly low and very high volume fraction of particles. First, a general introduction is given on theoretical aspects of fracture mechanics of gels, which involves an essential extension of classical fracture theories. The relationship between large-deformation and fracture behaviour of a gel and its structure proves to be far more complicated than for small-deformation properties. The main reasons for this difference are: (i) the much more important effect of relatively large inhomogeneities on fracture properties and (ii) some very different causes for the strain rate dependence. Not only are the average distance between cross-links and the average stiffness of the strands connecting them of importance, but also the distribution of these parameters. Moreover, inhomogeneities, be it defects (of mu m to mm scale) or weak regions (e.g. in composite gels) may have an overriding effect on the fracture properties. To understand the strain rate dependence, one should consider the energies involved as a function of the deformation rate and distinguish between the amount elastically stored during deformation, the amount dissipated due to viscous flow or due to friction processes and the net fracture energy. Moreover crack initiation and fast 'spontaneous' crack growth (crack propagation) have to be distinguished. The factors mentioned cause large deformation and fracture properties to be much more strongly dependent on the physical structure of a gel than are the small deformation properties.