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Pre-overloading to Extend Fatigue Life of Cast Clasps

Department of Conservative Dentistry and Prosthodontics, Dental School, University of Jordan, Amman 11942, Jordan; anabtawime{at}yahoo.com

View larger version (37K): [in this window] [in a new window]   Figure 1. Schematic representation of the basic test specimen (right). Each specimen was deflected by being loaded through a sphere located at its tip. The loading was radial to the outside in a direction that made a 30-degree angle with the cylinder’s cross-sectional plane (rectangular insert). To the left is a picture of a representative finite element model. The models were reproduced based on the dimensions of the real specimens.

View larger version (28K): [in this window] [in a new window]   Figure 2. Flow curves, mathematical models validation, and results of the bending tests and finite element simulations. (A) Plot of the "multi-linear material property" flow curves (in which each stress-strain curve is represented by connected successive lines of decreased slope) used in the finite element analysis for clasps made from the 3 alloys. The 458-MPa, 685-MPa, and 883-MPa underlined values represent the proportional limit values used for Gold Type IV, Co-Cr, and Ti-6Al-7Nb alloys, respectively. The elastic modulus values were 229 GPa, 92.6 GPa, and 115.8 GPa for Co-Cr, Gold Type IV, and Ti-6Al-7Nb alloys, respectively. (B) Plot of real deflection-predicted deflection corresponding to different levels of permanent deformation for the 9 bending test specimens and their corresponding simulation models (3 specimens for each alloy). The straight line represents an ideal relation when the predicted and real values match perfectly. (C) Plot of [ Permanent deformation/ von Mises (VM) plastic strain] ratios as a function of von Mises (VM) plastic strain on the inner upper corner of 3 representative models for the 3 alloys. (D) Plot of [ von Mises (VM) residual elastic strain/ von Mises (VM) plastic strain] ratios as a function of von Mises (VM) plastic strain on the inner upper corner of 3 representative models for the 3 alloys. (E) Plot of permanent deformation as a function of deflection for the 9 bending test specimens (3 specimens for each alloy). The average amount of deflection at which permanent deformation started to appear was 275 ± 12 µm, 490 ± 18 µm, and 656 ± 13 µm for Co-Cr, Gold Type IV, and Ti-6Al-7Nb alloy clasps, respectively. (F) This Fig. shows the changes in the first principle stress value (S1 -maximum tensile stress) for a representative Ti-6Al-7Nb model. The maximum S1 in the clasp and the S1 at the inner upper corner were recorded when the model was deflected from its rest position for 0.5 mm after each permanent deformation increment. The amount of reduction in the S1 value at the inner upper corner of the clasp was almost equal to the absolute value of residual 3rd principle stress (S3 - maximum compressive stress) there.

View larger version (47K): [in this window] [in a new window]   Figure 3. von Mises equivalent stress distribution patterns for a representative Ti-6Al-7Nb model. The scale at the bottom describes the ranking of colors. The dark blue indicates the lowest stress values close to zero, and the red indicates the highest; areas in gray have stress values exceeding the proportional limit and therefore represent areas undergoing plastic deformation. A and C represent the stress distributions on loading for deflections that resulted in 25 µm and 350 µm permanent deformations, respectively, while B and D represent those on unloading. The inner upper corner and the opposing outer side, both showing the highest stress values, were especially projected. Each model was split at an 80-degree location to show the stress distribution inside the clasp. When unloaded, the residual stress patterns had a four-layer configuration: The 2 surface layers had stresses opposite those when loaded, while the deep 2 layers had the same stresses (blue arrows, compressive; red arrows, tensile). The inner upper corner showed the highest von Mises residual stress values (compressive). For clarity, different scales were used for the 4 Figs.