Fracture behavior of ferroelectric ceramics during in-plane and out-of-plane 90 degrees domain switching was studied using the moir6 interferometry technique. The specimens used in the experiment were three-point-bending beams, each with a single through-notch, which were subjected to a mechanical load, an electrical load and a combined electrical and mechanical load, respectively. The main subject of interest is to investigate the influence of 90 degrees domain switching induced by the electrical load on fracture toughness and material brittleness. In the experiment, compared with out-of-plane 90 degrees domain switching, in-plane 90 degrees domain switching occurs in the region of approximately a 45 degrees band. It causes larger in-plane tensile strain epsilon(xx) in almost all regions of a specimen, especially in the 45 degrees band. In-plane 90 degrees domain switching greatly decreases fracture toughness and weakens the material brittleness of ferroelectric ceramics. The out-of-plane 90 degrees domain switching does not exhibit such a great influence on fracture toughness and material brittleness as the in-plane 90 degrees domain switching does. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.