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R-Curve Behavior of Dental Ceramic Materials

Department of Dental Prosthetics, Section of Dental Materials, University of Technology Aachen, Pauwelsstrasse 30, D-52074 Aachen, Germany;

View larger version (17K): [in this window] [in a new window]   Figure 1. Observed bending strength (MPa) as a function of indentation load P (N). The solid lines are least-squares fits of the logarithm of Eq. 6. The dashed line has a slope of -1/3 (β = 1/3). This slope represents a material without R-curve behavior.

View larger version (20K): [in this window] [in a new window]   Figure 2. Fracture resistance KR (MPam0.5) as a function of crack extension a (mm). The R-curves of the observed ceramic materials appear as straight lines in the double-logarithmic presentation. The R-curve steepness is displayed as the straight-line slope, and the R-curve level can be judged by its value of fracture resistance KR at a specific value of crack extension a, respectively. The dashed segments of the R-curves denote extrapolations beyond the upper limit of Eq. 2.

View larger version (18K): [in this window] [in a new window]   Figure 3. Stress intensity factor K (MPam0.5) as a function of crack extension a (mm) for Vita Omega Opaker. The R-curve (bold line, KR) is calculated according to Eq. 2. The stress intensity curve due to the bending stress Kb, the stress intensity curve due to the indentation stress Kr, and the superposed [Kb+Kr] curve in the left diagram (A) are plotted for an indentation load of P = 70.6 N. The point of tangency (Kc) denotes the onset of crack-extension instability, which defines the fracture toughness value. The 6 [Kb+Kr] curves in the right diagram (B) result from the observed data at different indentation loads (6.9-70.6 N). The dashed segments of the curves denote extrapolations beyond the upper limit of Eq. 2.

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