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.     

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

This paper describes an experimental and analytical study on the static fatigue behavior of piezoelectric ceramics under electromechanical loading. Static fatigue tests were carried out in three-point bending with the single-edge precracked-beam specimens. The crack was created perpendicular to the poling direction. Time-to-failure under different mechanical loads and dc electric fields were obtained from the experiment. Microscopic examination of the fracture surface of the piezoelectric ceramics was performed as well. A finite element analysis was also made, and the applied energy release rate for the permeable crack model was calculated. The effect of applied dc electric fields on the energy release rate versus lifetime curve is examined. The most important conclusion we reach is that the lifetimes for the piezoelectric specimens under a positive electric field are much shorter than the failure times of specimens under a negative electric field for the same mechanical load level.     

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.     

Fatigue life prediction for circular rubber bearings subjected to cyclic compression

The fatigue life of circular rubber bearings under cyclic compression is theoretically and numerically analyzed based on a previously proposed fatigue failure mechanism. The energy release rate at any point in circular rubber bearings under cyclic compression, which depends on the cracking energy density and crack length along the predicted crack propagation path, is derived first theoretically. Then, the corresponding fatigue crack growth rate and fatigue life are determined numerically by introducing the fatigue parameters of three different rubber compounds before and after suffering from thermal aging. Meanwhile, the effects of intrinsic flaw size and maximum compressive stress on the fatigue life of circular rubber bearings are also investigated. It is found that the enlargement in the Regime 1 range of the crack growth rate of rubber increases the fatigue resistance of circular rubber bearings. Therefore, the effects of the mechanical properties, intrinsic flaw size, threshold value, and maximum cyclic compressive stress on fatigue life are significant and should be taken into account in designing rubber bearings. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Fatigue Crack Propagation at Polymer Adhesive Interfaces

Fatigue (slow) crack growth in epoxy/glass, epoxy acrylate/glass and epoxy/PMMA interfaces was studied under constant and cyclic loading at both high and low humidities using the interfacial, four-point flexure test. Finite element analysis was used to determine the energy release rate and phase angle appropriate for the different crack geometries observed. The experimental results show that for the polymer/glass interfaces, the primary driving force for fatigue crack growth is the applied energy release rate at the crack tip and that increasing test humidity enhances crack growth under constant loading but has an insignificant effect under cyclic loading. At low humidity the crack growth rates under cyclic loading are significantly greater than under constant loading. For epoxy/PMMA interfaces the crack growth results were independent of the applied energy release rate, relative humidity, and cyclic vs. constant loading, within experimental scatter. In addition, for polymer/glass interfaces the effect of phase angle (13 to 54°) on crack growth rates is not significant. However, for epoxy/PMMA interfaces the applied energy release rate for the initiation of crack growth is considerably greater for a phase angle of 66° than for 5°, indicating that increasing shear at the crack tip makes the initiation of crack growth more difficult. These results are discussed in terms of possible mechanisms of fatigue crack growth at polymer adhesive interfaces.     

Cyclic Fatigue in Ceramics: A Balance between Crack Shielding Accumulation and Degradation

Cyclic fatigue growth rates in R-curve ceramics have been observed to depend very strongly on the maximum applied stress intensity, K _max, and only weakly on the stress intensity range, Δ This behavior is rationalized through measurement of crack wake shielding characteristics as a function of these fatigue parameters in a gas-pressure-sintered silicon nitride. In particular, evidence for a mechanical equilibrium between shielding accumulation by crack growth and shielding degradation by frictional wear of sliding interfaces is found for steady-state cyclic fatigue. This equilibrium gives rise to a rate law for cyclic fatigue. The data suggest that the accumulation process is the origin of the strong K _max dependence, and that the degradation process is the origin of the weak Δ dependence. These features are shown to be related to the "cyclic" R -curve and to the cyclic crack opening displacement, respectively.     

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.     

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.     

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.     

Retardation of Fatigue Crack Growth in Ceramics by Glassy Ligaments: A Rationalization

In high-temperature fatigue crack growth (FCG) experiments on ceramic materials containing amorphous grain boundary phases, the crack growth rates under cyclic loads were observed to be lower than those predicted solely on the basis of crack growth velocities measured under static loads. In this paper, a rationalization was offered for such a behavior by means of a phenomenological glass-bridging model which takes the relaxation behavior of glass into account. In ceramics which exhibit subcritical crack growth through cavitation ahead of the crack tip, the maximum stress intensity factor of the fatigue cycle required to initiate FCG was observed to be always greater than or equal to the threshold stress intensity factor for crack growth under sustained far-field loads. This trend was also explained with the aid of the glass-bridging model and invoking the equivalence between bridging and damage zones. The elevated temperature FCG behavior of nitride-based ceramics which exhibit grain bridging in the wake during crack propagation was discussed and contrasted with oxide-based ceramics which show glass bridging.     

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.     

Model for Fatigue Crack Growth in Grain-Bridging Ceramics

A model for fatigue crack propagation based on sliding wear of bridging grains is analyzed for polycrystalline ceramics. Taking into account damage development and crack tip energy balance, we have obtained rigorous solutions for equilibrium and compatibility equations in the crack wake under monotonic and cyclic loading/unloading conditions. Fatigue mechanics in ceramics is found to be formally similar to elastic-plastic mechanics of a path-dependent hardening material, due to the frictional resistance to reverse sliding. It features a load-displacement hysteresis causing energy dissipation and wear, and a longer cohesive zone required for supporting the same peak load with the wear-reduced bridging stresses. The unloading crack opening displacement is more strongly dependent on K _max than on Delta K ; such displacement causes wear on the bridging grains. Meanwhile, incremental crack growth brings in new bridging grains that has a shielding effect on the crack tip stress field; such an effect is strongly dependent on K _max but independent of Delta K . At steady state, when shielding accumulation and shielding degradation are balanced, the fatigue crack growth rate has a form d a /d N = A ( K _max) ^b (Delta K ) ^c , where A, b , and c are material-dependent parameters. Fatigue is predicted to have a very high b , a modest c , a higher fatigue resistance for tougher ceramics, and a stronger K _max dependence for less tough ceramics. These predictions are in agreement with experimental observations.     

Crack-Bridging Analysis for Alumina Ceramics under Monotonic and Cyclic Loading

Frictional degradation of grain-localized bridges behind a crack tip has been recognized as the major cyclic fatigue mechanism in alumina ceramics. Such a fatigue mechanism implies that the crack growth resistance ( R ) curve behavior during cyclic fatigue is different from that of monotonic loading due to the reduction in crack-tip shielding. A recent crack-bridging theory based on crack compliances is used to study the bridging stresses under monotonic loading and during cyclic fatigue. The bridging-stress distributions of two coarse-grained aluminas under monotonic loading are determined using compliance measurements. Because the interlocking grain bridges at the crack wake are subject to frictional damage from cyclic loading, the bridging-stress distribution evaluated during cyclic fatigue is distinct from that for monotonic loading. These results indicate that it is incorrect to incorporate the R -curve behavior from monotonic loading to the analysis of cyclic fatigue of alumina ceramics.     

Fatigue Behavior of Alumina–Zirconia Multilayered Ceramics

The influence of sustained and cyclic loading on the crack growth behavior of a multilayered alumina–zirconia composite exhibiting high internal compressive stresses is investigated. The study was conducted on precracked notched samples and focused on evaluating the static and cyclic fatigue resistance to crack extension beyond the first arresting interface (threshold) as well as the mechanisms involved during stable crack growth through the layered structure for each loading condition studied. Although it is found that the layered composite is prone to subcritical crack growth, the effectiveness of operative toughening mechanisms, i.e., compressive residual stresses as well as crack bifurcation and delamination at interfaces, is observed to be independent of the loading conditions. As a consequence, fatigue degradation of the multilayered ceramics studied is restricted to the intrinsic environmental-assisted cracking of the individual layers, pointing them out as toughened composites practically immune to variable stresses and much less static and cyclic fatigue sensitive than other structural ceramics.     

Fatigue peeling at rubber interfaces

Abstract

A fatigue peeling test has been developed to evaluate the failure of rubber to rubber interfaces under cyclic loading. Results obtained through this method have been compared to those of a typical fatigue crack growth experiment. The results show that the trends between these two failure modes are similar with the peeling necessary to drive the crack being slightly higher than the strain energy release rate at the same crack growth rate. Cyclic and time dependent contributions to the fatigue crack growth behaviour have been calculated using this test for an styrene–butadiene rubber compound and the results appear to be consistent with previous work although the origin of the cyclic contribution remains uncertain. The influence of pressure at the interface during vulcanisation has also been investigated and it has been observed that the fatigue peel behaviour is proportional to the surface area of contact developed during the curing cycle.     

Detection and Analysis of Crack Propagation in PZT Multilayer Ceramics

The flaw propagation in Lead zirconate titanate (PZT) multilayer ceramics under mechanical load was examined using impedance spectroscopy and three‐point bending studies. Initial flaws were generated by applying a positive sinusoidal electric field to the specimens. The cracks were sequentially propagated and after the release of the external mechanical load, impedance spectroscopy was conducted. The shift in the resonance frequencies and the subresonance height of the impedance spectroscopy were used as a measure of flaw extension. A functional dependence of the resonance frequency and the phase shift on the crack length was found. The crack propagation was studied on flaws starting at the positive and negative electrode, respectively. The maximum fracture strength, as well as the crack path, depends on the electrode potential. The variation in the fracture strength was caused by different observed fracture mode: interface cracking, matrix‐cracking, or a combination of both. The morphology of the fracture surfaces was ascribed to a textured microstructure, which is created by the sample processing, for example, by the poling process. A modified poling procedure with a lower poling temperature was analyzed, which yielded a reduction of the anisotropy of the electrode strength. Impedance spectroscopy was found to be a reliable measurement tool for automated flaw detection in PZT multilayer ceramics.     

Fatigue Crack Growth of Silicon Nitride at 1400°C: A Novel Fatigue-Induced Crack-Tip Bridging Phenomenon

high-strength Si₃N₄with elongated β-Si₃N₄ and equiaxed α-sialon was tested in cyclic and static fatigue at 1400°C. At low stress intensity factors and high frequencies, the pullout process of the elongated grains was enhanced, which suppressed the crack growth. This provides a possible explanation for the increased lifetime under cyclic leading conditions reported for ceramics by several investigators. While crack-healing by high-temperature annealing was found to greatly reduce the subsequent static fatigue crack growth rate, it had only a modest effecf on cyclic fatigue and none at high frequencies.     

Acoustic Emission from PZT 5A Ceramics during Electric Fatigue

Variation of Acoustic Emission (AE) and piezoelectric parameters has been studied during electric fatigue in commercial PZT 5A ceramics. AE was found to occur mainly due to domain switching in these ceramics during the application of ac fields. In general, AE activity was found to decrease as fatigue increases and a threshold field, which increases with increasing fatigue, was observed for these signals to occur. These are explained on the basis of domain pinning. Time frequency distribution analysis performed on AE signals captured at different fatigue cycles revealed microcracking to be another source mechanism for the observed AE apart from domain switching. Additionally, this analysis has shown that microcracks might facilitate the domain switching and vice versa.     

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.     

Fatigue Crack Growth in Structural Adhesives

Fatigue crack growth rate studies of two high temperature structural adhesives, EA-9649 and AF-163, are described. It is shown that crack closure loads develop as a result of fatigue crack propagation. The fatigue crack propagation rates have been correlated with the strain energy release rate range, AG and with an effective strain energy release, ΔG_aff. The effective strain energy release rate range subtracts the strain energy due to crack closure from the applied strain energy release rate range.