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On the in vitro Fatigue Behavior of Human Dentin: Effect of Mean Stress

¹ Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Materials Science and Engineering, Hearst Mining Building, University of California, Berkeley, CA 94720; and
² Department of Preventive and Restorative Dental Sciences, University of California, San Francisco, CA 94143;

View larger version (25K): [in a new window]   Figure 1. Fatigue theory and experimental set-up. Schematic illustrations of (a) the definition of the various stresses associated with a fatigue cycle, (b) typical stress-amplitude (a)-life (Nf) (S/N) plots for different mean-stress (m) levels, and (c) constant life (a-m) curves. (d) Load-displacement curve obtained in bending, showing loads used to obtain y (at deviation from linearity of elastic part of the curve) and ts (at maximum load). (e) Schematic of cantilever-beam geometry used for fatigue stress-life testing. Each dentin beam, which is mechanically clamped at one end, included some root dentin and coronal dentin. Inset shows sections cut from the molar in relation to the tubule curvature. For tension-compression loading, the tension-tension loading rig was modified so that the plunger had a line contact with both the top and bottom surfaces of the beam, instead of line contact with just the top surface.

View larger version (24K): [in a new window]   Figure 2. Stress-life (S/N) data obtained for human dentin in HBSS at 37°C in the form of the stress-amplitude, a, as a function of the number of cycles to failure, Nf. Results are shown (a) for various stress-ratios at a constant frequency (10 Hz), and (b) for various frequencies at constant stress-ratio (R = 0.1). Horizontal arrows represent samples that did not fail ("run-outs").

View larger version (53K): [in a new window]   Figure 3. Scanning electron micrographs of the typical fracture surfaces for the three stress-ratios investigated. Note that there are no significant differences between the appearance of the fracture surfaces for the tensile stress-ratios, R of 0.1 and 0.5, as shown in (a) and (b), respectively. For the tension-compression stress-ratio, R of -1 in (c), damage from fracture surface contact is evident. The black arrow in each fractograph gives the direction of nominal crack growth.

View larger version (59K): [in a new window]   Figure 4. Fatigue data re-plotted as stress-amplitude vs. mean-stress, a - m combinations that result in similar fatigue lifetimes, Nf. Also shown are the (a) Gerber, (b) Goodman, and (c) Söderberg models. (d) The Goodman and Gerber models are shown encompassing the fatigue data; the safe and unsafe regions for 106-cycle lifetimes are also indicated.