Cyclic Fatigue Crack Growth in Three-Point Bending PZT Ceramics under Electromechanical Loading

This paper investigates experimentally and analytically the cyclic fatigue crack growth in piezoelectric ceramics under electromechanical loading. Cyclic crack growth tests were conducted on lead zirconate titanate (PZT) ceramics subjected to dc electric fields, and a finite element analysis was used to calculate the maximum energy release rate for the permeable crack model. Based on bending experiments using single-edge precracked-beam specimens, cyclic fatigue crack growth rates are found to be sensitive to the maximum energy release rate and applied dc electric fields. Possible mechanisms for crack growth were discussed by scanning electron microscope examination of the fracture surface of the PZT ceramics.     

Dynamic fatigue behavior of cracked piezoelectric ceramics in three-point bending under AC electric fields

This paper describes an experimental and analytical study on the dynamic fatigue behavior of cracked piezoelectric ceramics under AC electric fields. Constant load-rate testing was conducted in three-point bending with the single-edge precracked-beam specimens. The crack was created normal to the poling direction. The effects of AC electric fields and loading-rate on the fracture load were examined. A phenomenological model of domain wall motion was also used in finite element computation, and the energy release rate for the permeable crack model was calculated. The effect of AC electric fields on the critical energy release rate was then examined. The results suggest that (1) the fracture load of PZT ceramics decreases as the load-rate decreases; (2) an overall decrease in the fracture load occurs when testing under AC electric fields; and (3) the critical energy release rate is not very affected by the AC electric fields.     

Fracture Criteria for Piezoelectric Ceramics

Fracture criteria for piezoelectric materials were investigated. Mode I and mixed mode fracture tests were performed on PZT-4 piezoelectric ceramics to verify the validity of the mechanical strain energy release rate as a fracture criterion. Experimental results indicated that crack extension could be aided or impeded by an electric field, depending on the field direction. Further, the direction of crack extension was studied. A crack closure method, together with finite element analysis, was introduced to calculate the mechanical strain energy release rate. The maximum mechanical strain energy release rate was used to predict fracture loads under combined mechanical and electrical loads. It was found that the mechanical strain energy release rate criterion is superior to other fracture criteria and predicts fracture loads fairly accurately.     

Fatigue crack growth law for ferroelectrics under cyclic electrical and combined electromechanical loading

New data sets of crack propagation in lead-zirconate-titanate DCB specimens under cyclic electric loading combined with a constant mechanical load have been obtained. Both an increasing mechanical load as well as an increasing field amplitude resulted in an enhanced crack propagation rate. The experiment was modelled with a Finite Element Analysis that used special crack tip elements and assumed a finite permeability of the crack. The calculations revealed a dielectric crack closure effect, explaining the experimentally observed threshold of fatigue crack growth for the electric load. Fracture quantities suitable for cyclic loading by electric fields above the coercive field were discussed and a Mode-IV intensity factor considered as appropriate. The resulting correlations were applied to the experimental results and a power law relationship for the crack growth rate versus the range of the Mode-IV intensity factor was found.     

Evaluation of Electric Fracture Properties of Piezoelectric Ceramics Using the Finite Element and Single-Edge Precracked-Beam Methods

Single-edge precracked-beam (SEPB) tests were performed on a commercial lead zirconate titanate (PZT) ceramic. Mechanical loading was applied by the crosshead displacement control of a screw-driven electromechanical test machine. The fracture toughness parameter K _C was determined for various electric fields. A finite element analysis was also done to calculate the total potential energy release rate, mechanical strain energy release rate, and stress intensity factor for three-point flexure piezoceramic specimens with permeable and impermeable cracks under displacement and load control conditions. Numerical investigation and comparison with test data indicate that the energy release rate, upon application of the permeable model, is useful for predicting crack growth in PZT ceramic under electromechanical loading. Based on current findings, we suggest that the energy release rate criteria for the permeable crack are superior to fracture criteria for the impermeable crack.     

Electric-Field-Induced Fatigue Crack Growth in Piezoelectrics

When subjected to large alternating electric fields, ferroelectric ceramics may experience cracking and mechanical degradation. This article describes an experimental procedure for characterizing crack extension from preexisting flaws in such materials subject to high-amplitude, alternating electric fields. A new mode of electric-field-induced fatigue crack growth is identified. Fracture mechanics concepts are applied to interpret the observed cracking.     

Fatigue Crack Growth in Ferroelectric Ceramics Driven by Alternating Electric Fields

Electric-field-induced fatigue crack growth in ferroelectric ceramic PZT-5 with precracks was investigated. The experimental results showed that there were two distinct characteristics in the crack growth under electric loading. Under low electric loads, microcracks located ahead of the main crack emerged and grew and, as a result, impeded the growth of the main crack. On the other hand, under high electric loads, microcracks were absent, and the main crack was the only mode of fatigue cracking. The main crack grew macroscopically along the original path perpendicular to the electric field. Microscopically, the crack grew along the grain boundaries and grain breakaway was observed. The crack growth rate was nonlinearly related to the cyclic electric load. Similar to mechanical fatigue, there existed a crack growth threshold in the applied electric-field amplitude below which the crack ceased to grow. A steady crack growth occurred when the applied electric field exceeded this threshold. An empirical model for crack growth was obtained. Domain-switching effect and fracture-mechanics concepts were used to explain the observed crack closure and crack growth under electric loads.     

Dynamic fracture behavior of piezoelectric ceramics under impact: Force-electric response and electrical breakdown

The brittle fracture may occur in the application of piezoelectric ceramics, but the traditional research is still limited to the static fracture of the materials. Based on the improved Hopkinson pressure bar loading system and high-speed photography technology, the experimental study on the fracture behavior of piezoelectric ceramics under impact loading was carried out. The dynamic mechanical and electrical response of lead zirconate titanate (PZT) and the possible electric breakdown phenomenon were analyzed. The experimental results show that the output voltage is stable and the maximum output voltage is 889 V when the impact load does not cause the material to fracture. When the material breaks, its macroscopic output voltage fluctuates due to electric breakdown. Combined with the finite element simulation of the impact fracture process, the distribution characteristics of the stress field and electric field near the crack during the fracture process were analyzed. The results show that the sliding between grains formed the crack cavity parallel to the electric field during the impact process. Furthermore, based on the theory of dielectric breakdown, the possibility of electric breakdown in the initial defect and the elliptical cavity formed by the impact is analyzed.     

含界面边裂纹压电材料反平面问题的应力强度因子

研究了含界面边裂纹的不同压电介质组成的复合材料在反平面荷载和平面内电场作用下的电弹场,得到了级数形式的基本解和应力强度因子,最后用边界配置法求解了应力强度因子。结果表明,在外加剪切荷载的作用下,应力强度因子与外加电场无关。     

Effects of an Applied Electric Field on the Modulus of Rupture of Poled Lead Zirconate Titanate Ceramics

The effects of applied dc electric fields of ±10 kV/cm on the modulus of rupture of poled PZT-841 ceramics were studied using three-point bending tests. At each level of the applied electric field, 54 or 55 samples were tested and the data were statistically analyzed. The results showed that the measured moduli of rupture followed a Weibull distribution with a Weibull modulus of 10.6 when no electric field was applied. When the applied electric field was either parallel or antiparallel to the poling direction, the distribution of the modulus of rupture revealed two peaks and was fitted by a two-peak Weibull distribution. One peak occurred at about 95 MPa, approximately the same as that without the presence of the applied electric field, while the other peak occurred around 50 MPa under either a positive or a negative electric field. Obviously, either a positive or a negative electric field assisted the applied mechanical loading to fracture the samples.     

Poling of Lead Zirconate Titanate Ceramics and Flexible Piezoelectric Composites by the Corona Discharge Technique

In the conventional poling method, piezoelectric ceramics and composites are poled by applying a large dc voltage. Poling of composites having a polymer matrix with 0–3 connectivity is especially difficult because the electric field within the high-dielectricconstant grains is far smaller than in the low-dielectric-constant polymer matrix. Therefore, very large electric fields are required to pole these types of composites. However, large electric fields often cause dielectric breakdown of the samples. In this study for improved poling, the corona discharge technique was used to pole piezoelectric ceramics, fired PZT composites, and 0.5PbTiO₃· 0.5BiFeO₃ 0–3 polymer composites. An experimental setup for corona poling is described. The dielectric and piezoelectric properties of materials poled by the corona discharge technique were comparable to those obtained with the conventional poling method.     

Electrical fatigue behavior of lead zirconate titanate ceramic under elevated temperatures

The electrical fatigue behavior of lead zirconate titanate (PZT) ceramics is investigated under different temperatures. A bipolar triangular electric field with the amplitude of ±1.5 kV/mm and the frequency of 50 Hz is applied to samples up to 1 × 10⁶ cycles. The fatigue rate is found to be temperature dependent, and the fatigue degradation is represented by the loss of remnant polarization, dielectric constant, and piezoelectric constant increased with loading cycle numbers. The degradation, involving surface damage and crack propagation, is more pronounced in samples cycled at lower temperatures, and increases with increasing number of cycles. The temperature effect on fatigue degradation of the properties is described based on the field shielding effect caused by surface damage and fatigue-induced cracks. The effect is more dominant in case of higher cycling numbers and lower temperature fatigue due to higher strain mismatch between switchable and non-switchable domains. Moreover, Raman spectroscopy is used to determine the influence of fatigue on the ferroelectric domains in different areas of the specimens.     

A study of the effects of vibration on the electric power generation properties of lead zirconate titanate piezoelectric ceramic

To better understand how the electric power generated from PZT piezoelectric ceramics is affected by mechanical loading conditions the power generation was examined during cyclic loading under various loading conditions. The electric power generation was continuously examined using a monitoring system that we have recently developed. This system revealed that the electric power increased with increase of the applied load but then decreased when the applied load exceeded a certain level. In addition, greater electric power was generated with a simple beam configuration compared with a cantilevered beam. The change of electric power generation was directly related to the stress direction; high stress in the tetragonal structure parallel to the c-axis gave rise to high electric power generation. On the other hand, material failure, including domain switching and crack generation, caused a reduction of the electric power generated. Based upon our experimental data, suitable loading conditions to give high piezoelectric energy generation have been clarified.     

High Bipolar Fatigue Resistance of BCTZ Lead‐Free Piezoelectric Ceramics

(Ba, Ca)(Ti, Zr)O₃ ceramics have been considered as a potential lead‐free alternative to commonly used lead‐based piezoelectric ceramics due to their high piezoelectric performance at room temperature. In this study, the bipolar fatigue behavior of this material is investigated at room temperature. Two compositions were cycled with a bipolar electric field signal at 10 Hz with a maximum of three times the coercive field for up to approximately 10⁷ cycles. Both investigated compositions exhibited high bipolar fatigue resistance compared to other ceramics reported in the literatures. The high fatigue resistance originates from the lack of mechanical damage and a weak domain wall pinning effect due to their location in the phase transition region. It was also found that pore morphology affected bipolar fatigue behavior.     

Experimental investigation of electrostrictive polarization biased direct apparent piezoelectric properties in polyurethane elastomer under quasistatic conditions

Polyurethane elastomer was recently discovered to demonstrate a very high field induced electrostrictive response. In this work an experimental setup, consisting of an electric circuit and a mechanical system, was designed and constructed for the measurement of the electrostrictive polarization biased apparent piezoelectric response of polyurethane elastomers in a direct piezoelectric effect under quasistatic conditions. The electric circuit design allows the application of a direct current (dc) bias electric field to the sample and the possibility of picking up the generated quasistatic electrical signal separately. The mechanical system provides the function of a vibration source from which the stress and strain of the sample can be measured. Therefore, such effective piezoelectric properties as d₃₁ and k₃₁ can be measured. The electromechanical coupling coefficient was derived by two different methods. One was from the deduction based on the piezoelectric equations. The other was from the calculation based on the basic definition of the electromechanical coupling coefficient (i.e., through the exact measurement of input mechanical energy and output electric energy). In the latter case, the internal resistance of the sample and the dc bias blocking capacitor were found to be the critical factors for precision determination of the total electrical energy output. The different approaches led to close agreement. The effective d₃₁ can be 184 pC/N under a 25 MV/m bias electric field in a 30-µm thick sample, which is much higher than that of typical piezoelectric polymers. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2603–2609, 1999     

Electric Fatigue of Lead‐Free Piezoelectric Materials

A considerable body of knowledge now exists from studies involving the development of lead‐free piezoelectric ceramics and a number of high potential alternatives to current lead‐based materials have been identified. Stability under cyclic electric fields is an important property of piezoelectric materials. Here, we review the research to date which shows that fatigue under cyclic electrical loading is prevalent in many lead‐free piezoelectric ceramic compositions. However, the variety of compositions and mechanisms for piezoelectric behavior in these materials corresponds to significant variances in the nature of fatigue degradation and the likely mechanisms thereof, which do not directly parallel those of well‐studied lead‐based materials. In particular, the use of field‐induced phase changes as an actuation mechanism provides distinctive fatigue behaviors. Particular attention is given to fatigue of ferroelectric and relaxor (ergodic and nonergodic) structures and their dependence upon temperature and electric field and the potential design of materials with high fatigue resistance.     

Fracture behavior of poled piezoelectric PZT under mechanical and electrical loads

Four point bending samples were poled parallel to the long axis, notched and fractured. During mechanical loading, a constant electric field was applied parallel or antiparallel to the poling direction (perpendicular to the crack surface). Assuming electrical crack boundary conditions of (i) an impermeable or (ii) a completely permeable crack, the stress intensity factors K_I and the field intensity factors K_IV at failure were determined by linear-piezoelectric finite element calculations. The fracture curve K_IC(K_IV) for the impermeable crack model does not comply to fracture criteria based on the total energy release rate or on the mechanical energy release rate. Within the completely permeable crack model, it appears generally impossible to describe electric field effects on the fracture resistance. Some theoretical extensions of the crack models are discussed which might contribute to resolve the aforementioned problems.     

Influence of electric field and current on the strength of depoled GaN piezoelectric semiconductive ceramics

By using three-point bending tests, the effects of an applied DC electric field and current on the strength of depoled GaN piezoelectric semiconductive ceramics are investigated. Under combined mechanical-voltage-electrical current loading, the corresponding stress and electric fields and carrier distribution in specimens are analyzed based on the finite element method. It is shown that, when an electric field of 0.95 kV cm^–1 is applied, the bending strength decreases by 14.7% and then, remains unchangeable with further increase of the electric field. In contrast, the bending strength decreases from 11.5 to 8.5 MPa as the applied electric current increases from 0 to 5 × 10⁴ A m^–2. The results imply that there is a strong correlation between the bending strength and electric field or current for piezoelectric semiconductive ceramics.     

Direct observation of domain switching and crack nucleation in a piezoelectric material

The effect of an electric field on domain switching and fatigue induced crack nucleation and growth in a piezoelectric material of nominal composition Pb(Zr_0.5, Ti_0.5)O₃ has been investigated. The ceramic was subjected to localised static and cyclic electric fields, which were applied via pairs of closely spaced surface-mounted electrodes, while simultaneously imaging the microstructure in the SEM. Electric field–polarisation hysteresis loops were also collected from the local region using the same electrodes.Domain wall mobility was observed above a threshold electric field strength, as was microcracking. Cracks were seen to nucleate at grain boundaries, and were sometimes associated with microstructural features, such as pores. Crack propagation was mainly intergranular, and occurred preferentially in a direction parallel to the local field direction. Transgranular fracture was also observed, with the crack path being influenced by interaction with domain boundaries. Factors affecting domain switching and crack propagation are discussed in the context of the locally applied electric field.