Pre-overloading to extend fatigue life of cast clasps.

Permanent deformation in bending is associated with the development of residual stresses. The objective of this study was to characterize those residual stresses and test whether they can be manipulated to extend the fatigue lives of cast clasps. Simulations with validated non-linear finite element models were used to characterize the residual stresses in clasps cast from Ti-6Al-7Nb, Co-Cr, and Type IV gold alloys. In addition, two groups of as-cast and pre-overloaded (subjected to a load that produced 20 microm of permanent deformation) Ti-6Al-7Nb clasps (10 specimens each) were subjected to cyclic 0.5-mm deflections at 5 Hz until fatigue. Pre-overloaded specimens demonstrated significantly longer fatigue lives (32,200 +/- 17,300 cycles) than did those tested in the as-cast condition (17,900 +/- 7600 cycles), consistent with the maximum tensile stress values revealed by finite element analysis.