Enamel is the hardest tissue in the human body covering the crowns of teeth. Whereas the underlying dental material dentin is very well characterized in terms of mechanical and fracture properties, available data for enamel are quite limited and are apart from the most recent investigation mainly based on indentation studies. Within the current study, stable crack-growth experiments in bovine enamel have been performed, to measure fracture resistance curves for enamel. Single edge notched bending specimens (SENB) prepared out of bovine incisors were tested in 3-point bending and subsequently analysed using optical and environmental scanning electron microscopy. Cracks propagated primarily within the protein-rich rod sheaths and crack propagation occurred under an inclined angle to initial notch direction not only due to enamel rod and hydroxyapatite crystallite orientation but potentially also due to protein shearing. Determined mode I fracture resistance curves ranged from 0.8-1.5 MPa*m(1/2) at the beginning of crack propagation up to 4.4 MPa*m(1/2) at 500 microm crack extension; corresponding mode II values ranged from 0.3 to 1.5 MPa*m(1/2).

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By measuring the curvature of thin disks of vitreous silica that have been penetrated by water from one side only, we determined the volume expansion of the silica and the effect of this volume expansion on its strength. We found that the waterstrengthening process depended on crack-size, temperature, and the amount of swelling of the silica. We also evaluated the diffusivity of water in vitreous silica, using the swelling stresses as the diffusion metric. Diffusivity values, so obtained, are close to the accepted values for the diffusion of water in vitreous silica, as is the activation energy for the diffusion process. Our data suggest that swelling and the consequent bending of the disks is caused by silanol group formation in the silica structure; molecular water plays little role in the swelling process.Si-O-Si þH 2 O $ 2ð SiOHÞ:(1)At high temperatures, > 500°C, the relative amounts of H 2 O and SiOH are given by the equilibrium constant K w0 :where [H 2 O] is the concentration of water and [SiOH] is the concentration of the silanol groups in the glass, in units of moles per volume of glass (M/m 3 ). Equation 2 is based on the assumption that the silanol groups are unassociated, which is to say the groups are far enough apart to act as independent molecular entities. At lower temperatures, T < 250°C, 16 the silanol groups are usually associated so that the two silanol groups are sufficiently close together that they can behave as a single chemical entity. Equation 2 then takes the following form:as shown by Oehler and Tomozawa 16 in a study of the diffusion of water into silica glass. These authors measured the J. Mauro-contributing editor Manuscript No. 35176.

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