Two R Curves for Partially Stabilized Zirconia

The enhanced fracture toughness due to stress‐induced transformation can be explained from two viewpoints: (1) the increase can be attributed to the need to supply a work of transformation, or (2) the transformation can be considered to result in internal stresses which oppose crack opening. Experimental results for magnesia‐partially‐stabilized zirconia are presented for the two experimental measures of toughness corresponding to these two viewpoints, namely (1) the specific work of fracture, R , and (2) the nominal stress intensity factor, K R . It is observed that these two measures are not equivalent during the initial stage of R ‐curve behavior, prior to reaching steady‐state cracking. The theoretical reason for this difference is discussed. In particular, it is noted that the usual definition for the crack extension force does not correspond to the experimentally measured work of fracture in the presence of stress‐induced (or pre‐existing) sources of internal stress.