The effect of grain boundaries on the electromechanical response of a ferroelectric polycrystal subjected to an electric field or stress is investigated numerically by using a phase field model. The grain boundaries in the phase field model are regarded as dielectrics in which the ferroelectric properties are degraded completely. The phase field simulations show that the presence of dielectric grain boundaries results in a large build-in depolarizing field in grains. The depolarizing field has a significant influence on the coercive field, the switching behaviour of ferroelectric domain under an electric field or stress, and the piezoelectric and dielectric properties of the ferroelectric polycrystal. It is found that both coercive field and remnant polarization decrease with the increase of the thickness of dielectric grain boundary. However, the piezoelectric coefficient and permittivity of the ferroelectric polycrystal become larger when the thickness of dielectric grain boundary increases. The enhancement of dielectric and piezoelectric properties by the dielectric grain boundary suggests a new degree of freedom to tune the electromechanical response of ferroelectric polycrystalline materials.

No abstract

No abstract

No abstract

To provide the desired thrust and prevent the engine from exceeding any safety or operational limits, a min-max selector with linear limiters is widely employed in current aircraft engine control logic. However, with the further requirements of engine performance, the traditional linear limiters should be improved. Though there are many researchers working on the development of improvement methods, none of those methods consider the limitation of core shaft acceleration. In this paper, a novel control scheme for aircraft engine based on sliding mode control with acceleration/deceleration limiter is proposed. Above all, the controller construction process is introduced, and the asymptotic stability of the whole controller is given. Then, with linearized model of JT9D turbofan engine, the control performance of the new approach is presented, which is also compared with the traditional methods. The simulation results show that the proposed method is efficient, and it can ensure all outputs of the controller, including the core shaft accelerationṄ c , high-pressure turbine outlet temperature increment T 48 , high-pressure compressor stall margin increment SmHPC, and so on, are well controlled. INDEX TERMS Min-max logic, sliding mode control, aircraft propulsion, acceleration/deceleration limit.

The wide use of fir-tree root and groove in turbine structures prompts the expectation to find optimum configurations, which ensure that stresses are in the safe limits to avoid mechanical failure. To perform the optimization, the reasonable characterization of root configuration is required. The existing researches characterized the fir-tree root with straight line, arc or even elliptic fillet, then the parameters of these features were defined as design variables to perform root profile optimization. However, this feature-based optimization technique yields configuration which is only optimal under the feature assumption, the question why choose these feature and whether there is a better feature modeling technique is difficult to answer. In this work, instead of the feature-based method, spline curves technique is involved to characterize the root and groove configuration, and the horizontal coordinates of the control points are selected as design variables, which are modified in the vicinity of their initial values during optimization process. The objective function is to minimize the peak stress in the root and groove regions. With the Multi-island genetic algorithm, the optimal fir-tree root configuration can be obtained with better stress distributions and low stress concentrations. The proposed spline-based optimization approach may shed lights on the conceptual design of blade root and can be easily extended to other industrial equipment design.

The effect of grain boundary conductivity on the poling process and the nonlinear electromechanical behaviors of ferroelectric polycrystals are investigated through the use of a phase field model. The grain boundary is modeled as a semiconductor in the phase field model via Maxwell’s equations, which consider the drift of free charges under an electric field. The simulation results show that the poling electric field of the ferroelectric polycrystal with the semiconducting grain boundary is much larger than that of an insulating grain boundary, which is due to the screening of polarization-induced charges at the grain boundaries. The P-E hysteresis loop becomes narrow, and the ferroelectric property degrades in the presence of a semiconducting grain boundary; this indicates that the grain boundary conductivity has a significant influence on the nonlinear behavior of the ferroelectric polycrystals. On the other hand, the grain boundary conductivity, however, has less effect on the response of the ferroelectric polycrystals to a mechanical load. The present work provides an insight into the effect of the charge leakage, which is induced by the material defects, e.g., the grain boundary, on the electromechanical properties of piezoelectric ceramics.

No abstract