13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Fig. 2. General aspect of the three kinds of MVs studied in this research: (a) a healthy MV, (b) a moderately calcified MV, (c) a strongly calcified MV 2. Experimental techniques The MCT were excised from the MVs preserving part of the leaflets and the papillary muscle in order to facilitate the adhesion of the specimen to the set up designed to perform the tests. To avoid the slipping of the tissue from the clamps, and adequate set up was designed and fabricated. In all cases, calibrated photographs of the specimens were taken to determine the average cross-sectional area (A0) and the initial gauge length (L0) of the specimen [13]. The tensile tests were performed under displacement control conditions, applying a constant rate of 1 mm/min following the procedure described by Ritchie [24, 7]. The stress – strain curve of each test was obtained and the relevant mechanical parameters of the material were determined: the Young’s modulus (E), the secant modulus (Es), the proportional limit stress (σ P), the ultimate strength (σ R), the strain at fracture (ε R) and the density of energy (energy per unit volume of the specimen) stored up to maximum load (Ea). To detect the calcium deposits in the MCT, some specimens were observed by means of SEM with a JEOL JSM 5800 scanning electron microscope. AAS technique was used to determine the concentration of calcium in some selected samples [9]. 3. Experimental results 3.1. Tensile tests The force vs. elongation curve represented in Fig. 3 (a) corresponds to a tensile test performed on one of the functional MCT analysed. The figure includes a text box with the geometric dimensions of the chord as well as some relevant points (A, B, C and D, represented as solid points). The stress vs. strain curve (engineering variables) is represented in Fig. 3 (b), where the most representative mechanical parameters are included (E, Es, σ P, σ R, ε R and Ea). This example allows the general features of the mechanical response of the MCT to be appreciated: Fig. 3. Typical curve of mechanical behaviour of one of the MCT, showing the relevant features: (a) force vs. elongation; (b) stress vs. strain (engineering variables)
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