Fatigue Behavior of Crystalline-Reinforced Glass-Ceramics

Purpose: To evaluate the fatigue behavior of two crystalline-reinforced ceramics: leucite-reinforced (VL) and lithium disilicate-based (VD) glass-ceramics. 

Materials and Methods: Bar-shaped specimens (16 × 4 × 1.2 mm) were produced for each ceramic using prefabricated CAD/CAM blocks. For each group, 30 specimens were subjected to a three-point flexural strength test in a universal testing machine. For VL and VD, 36 and 41 specimens were subjected to a cyclic fatigue test, respectively. The cyclic fatigue test was performed with a pneumatic mechanical cycling machine (1 Hz; 37°C distilled water). Specimens were tested at two stress levels for each preset lifetime (10³ and 10⁴ cycles for VL; 10⁴ and 10⁵ cycles for VD) following the boundary technique. Fractography was performed with a scanning electron microscope. Data were analyzed with Weibull analysis. 

Results: There were significant differences among groups for characteristic strength (σ₀) and Weibull modulus (m), as the confidence intervals did not overlap. The VD group presented the highest values of σ₀, but the lowest Weibull modulus. Both groups showed a reduction of approximately 60% of the initial flexural strength (σ_f) after cycling for 10⁴ cycles. For VD tested in fatigue, there was no degradation of σ_f when the number of cycles was increased from 10⁴ to 10⁵. The VL group showed an 18% decrease in σ_f when the number of cycles increased from 10³ to 10⁴. Flexural strength values estimated for a 5% probability of failure were 36 MPa for VL and 55 MPa for VD, after 10⁴ cycles. 

Conclusion: Both glass-ceramics showed similar strength degradation (60%) after a lifetime of 10⁴ cycles, despite their distinct mechanical properties. Mechanical cycling in humid conditions proved to be an important factor for the degradation of the mechanical properties of crystalline-reinforced glass-ceramics.