13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- detected in the experimental curves. These tests consist of stretching some chords until the first unload occurs (therefore, before the final fracture); immediately after that, the test is stopped and the chord is longitudinally opened to be examined in the SEM. 4.3. Determination of the calcium content present in the chordae through atomic absorption spectroscopy In Fig. 9, the relation between the mechanical properties of normal and moderately calcified MCT as a function of calcium content is represented (notice that a logarithm scale was used in all cases). The data included in each of the graphs were fitted through a potential law function (a linear fitting on a log-log scale) to show the trends of the different families of data. The equation of the fitting and the R-squared value are shown on a chart box. This set of figures serves to confirm some of the aspects reported previously in Section 1; thus, the influence of the calcium content is clear for the mechanical parameters E, Es, σ P, σ R and Ea. In contrast, the effect of the amount of calcium on the strain at fracture, ε R, seems to be negligible. Fig. 8. SEM micrographs comparing the collagenous core of a non previously tested MCT, (a), with another chord tested up to fracture, (b). Picture (c) shows some micro-fractures detected in the collagen fibres of MCT previously subjected to interrupted tensile tests whereas in (d), the retraction of some broken fibres is appreciated (see the arrows). Micrograph (e) allows the detailed appearance of the fracture region of a MCT to be appreciated; the outer spongy layer of the chord is retracted (arrows) due to its higher elasticity.
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