Application of fracture mechanics in electrical/mechanical failures of dielectrics
by Liu Guoning
Ph.D. Mechanical Engineering
xiv, 153 leaves : ill. ; 30 cm
Theoretical and experimental study of the mechanical/electrical fracture behavior of dielectric materials, piezoelectric ceramics PZT 8, PZT 4, and polymeric material poly vinyl chloride (PVC), was made in this work....[ Read more ]
Theoretical and experimental study of the mechanical/electrical fracture behavior of dielectric materials, piezoelectric ceramics PZT 8, PZT 4, and polymeric material poly vinyl chloride (PVC), was made in this work.
The charge-free zone (CFZ) model proposed by Zhang et al. for the failure of conductive cracks in dielectrics, which was well verified by the experimental results of depoled piezoelectric ceramics PZT 4, was extended to predict the failure behavior of conductive cracks in piezoelectric ceramics. Piezoelectric ceramics were treated as mechanically brittle and electrically ductile materials in the charge-free zone model. The failure criterion, developed from the CFZ model, for conductive cracks in piezoelectric ceramics under mechanical and/or electrical loading has an elliptic shape in terms of the normalized electric intensity factor and the normalized stress intensity factor.
To verify the theoretical prediction from the CFZ model for piezoelectric ceramics, experiment was conducted to study the failure behavior of electrically conductive cracks (deep notches) in poled lead zirconate titanate PZT-8 ceramics. When the critical stress intensity factor was normalized by the critical stress intensity factor under purely mechanical loading and the critical electric intensity factor was normalized by the critical electric intensity factor under purely electric loading, the experimental results revealed that the failure behavior of the conductive cracks in the ceramics was described by an elliptic function of the normalized electric intensity factor versus the normalized stress intensity factor under combined mechanical and electric loading. The experimental results verified well the theoretical predictions from the CFZ model.
The relationship between strain (deformation) and electric field (voltage) is linear when electric filed (voltage) is low. The relationship becomes nonlinear at high electric field (voltage). In order to adopt the linear constitutive equations which have simple forms, and to cover the actual applying procedure at the same time, a novel concept of secant piezoelectric constant was introduced to refine the CFZ model.
In addition, the concepts of fracture mechanics, which were successfully applied in the study of poled PZT 4, PZT 8 and depoled PZT 4, were introduced to study the electrical failure behavior of polymeric material poly vinyl chloride (PVC). Two kinds of samples, compact tensile (CT) samples and the double notched samples, were used to obtain the electrical fracture toughnesses. The electrical fracture toughnesses obtained from CT samples were higher than those obtained from the double notched samples. This showed that for polymers, electrical fracture toughnesses were not only related to the tested materials, but also related to the conditions of how the notch tips were electrically biased.