An estimate is made of the upper limit of the toughening that might be expe
cted by the crack tip stress induced reorientation of domains in ferroelect
ric ceramics. The calculation is based on obtaining the shielding stress in
tensity factor produced by the shear transformation of a zone surrounding t
he crack tip and extending over the crack surfaces. The estimate suggests t
hat the toughening that might be expected is less than 10% even in the most
ideal conditions. This case is contrasted with the much greater toughening
which is expected from the crack tip stress induced phase transformations
which are largely driven by the chemical, or bulk, free energy change of th
e transformation. (C) 2001 Elsevier Science Ltd. All rights reserved.