13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Fig. 7. Values of the calcium content measured through AAS in functional, moderately calcified and strongly calcified MCT 4. Discussion 4.1. Tensile tests After carefully analysing the tensile curves, the following systematic characteristics were identified: i) The curves show a pronounced nonlinear behaviour in their initial region. For reduced loads, the collagen fibres are in relaxed conditions and appear wavy and crimped (Section 4.2 includes some additional microscopic information) [5, 22, 24]. ii) Next, a region of linear behaviour develops from point A to point B. The points included in this interval were used to obtain E, see Fig. 3 (b). From a microstructural point of view, the crimp patterns disappear and the collagen fibres are progressively straightened and aligned by the applied force. iii) From point B, the curve starts to bend until point C, where a sudden fracture takes place. The coordinates of point C correspond to ε R and σ R. iv) In general, irregularities consisting of partial unloads were observed in the experimental curves (see the arrows in Fig. 3 (a)). Taking into consideration the information gathered in Fig. 5, the following aspects can be drawn: i) A great difference between functional and moderately calcified MCT is appreciated when the mechanical parameters related with the stiffness (E, Es) or the resistance (σ P, σ R or Ea) are considered. Indeed, observing the mean values of these parameters reveals that, on average, the normal MCT are between three and seven times stiffer or more resistant than the moderately calcified chords. ii) Although there is not enough information available to conduct a statistical analysis of the distribution of minimum values, the results clearly suggest that these are noticeably lower for the moderately calcified chords than for the functional ones, whenever the stiffness or resistant parameters (E, Es, σ P, σ R or Ea) are being considered. iii) The results obtained for the strain at fracture, εR show a rather different picture: in this case, see Fig. 5 (e), no significant differences can be seen between normal and moderately calcified MCT. 4.2. SEM and EDS analysis In Section 4.1, the relevant aspects of a typical stress vs. strain curve were described and explained because of the microstructural changes that take place as a MCT is stretched. In order to confirm the validity of the above mentioned explanations, several previously tested MCT were fixed in 3% glutaraldehyde, carefully opened in longitudinal direction with the help of a scalpel, and examined by SEM. In Fig. 8, a comparison between the arrangements of collagen fibres in a non-previously tested MCT (Fig. 8 (a)), and a MCT tested up to fracture (Fig. 8 (b)) is shown. In addition, several interrupted tests were conducted to examine the validity of the explanation of the sudden unloads
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