13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Fig. 9. Graphs representing the influence of the calcium content on the mechanical parameters analysed for normal and moderately calcified MCT: (a) Young´s modulus; (b) secant modulus, (c) proportional limit stress; (d) tensile strength; (e) strain at fracture; (f) density of energy up to maximum load. 5. Conclusions In general, although large scatter was systematically obtained, a great difference can be observed between functional and moderately calcified MCT when the mechanical parameters related with the stiffness (Young’s modulus, secant modulus) or the resistance (proportional limit stress, ultimate strength and density of energy stored up to maximum load) are considered. In contrast, no significant differences can be seen between normal and moderately calcified MCT when comparing the strain at fracture. The amount of calcium present in normal, moderately and strongly calcified chords was measured through AAS; noticeable differences in calcium content were detected between them, although large scatter was present in the results. This analysis allowed the correlation between the measured mechanical properties and the level of calcification of normal and moderately calcified chords to be established. Finally, the relevant features associated with the fracture process and the calcium deposits in the MCT were examined with the SEM. In this sense, the calcium deposits (presumably in the form of hydroxyapatite) present in the moderately and strongly calcified chords were observed; in this latter case, some deposits of deeply rooted substances were appreciated. Moreover, several previously tested MCT together with some chords subjected to interrupted tensile tests were opened and examined in the SEM. As a result, some explanatory mechanisms were proposed in order to justify the typical aspects observed in the stress vs. strain curves.
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