13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Effects of Electric Field and Poling on the Cyclic Bending Fatigue in Cracked Piezoceramics Yasuhide Shindo1,*, Fumio Narita1, Masayuki Sato1 1 Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan * Corresponding author: shindo@material.tohoku.ac.jp Abstract We investigates the fatigue behavior of cracked piezoelectric ceramics in cyclic bending under electric fields both theoretically and experimentally. Fatigue tests were carried out in three-point bending with the single-edge precracked-beam (SEPB) specimens. The crack was created normal to the poling direction. Number of cycles to failure was measured under electric fields. Plane strain finite element analysis was also performed, and the effect of polarization switching on the energy release rate was discussed under high negative electric field. In addition, mechanisms for crack growth were discussed by scanning electron microscope examination of the fracture surface of the piezoelectric ceramics. Keywords Piezomechanics, Finite element method, Material testing, PZT ceramics, Fatigue 1. Introduction Research on fracture and fatigue of the lead zirconate titanate (PZT) ceramics has increased during the past decade. In recent years, the dynamic fatigue of SEPB hard PZT ceramics was studied under direct current (DC) electric field [1]. In the case under cyclic mechanical loading, Narita et al. [2] studied the fatigue crack growth in SEPB hard PZT ceramics under sinusoidal mechanical load and DC electric field, and discussed the effect of electric field on the crack growth rate versus maximum energy release rate curves. It is expected that the polarization switching under high negative electric field affects the crack behavior of piezoelectric ceramics [3]. In particular, the polarizations are easy to switch in soft PZT ceramics, whereas hard PZT ceramics are very difficult to switch. Literature studies of fatigue in the PZT ceramics under high negative electric field are sparse and inconclusive. The main aim of this work is to discuss the effects of the electric field and polarization switching on the fatigue of cracked soft PZT ceramics in cyclic bending. 2. Experimental Procedure Commercially supplied soft PZT C-91 ceramics (Fuji Ceramics Co. Ltd., Japan) were used. The material properties are listed in Table 1, and the coercive electric field is about Ec = 0.35 MV/m. Fatigue tests under zero or negative electric fields were carried out using SEPB specimens. The specimen and testing set up are shown in Figure 1. PZT ceramics of 5 mm×5mm×5 mm were cut, and the specimen was produced by first poling a PZT and then bonding it between two unpoled PZTs. The size of the specimens was 5 mm thick, 5 mm wide, and 15 mm long. Vickers indents were introduced using the commercial microhardness testing machine. The specimens were compressed until a precrack was formed, and the crack has initial length of about 0.5 mm. Three-point bending apparatus with 13 mm span was used, and load P with constant-amplitude sinusoidal variation was applied with 250 N load cell (resolution : 0.01 N) to the specimens at constant frequency f of 1 or 50 Hz. The load ratio, defined as the ratio of minimum load Pmin to maximum load Pmax of the fatigue cycle, was R = Pmin/Pmax = 0.5, and the maximum load Pmax was 90, 100 or 110 N. Number of cycles to failure under constant applied load Pmax and electric field was then measured. After the tests, scanning electron microscope (SEM) was used to examine
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