Papers by Author: Katsuhiko Watanabe

Abstract: The effect of electromechanical loadings including their sequence, which was motivated by the property of crack energy density for piezoelectric material, was experimentally investigated. Three-point bending fracture test was performed for two piezoelectric ceramics under different electromechanical loading conditions. It was found that the fracture loads under closed circuit condition are greater than those under open circuit condition. Effect of applied electric field on fracture load in the test varied with materials. Applied electric field always enhanced crack extension in C-21 ceramics regardless of their directions, while it produced very little effect on crack extension under negative electric fields in C-2 ceramics. It was also found that electromechanical loading sequence clearly affects fracture strength, although its effect varies also with materials.

Abstract: An electrical nonlinear constitutive model for piezoelectric material is studied in this paper. Electric yielding criterion and yielding effect on CED are investigated. Using the mechanical part of CED as a fracture parameter, it is shown that the predicted fracture loads through FEA for compact tension specimen is in accordance with corresponding experimental results.

Abstract: Creep crack growth (CCG) rate has been organized frequently by C* or Ct parameter However, crack behavior of early stage under unsteady state condition has not been explained. Crack energy density (CED), which has been proposed as a parameter that can provide a unified description of crack behavior with no restriction on constitutive equation, can give the general expression about creep crack growth rate. By applying Ct and the concept of CED to the results, we showed that creep crack growth rate for all ranges of creep can be explained in a unified way by CED and its derivatives. Moreover, the physical meaning of the Ct is clarified in the discussion.

Abstract: In the recent years, the studies on the mechanical behaviors of various materials subjected to biaxial loading have been worked since they are more complicated and intrinsically different from those under the simple uniaxial condition. The cruciform specimen without any slots has been commonly used for the goal so far. We prepare improved biaxial specimen with slots and make sure its validity by means of finite element analysis and photoelastic experiment. Even though the equal load biaxiality was applied to the specimens, as the results, we found that the stress biaxiality ratios in central region of specimen differ according to the position from the center of them, the specimen with slots in the arms is more effective to make state stress uniform than the specimen without slot.