13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 3. Typical pattern of cracks obtained from nanoindentations carried out at different maximum penetration depths: a) 300 nm; (b) 2000 nm. Their corresponding indentation contact stiffness versus squared root of the contact area plots were put over each image. 3.1. Mechanical properties: elastic modulus, hardness and fracture toughness. Table 1 summarizes the hardness and elastic modulus of bovine enamel obtained from indentation tests at maximum penetration depth of 300 nm, where the loading did not cause the initiation of cracks. The values obtained in this study were very similar to those reported in preview works [11-13]. Data in Table 1 also include the fracture toughness of enamel according to the equations previously described (see Eqs. (1-5)). It is noteworthy that, in all cases, there was not any constant load step. Therefore, it was not possible to apply the energetic methodology as described in the introduction section. Elastic modulus , E (GPa) 92 ±6 Oliver-Pharr [10] Hardness, H (GPa) 4.7 ±0.3 Oliver-Pharr [10] Indentation fracture toughness, KC (MPa m) 0.39 ±0.14 Eq (1) [3] 0.5 ±0.2 Eq (2) [5] 0.6 ±0.2 Eq (3) [6] 0.7 ±0.2 Eq (4) [8] 2.5 ±0.7 Eq (9) Table 1. Experimental results of elastic modulus, E, hardness, H and indentation fracture toughness, KC. a) b) 0 1 2 3 4 5 0 2 4 6 8 10 S (105) (N/m) A1/2 (10-6) (m) No cracking zone Cracking zone 0 0.2 0.4 0.6 0.8 1 1.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 S (105) (N/m) A1/2 (10-6) (m) No cracking zone No cracking zone
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