Crack initiation and propagation in composite microstructure of dentin.

Dentin possesses unique hierarchical structure through long-term natural selection, which imparts exceptional mechanical properties to this hard tissue. In this study, the combined effects of ductile fracture of intertubular dentin (ITD) and brittle cracking of peritubular dentin (PTD) on the crack initiation and propagation in the composite microstructure of dentin are explored. A micromechanical model accounting for the unique two-layer microstructure of dentin is developed and numerical simulations of fracture of microstructured dentin are performed. It is found that microcracking of PTD prior to the penetration of main crack into PTD and crack deflection along the interface between PTD and ITD are two major mechanisms involved in the fracture of microstructured dentin, which is in a good agreement with experimental observations. The competition between the two mechanisms is governed by PTD strength. Furthermore, we reveal that the elastic and fracture properties of PTD and strain hardening of ITD greatly affect the fracture behavior of dentin. The decrease in elastic modulus and increase in tensile strength of PTD elevate the notch tip plasticity, increasing the propensity of crack initiation at notch tip. Whereas such variations in elastic modulus and tensile strength of PTD can lead to the transition of fracture mechanism from multiple void interactions to crack-void interaction, thereby enhancing toughness of dentin. Increasing the strain hardening exponent of ITD can also give rise to the amplified toughness. [ABSTRACT FROM AUTHOR]