Biomaterials 24 (2003) 5209–5221 Crack blunting, crack bridging and resistance-curve fracture mechanics in dentin: effect of hydration J.J. Kruzic a , R.K. Nalla a , J.H. Kinney b , R.O. Ritchie a, * a Department of Materials Science and Engineering and Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, MC #1760, Berkeley, CA 94720, USA b Lawrence Livermore National Laboratory, Livermore, CA 94550, USA Received 18 June 2003; accepted 21 June 2003 Abstract Few studies have focused on a description of the fracture toughness properties of dentin in terms of resistance-curve (R-curve) behavior, i.e., fracture resistance increasing with crack extension, particularly in light of the relevant toughening mechanisms involved. Accordingly, in the present study, fracture mechanics based experiments were conducted on elephant dentin in order to determine such R-curves, to identify the salient toughening mechanisms and to discern how hydration may affect their potency. Crack bridging by uncracked ligaments, observed directly by microscopy and X-ray tomography, was identified as a major toughening mechanism, with further experimental evidence provided by compliance-based experiments. In addition, with hydration, dentin was observed to display significant crack blunting leading to a higher overall fracture resistance than in the dehydrated material. The results of this work are deemed to be of importance from the perspective of modeling the fracture behavior of dentin and in predicting its failure in vivo. r 2003 Elsevier Ltd. All rights reserved. Keywords: Dentin; Fracture; Hydration; Toughening; Uncracked-ligament bridging; Crack blunting 1. Introduction Dentin is a calcified tissue that makes up the bulk of the teeth; it is physically located between the exterior enamel and the interior pulp chamber. Human dentin is a hydrated composite composed of nanocrystalline carbonated apatite mineral (B45% by volume), type-I collagen fibrils (B30% by volume) and fluid (B25% by volume). The mineral is distributed in the form of 5 nm thick crystallites in a scaffold created by the collagen fibrils (50–100 nm diameter). The inorganic mineral is believed to provide the strength and the organic collagen the toughness [1]. The distinctive feature of the microstructure is the distribution of cylindrical tubules (1–2 mm diameter) that run from the dentin-enamel junction to the soft, interior pulp. These tubules are surrounded by a collar of highly mineralized peritubular dentin (B1 mm thick) and are embedded within a matrix of mineralized collagen, called intertubular dentin [2]. The mineralized collagen fibrils form a planar felt-like structure oriented perpendicular to the tubules [3]. The fluid is located mainly in these tubules (75%), with the rest being distributed within the intertubular matrix [4]. A mechanistic understanding of the mechanical properties of dentin is important from the perspective of developing a framework for failure prediction in human teeth, particularly in light of the effect of microstructural modifications from caries, sclerosis, aging and restorative processes. In this context, it has been recognized that such properties, especially resis- tance to fracture, are strongly influenced by the degree of hydration [5]. Indeed, from a clinical perspective, the higher incidence of fracture in endodontically repaired teeth (with pulp replaced) has been attributed to decreased hydration, although this is still a controversial issue [6–8]. Despite the obvious importance of the effect of hydration on fracture behavior, there are only a handful of studies to date that attempt to address this issue. Early studies [9,10] used a ‘‘work of fracture’’ (defined as the work per unit area to generate new crack ARTICLE IN PRESS *Corresponding author. Tel.: +1-541-486-5798; fax: +1-510-486- 4881. E-mail address: roritchie@lbl.gov (R.O. Ritchie). 0142-9612/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0142-9612(03)00458-7