Effect of Crystallites on Subcritical Crack Growth and Strain‐Rate Sensitivity of Strength of Cordierite Glass‐Ceramics

Abstract

A study was conducted of the subcritical crack growth in a series of cordierite glass‐ceramics, including the original glass, by means of the double‐torsion method and measurement of the strain‐rate sensitivity of the strength of specimens with controlled indentation cracks subjected to biaxial flexure. The double‐torsion data showed that the same rate of crack growth required a stress intensity factor for the fully crystallized glass‐ceramic about 3 times higher than for the original glass, accompanied by a simultaneous increase in the stress intensity exponent (N). The suppression in the rate of crack growth in the crystallized material at any value of stress intensity factor was attributed to enhanced crack deflection around the crystallites. Limited observations suggested that the increase in N on crystallite formation may be due to a greater degree of crack deflection at the lower crack velocities than at the higher velocities. For strength specimens of the as‐received and crystallized glass with large surface flaws, the N values obtained by the strain‐rate measurements and the double‐torsion test showed good agreement. However, for the fully crystallized material with surface flaws too small to interact significantly with the coarser crystallites, the N value for the strain‐rate data was well below the value for the double‐torsion method. Indentation‐fracture data of the double‐torsion specimens confirmed the existence of crack‐size dependent fracture toughness. Limited fracture toughness data coupled with SEM fractography indicated that with increasing crystallite size toughening occurs by crack deflection as well as by microcrack toughening counteracted by fracture through the crystallites.