Enamel and dentin compose the crowns of human teeth. They are joined at the dentinoenamel junction (DEJ) which is a very strong and well-bonded interface unlikely to fail within healthy teeth despite the formation of multiple cracks within enamel during a lifetime of exposure to masticatory forces. These cracks commonly are arrested when reaching the DEJ. The phenomenon of crack arrest at the DEJ is described in many publications but there is little consensus on the underlying cause and mechanism. Explanations range from the DEJ having a larger toughness than both enamel and dentin up to the assumption that not the DEJ itself causes crack arrest but the so-called mantle dentin, a thin material layer close to the DEJ that is somewhat softer than the bulk dentin. In this study we conducted 3-point bending experiments with bending bars consisting of the DEJ and surrounding enamel and dentin to investigate crack propagation and arrest within the DEJ region. Calculated stress intensities around crack tips were found to be highly influenced by the elastic modulus mismatch between enamel and dentin and hence, the phenomenon of crack arrest at the DEJ could be explained accordingly via this elastic modulus mismatch.

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International audienceIn this paper, we discuss the effect of water on the strength and static fatigue of silica glass. When a crack is formed in silica glass, the surrounding environment rushes into the crack; water then diffuses from the environment into the newly formed fracture surfaces to generate a zone of swelling around the crack tip. Because the swollen material is constrained from expanding by the surrounding glass, a zone of compressive stress is generated at the fracture surface around the crack tip. The results are similar to those found for transformation toughened zirconium oxide, with the exception that the transformation zone in silica glass grows with time, so that the effect gets progressively stronger. Using diffusion data from the literature, we show that the diffusion of water into silica glass can explain several significant experimental observations: the reported strengthening of silica glass by soaking in water at 88 degrees C; an increase in the slope of dynamic fatigue curve by prior exposure to water at 88 degrees C; the observation of a static fatigue limit in silica glass at very low values of the applied stress-intensity factor; and the observation of crack face displacements caused by water penetration into the glass at the crack tip

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Thin films of bismuth vanadates are deposited by chemical vapor deposition (CVD) on α-Al2O3 substrates using an O2 atmosphere and vanadyl(IV) acetylacetonate and triphenylbismuth as precursors. The microstructure of the samples is studied by XRD and Raman spectroscopy and their chemical composition is investigated by XPS and SIMS. AFM is used to analyze the surface morphology of the samples. All the samples show a nonohmic behavior beyond a threshold voltage, V th, which is linearly dependent on the V4+/V5+ ratio. Impedance spectroscopy measurements indicate that the obtained samples are oxide ion conductors at room temperature and that the mechanism of ion conduction occurs by means of hopping between vacancies. Furthermore, ferroelectric−paraelectric transitions take place in the materials at low temperatures.

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