Fatigue failure of lithium disilicate and translucent zirconia crowns with different occlusal thickness
Step-stress accelerated life testing (SSALT) (36), which comprised 18 specimens distributed into three profiles: mild (n=9), moderate (n=6), and aggressive (n=3). Each profile started at 300 N and finished at 3,000 N. The mild profile was designed with 200 N load increase at each 20,000 cycles up to 320,000 cycles; the moderate profile was designed with a 250 N load increase each 15,000 cycles up to 195,000 cycles; the aggressive profile had 300 N load increase at each 10,000 cycles up to 110,000 cycles. The data were analyzed using an underlying life distribution to describe the life data collected at different stress levels and a life-stress relationship to quantify how life distribution changed across different stress levels(36)(37). Thus, the Weibull Distribution was chosen to fit the life data collected in SSALT. Considering the time-varying stress model of SSALT, the inverse power law relationship was selected to extrapolate a use level condition considering the cumulative effect of the applied stresses, commonly referred as the cumulative damage model. From the extrapolated use level condition, a variety of functions could be derived. Hence, the use level probability Weibull curves (probability of failure versus number of cycles) with a set load of 300 N at 90% two-sided confidence interval were calculated and plotted (CI: 90%) (Synthesis 9, Alta Pro, Reliasoft, Tucson, AZ, USA).The reliability was calculated for completion of a mission of 100,000 cycles at 300, 600, 900, and 1200 N and the differences between groups were identified based on the non-overlap of the CI. Parameters estimation for all analyses was calculated via maximum likelihood estimate (MLE) method, and 90% two-sided confidence interval (90% CI) was approximated using the Fisher matrix approach..