Fracture and fatigue behaviour of hydrogels for cartilage tissue replacement

In newly developed biomedical applications, hydrogel composites are widely used as load-bearing components under complex loading conditions, such as high stretches and cyclic deformations. These loading conditions can cause fracture and fatigue of hydrogel composites. The mechanical behaviours of many hydrogel composites show significant time dependence. The underlying mechanisms for the time-dependent deformation, fracture and fatigue crack growth behaviour remain elusive and inconclusive. Deeply understanding these mechanisms will greatly improve the anti-fracture and anti-fatigue capability of hydrogel composites and thereby lengthen the service life. The objective of this dissertation is to develop a theoretical and computational framework to describe the complex constitutive behaviour, analyse the time-dependent fracture mechanisms, and predict the fatigue crack growth behaviour of hydrogel composites. For these purposes, an extended poro-visco-hyperelastic-damage model is developed within the framework of the Theory of Porous Media (TPM) at finite strains. The non-equilibrium mechanical response is described by introducing internal variables based on the multiplicative decomposition of the deformation gradient tensor into elastic and inelastic parts. A continuum damage model is utilised to describe the mechanical degradation of polymer networks and fibres. The time-dependent breaking/reforming kinetics of physical bonds is described by a Bell model-based chain evolution law. Moreover, an energy-based fatigue crack growth model is proposed to predict the fatigue crack growth behaviour of hydrogels. The averaged elastic energy density surrounding the crack tip is evaluated as the driving force via a volume averaging method. The evolution of the energy density with cycles is considered. The developed damage model and the fatigue crack growth model are first validated with experimental data. The simulations for the delayed fracture and fatigue crack propagation of hydrogel specimens are accomplished under ...