13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 3 2 2 3 1 2 V C c P H E l a x K 1.073 ⋅ = (4) Other alternative method to determine the fracture toughness consists on calculating the energy released during cracking, as a measure of the fracture toughness (Li et al. 1997) [9]. This method is based on the formation of a constant force step during the application of indentation load. This force step is associated with the sudden formation and propagation of cracks. The step length depends on how much the indenter and cracks suddenly advance into the material. This energetic method calculates the fracture toughness comparing the area limiting by the load-displacement curve with crack generation and the hypothetical one if cracking does not occur (Fig. 2). If the Mode I fracture is assumed, the threshold stress intensity required for the fracture can be written as: ( ) 2 1 2 IC A ∆U ν 1 E K ⋅ − = (5) where KIC is the indentation fracture toughness of the material, ΔU is the strain energy obtained from the area between extrapolated load-displacement curve and the step of experimental one (Fig. 2), E is the elastic modulus, ν is the Poisson´s ratio and A represents the cracked area. Figure 2. Strain energy obtained from the area between extrapolated load-displacement curve and the step of experimental one. This method can be used even when a particular cracks pattern is not obtained. However, a constant load step during the loading branch is necessary in order to apply the Eq. (5). Additionally, a good estimation of cracked area is essential to obtain reliable results. Therefore, the aim of this study was to evaluate the reliability of the energetic methodology using depth sensing indentation technique to estimate the indentation fracture toughness of bovine dental enamel. These values were compared with those traditionally obtained by applying the semi-empirical equations based in a specific pattern of cracks, Eqs. (1)-(4). 2. Experimental procedure Eight incisors were extracted from bovines of two years old. The teeth were cleaned and stored in artificial saliva prior to the tests. The labial surfaces of the specimens were polished using a
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