Glass ceramics for structural applications

Compared to glass, glass-ceramic (GC) is a promising structural material because of its higher fracture toughness, excellent resistance to high temperature (fire resistance) and reduced sensitivity to static fatigue. An experimental campaign for the statistical characterization of bending strength of a commercial GC was composed of: (a) Ring on Ring biaxial bending tests (UNI-EN 1288-5) and (b) 4PB test (UNI-EN 1288-5). Two batches of samples of thicknesses 6 and 8 mm were tested, each one composed of 50 and 20 specimens for test type (a) and (b), respectively (140 specimens in total). Classical Weibull probability distribution can accurately interpret the data, showing much lower dispersion with respect to glass. The strength of GC is comparable with that of tempered glass, but it breaks into large pieces like annealed glass. In order to evaluate the effects of static fatigue, additional 4PB tests were performed at four different load rates (0.02, 0.2, 2, 20 MPa/s) following ASTM C1368. A phenomenological model of equivalent-crack growth is applied in order to characterize the initial equivalent defects of the material and make predictions of the decay of strength with loading time, calculating the corresponding reduction coefficients kmod. As expected, the present GC results much less sensitive than glass to static fatigue.