Studies On Partial Discharge And Its Effects In Dielectric/Piezoelectric/Electrostrictive Materials With Defects

Smart materials (such as piezoelectric materials, electrostrictive materials) and the related deviceshave been widely used in the field of modern high technology, due to their unique advantagescompared with the traditional materals. However, a variety of defects, such as holes, cracks and pores,inevitably exist during the production and manufacturing process. Existence of those defects results inhigh electric field and stress concentration when the materials are subjected to the applied mechanical,electrical, magnetic and temperature loadings, and it may lead to crack growth. On the other hand, thepartial discharge and a series of discharge effects may be induced when the electric field inside thedefects exceeds the critical electric breakdown field, finally resulting in fracture or failure of thesematerials.Based on the complex variable theory and combined with Stroh formalism and the critical electricbreakdown conditon in the view of Paschen law, in this paper we study the electric breakdown probemsin the dielectric materials, piezoelectric materials and electrostrictive materials containing differentdefects, respectively. The electric field inside the defects, the criteria for the occurrence of partialdischarge and the effects of discharge are analyzed, respectively. This paper consists seven chaptersand the main contents are as follows:In the first chapter, a brief introduction to the electric induced failure problems of dielectricmaterial, piezoelectric materials and electrostrictive materials is given and the problems needed to besolved are outlined.In the second chapter, the electric breakdown within the defects of those materials is summarizedfrom the different aspects of discharge, such as the discharge style, the discharge mechanism and thedischarge effects. And the critical condition for the dielectric materials, piezoelectric materials andelectrostrictive materials containing defects is introduced based on the law of Paschen.In the third chapter, the thermal stress induced by partial discharge inside an air-containing cavityin an infinite dielectric material is studied. In the solution process, the complicate discharge effects aresimplified as the thermal effect, and then the thermal stress induced by the heat dissipation is calculatedunder the assumption that all the electric energy stored inside the cavity will completely be convertedto thermal energy, which is uniformly distributed along the whole surface of the cavity.In the fourth chapter, the electric breakdown and its effects inside the crack of dielectric materialsare analyzed. According to the complex potential theory and the semi-permeable electric boundarycondition, the electric field inside the crack is derived, and then the critical condition for Townsend-type discharge is obtained. Considering the impact of the electric filed and discharge on thepropagation of the crack, the energy release rate of the crack is also given with and without theoccurrence of partial discharge.In the fifth chapter, the electric breakdown in the piezoelectric materials containing elliptic holeand crack is studied, and the effects of discharge inside the defects on the fracture of piezoelectricmaterials are discussed. By using the Stroh formalism, the electric field and the criterion of electricbreakdown inside the elliptic hole and crack are derived. Meanwhile, based on the energy release rate,the impacts of mechanical stress and the applied positive or negative electric field on the propagation ofthe crack are investigated.In the sixth chapter, the influences of the mechanical, electric loadings and the dielectric constantswithin the crack and the remote environment on the discharge in the electrostrictive materials arediscussed. According to the complex potential theory and the semi-permeable electric boundarycondition, the cubic equation containing the electric displacement inside the crack is first derived andthe electric field is obtained under the electric displacement and electric field relation. Then, the criticalbreakdown condition is derived. Finally, the numerical results are shown in the condition of differentenvironments under remote mechanical and electrical loadings to discuss the effects on the propagationof the crack.Finally, in the seventh chapter, the present work is summarized and the future works on the topicare prospected.