Precision of different fatigue methods for predicting glass-ceramic failure

This study aimed to characterize the fatigue behavior using two fatigue methods, boundary and staircase, and to predict the probability of failure (P_f) of zirconia-reinforced lithium silicate glass-ceramic (ZLS). Bar-shaped specimens of ZLS (18 ×4 ×1.2 mm) were fabricated. Thirty specimens were subjected to a three-point flexural strength test using a universal testing machine with 0.5 mm/min crosshead speed, in 37 °C distilled water. Flexural strength data were analyzed with Weibull statistics. Eighty-six bars were subjected to cyclic fatigue using boundary and staircase methods. Fatigue tests were performed in a pneumatic cycling machine (2 Hz, 37 °C distilled water) for 10 ³ and 10⁴ cycles. Fatigue data were analyzed using an inverse power law relationship and log normal-lifetime distribution. Fracture toughness (K_Ic) was determined using V-notched specimens (18 ×4 ×3 mm) and the short beam toughness method (n = 7). Vickers hardness (VH) was evaluated (4.9 N, 20 s). Fractographic and EDS analyses were also performed. ZLS showed a characteristic strength of 197 MPa, Weibull modulus of 4, VH of 6.67 GPa and K_Ic of 1.93 MPa m^1/2. After 10³ cycles, for both methods, there was a degradation of 78% of the initial strength. There was no significant degradation when the number of cycles increased from 10³ to 10⁴. Both methods resulted in similar P_f and precision at 40 MPa (~50% P_f). Yet, staircase shows good accuracy and precision in predicting the stress amplitude for a P_f near 50%; while boundary is also effective for P_f lower than 50%. The fatigue methods evaluated show similar accuracy and precision for predicting the P_f of a glass-ceramic when simulations were made in the range of stress levels and lifetimes used in the fatigue tests.