Micromechanics of Creep‐Crack Growth in a Glass‐Ceramic

The near‐tip strain and opening behavior of creep cracks in a glass‐ceramic were studied at temperatures for which grain‐boundary sliding is the dominant deformation mechanism. Using the stereoimaging displacement measurement technique, near‐tip creep strain and crack‐opening displacement (COD) increments were obtained as functions of distance from the crack tip and time of creep. Both the time and radial dependence of the strain and COD were observed to be consistent with the Riedel and Rice (RR) field when the time of creep and crack extension were small. Increasing creep time and crack extension led to the formation of a localized shear zone located directly ahead of the crack tip. Further crack extension tended to follow this shear zone within which creep damage accumulated. Neither the rate of COD nor the strain‐rate distribution within this shear zone could be described by the RR field or the Hui and Riedel (HR) field for growing cracks. Steady‐state, stable crack growth exhibiting both Mode I and II components was observed in the glass‐ceramic despite a creep exponent of ≅2. Varying the test temperature and applied stress intensity influenced both the development of the crack‐tip shear zone and the crack growth process. These observations are discussed on the basis of the interrelationships between the near‐tip field, creep damage accumulation, and grain‐boundary sliding.