13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- mechanical grinder (Labopol-5, Struers, Copenhagen, Denmark) with polishing cloths with alumina suspension slurry of 3 µm and OP-A (Struers, Copenhagen, Denmark). Afterwards, the specimens were kept fully hydrated in artificial saliva at room temperature before the indentation tests. Hardness and elastic modulus of bovine enamel surfaces were studied by nanoindentation (Nanoindenter XP-MTS System Corporation) using a Berkovich indenter with a tip radius of 100 nm. Before each batch tests, the Berkovich diamond indenter was calibrated on a standard fused silica specimen. A Continuous Stiffness Measurement module (CSM) was chosen. This method consists of applying multiple unloading cycles of very small displacement during the loading process. The Oliver-Pharr methodology [10] was applied on each of these partial unloading cycles, providing values of elastic modulus, E, and hardness, H, as a continuous function of load or penetration depth. A maximum penetration depth of 300 nm was fixed for all indentation tests. Three rows of 20 indentations were done on each sample. Each indentation was separated 100 µm to each other. During the loading branch, continuous loading-unloading cycles with amplitude of 2 nm and a frequency of 45 Hz were superimposed. Additionally, 5 indentations were done on each sample using the CSM module up to a maximum penetration depth of 2000 nm. This penetration depth was sufficient to generate a characteristic pattern of cracks, which extended from the corners of the Berkovich imprint and propagated along the enamel microstructure. After the indentation tests, the residual imprints were observed by Scanning Electron Microscopy (SEM) under low vacuum condition and taking care that the samples remained inside the microscope the shortest time in order to prevent an excessive dehydration and consequently, the cracks propagation due to residual stresses. 3. Results Figure. 3 shows two SEM images of typical residual imprints from depth sensing indentations at different maximum penetration depths and their corresponding contact stiffness versus contact depth curves: 300 nm (Fig. 3a) and 2000 nm (Fig. 3b). Indentations at maximum penetration depth of 300 nm were characterized by the absence of cracks in the edges of the residual imprint (Fig. 3a). Additionally, the contact stiffness, S, showed a linear trend with the square root of the contact area, A, between the indenter and the material surface (Fig. 3a), as predict the theoretical relation between them [10]. By contrast, the indentations obtained for a maximum penetration depth of 2000 nm were characterized by the presence of radial cracks from the edges of the residual imprints (Fig. 3b). All residual imprints showed a similar superficial cracks pattern, consequently, the semi-empirical equations described in the introduction section could be applied. In addition, in these indentation tests, the cracks deflected during their propagation through the enamel microstructure. It was also noted that the contact stiffness lost its linear dependency with the square root of contact area (Fig. 3b), coinciding with the formation and propagation of cracks during the indentation process.
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