Local fracture properties in poled [Pb(Zn<sub>1/3</sub>Nb<sub>2/3</sub>)O<sub>3</sub>]<sub>(1-x)</sub>-[PbTiO<sub>3</sub>]<sub>x</sub> (x=0.045, PZN-4.5%PT) ferroelectric relaxor single crystals were assessed. The crystals were cut along the , , and  planes. Scanning electron micrographs of Vickers indentations for two different crack orientations were used to determine the crack tip toughness (K<sub>tip</sub>) and the local critical energy release rate (G<sub>tip</sub>). Cracks oriented along the  and  crystal planes were found to have practically identical local fracture properties. These properties were determined using Stroh's formalism to account for the large anisotropic material coefficients.
We have used a kinetic approach to the fatigue phenomenon in ferroelectrics for the analysis of the evolution of switching current and strain hysteresis loops in bulk PZT ceramics and the switching current in PZN-PT single crystals during cyclic switching. It is proposed that fatigue is due to a redistribution of the local internal bias field during cycling (spatially non-uniform imprint effect). The model considered is based on the fact that during cycling the ratio of the states with opposite direction of polarization ranges over the sample area. The local value of this ratio defines the change of the internal bias field at the given point during the switching cycle considered. Thus the spatial distribution of the internal bias field depends on the domain evolution prehistory. We have investigated by computer simulation the self-consistent change of the internal bias field distribution function with cycling, which leads to fatigue. The mathematical treatment of the switching current data allows us to extract the information about the evolution of the field distribution function. The fatigue-induced change of the strain loops is explained by a strong unipolarity of the growing frozen domain area, which has been predicted by our simulations. The analysis of experimental data confirms the validity of our model.
R-curves were measured for ferroelectric ceramic lead zirconate titanate (PZT) with two different grain sizes using the surface crack in flexure technique. Larger grain size resulted in a higher plateau value of the R-curve. This was consistent with the larger amount of ferroelastic switching observed from the stress/strain curve.
The electric fatigue behavior of commercial lead zirconate titanate (PZT) was investigated by optical microscopy and instrumented Hertzian microindentation. Macroscopic delamination cracks were found near the electrodes after large numbers of electrical cycles. Additionally, macroscopics edge cracks were found to orientate from the boundary between electroded and unelectroded material. Instrumented Hertzian microindentation measurements on specimen cross sections show larger indentation depths just beneath the electrodes than in the center of the sample. This behavior may result form a higher microcrack density near the electrodes. The role of micro- and macrocracks in the electrical fatigue behavior of ferroelectric ceramics is discussed and compared to macroscopic material parameters and acoustic emissions.
Piezoelectric actuators are key components in many smart structures applications. Long term reliability of these actuators becomes increasingly important as research makes the transition to commercial applications. Fracture toughness is a material parameter that measures a material's resistance to crack propagation. This is perhaps, one of the most important parameters for reliable device design. This paper reviews recent work by the authors on fracture behavior of ferroelectric ceramics. Vickers indentation data were used to determine the fracture toughness of a coarse and a fine grained PZT and two compositions of PLZT. R-curve data were obtained for two compositions of PLZT. The R-curve data presented here were generated using four point bend specimens with controlled surface flaws. The R-curve results compare favorably with the Vickers indentation data. A comparison of the R-curves for a ferroelectric composition of PLZT and a quadratic electrostrictive composition of PLZT demonstrate the contribution of domain reorientation to the toughening process.
Conference Committee Involvement (2)
Active Materials: Behavior and Mechanics
15 March 2004 | San Diego, CA, United States
Active Materials: Behavior and Mechanics
3 March 2003 | San Diego, California, United States