Abstract: Unusual increase of displacement in special ring type piezoelectric ceramic actuators with transverse mode was found, which were made from three discrete
actuators using piezoelectric ceramics with tungsten bronze structure. The increasing values of displacement were as high as about 30%, as
compared with the composing discrete one. The discrete actuators are in press stress according to analyses and experiments, and the unusual
phenomenon is related to their stress condition. This mechanism can be described as follows: under the press stress the spontaneous polarization
P_s will departure from original direction (the pressure direction), due to some 90° domains reorientation or some distortion of oxygen-
octahedras in tungsten bronze structure, then as soon as electrical field E is applied, the P_s will return the original directions, that leads to
the increment of strain for the ring type actuator.

Key words: piezoelectric ceramics, tungsten bronze crystal structure, ring type piezo-actuator, converse piezoelectric effect, stress