Role of crack size in the bi-modal static fatigue failure of a cordierite glass and glass-ceramic

A study was conducted of the static fatigue behaviour at room temperature of a cordierite glass and glass-ceramic with model flaws of two different sizes introduced by the indentation fracture method. For the cordierite glass the stress intensity exponent, N, for sub-critical crack growth inferred from the dependence. of time-to-failure on stress was the same for the two different crack sizes and showed good agreement with the value of N for data obtained in an earlier study for dynamic fatigue and by the double-torsion method. For the cordierite glass-ceramic, which consisted of larger crystallites (a few micrometres in size) embedded in a primarily crystalline fine-grained matrix, the static fatigue response depended strongly on crack size, also observed during dynamic fatigue. This effect was attributed to the relative size difference between the cracks and the crystallites. The small cracks could propagate in a planar fashion, whereas the larger cracks were subject to toughening by crack-deflection around the crystallites and a corresponding decrease in the rate of sub-critical crack growth. An analysis for the case of idealized bi-modal crack-size dependent sub-critical crack-growth indicated that, for a cordierite glass-ceramic with small cracks, the role of the larger crystallites serves to introduce a pseudo fatigue-limit.On sabbatical leave from Department of Ceramic Engineering, Yonsei University, Seoul, Korea.