ANALYSIS OF DAMAGE NEAR A CONDUCTING CRACK IN A PIEZOELECTRIC CERAMIC

The finite element formulation for analyzing static damage near a conducting crack in a thin piezoelectric plate is established from the virtual work principle of piezoelectricity.The damage fields under various mechanical and electrical loads are calculated carefully by using an effective iterative procedure.The numerical results show that all the damage fields around a crack tip are fan-shaped and the electric field applied has great influence on the mechanical damage, which is related to the piezoelectric properties.

TRANSVERSELY ISOTROPIC DAMAGE AROUND CONDUCTING CRACK-TIP IN FOUR-POINT BENDING PIEZOELECTRIC BEAM

A static damage constitutive model was proposed on basis of the electrical enthalpy density, and then some characteristics of transversely isotropic damage were discussed. Finally, the effects of both crack depth and applied loads on damage distributions were investigated through numerically analyzing transversely isotropic damage in a four_point bending PZT_PIC151 beam with a central conducting crack. Some conclusions were given: 1) Crack depth and mechanical loading have great influence on both mechanical and electrical damages.With their increment,the damages at crack_tip obviously increase and their region sizes also expand. 2) Effects of electrical loading on the two kinds of damages are obviously different. Electrical loading monotonously changes magnitude but region size of mechanical damage, whose effect on electrical damage is very complex.

Temperature-Dependence of Microstructure Evolution in a Ferroelectric Single Crystal with Conducting Crack

The different temperature-induced nonlinear behavior near a conducting crack tip in a ferroelectric single crystal is studied based on a phase field approach containing the time-dependent Ginzburg-Landau equation.Since domain switching in a crack tip plays an important role in the fracture behavior,by using three-dimensional nonlinear finite element method,the temperature-induced domain switching behavior of a ferroelectric single crystal is simulated under applied electrical and mechanical loads.The simulations show that increasing the temperature will enhance the crack propagation under a strong electric field,which results in switching-weakening.In particular,increasing the temperature from 300°C to 600°C will impede the crack propagation under combined mechanical and electric field loading,which results in switching-toughening.Salient features of the results are consistent with many experimental observations.

Influence of residual surface stress on the fracture of nanoscale piezoelectric materials with conducting cracks

In this paper,we analyze the stress and electric field intensity factors affected by residual surface stress for conducting cracks in piezoelectric nanomaterials.The problem is reduced to a system of non-linear singular integral equations,whose solution is determined by iteration technique.Numerical results indicate that the residual surface stress can significantly alter the crack tip fields at nanometer length scales.Due to the residual surface stress,281he electric field can produce stress around crack tip.This suggests a strong electromechanical coupling crack tip field for nanoscale piezoelectric materials.Such a finding is considerably different from the classical fracture mechanics results.A transit electric field to stress load ratio is identified,for which influences of residual surface stresses vanish.The research is useful for the applications of nanoscale piezoelectric devices.

Effects of leachate infiltration and desiccation cracks on hydraulic conductivity of compacted clay

Both cracks in clay liner and the complex composition of landfill leachate might have effects on the hydraulic conductivity of a compacted clay liner. In this study, the hydraulic conductivities of natural clay and bentonite-modified clay with and without desiccation cracks were measured, respectively, using three types of liquids as permeating liquid： 2 500 mg/L acetic acid solution, 0.5 mol/L CaCl2 solution, and tap water. When tap water was adopted as the permeating liquid, desiccation cracks resulted in increases in the average value of hydraulic conductivity： a 25-fold increase for the natural clay and a 5.7-fold increase for the bentonite-modified clay. It was also found out that the strong selfhealing capability of bentonite helped to reduce the adverse impact of cracks on hydraulic performance. In contrast to tap water, simulated leachates（acetic acid and CaCl2 solutions） show no adverse effect on the hydraulic conductivities of natural and bentonite-modified clays. It is concluded that desiccation cracks and bentonite have more significant effects on hydraulic performance than simulated leachates.

A Kinetic Model of Dielectric Ageing

A kinetic model of dielectric ageing is presented. The central finding of this investigation is that there is a power-law relationship between the local electric field concentration and the rate of defect-tip initiated conducting crack growth. By applying such a power-law conducting crack growth rate expression to the evaluation of the life of solid dielectrics, the empirical classical ageing law of insulation materials can be derived theoretically as a lobical result. All the results are universal and agree with the experimental data of oxide films.

EFFECTS OF STATIC ELECTRIC FIELD ON THE FRACTURE BEHAVIOR OF PIEZOELECTRIC CERAMICS

The paper gives an overview on experimental observations of the failure behavior of electrically insulating and conducting cracks in piezoelectric ce- ramics.The experiments include the indentation fracture test,the bending test on smooth samples,and the fracture test on pre-notched(or pre-cracked)compact ten- sion samples.For electrically insulating cracks,the experimental results show a com- plicated fracture behavior under electrical and mechanical loading.Fracture data are much scattered when a static electric field is applied.A statistically based frac- ture criterion is required.For electrically conducting cracks,the experimental results demonstrate that static electric fields can fracture poled and depoled lead zirconate titanate ceramics and that the concepts of fracture mechanics can be used to mea- sure the electrical fracture toughness.Furthermore,the electrical fracture toughness is much higher than the mechanical fracture toughness.The highly electrical fracture toughness arises from the greater energy dissipation around the conductive crack tip under purely electric loading,which is impossible under mechanical loading in the brittle ceramics.