Switch-toughening of ferroelectrics subjected to electric fields

Electric fields can influence the fracture toughness of ferroelectrics. For example, poled ferroelectrics exhibit fracture toughness anisotropy: the material is tougher for a crack parallel to the poling direction but less tough for a crack perpendicular to it. When an electric field is applied to a poled sample, a positive field reduces its fracture toughness but a negative field enhances it. Previous investigations attribute these phenomena to polarization switching. This paper proposes a model of stress-assisted 90 degrees polarization switching to quantify the toughening process. Small scale switching and uniform electric fields are assumed. An analytical solution is presented for a mono-domain ferroelectric crystal undergoing a confined polarization switch. This solution and the domain orientation pattern enable us to estimate the fracture resistance against the steady stale crack growth in ferroelectrics by a Reuss-type multiple-domain assembly. A dimensionless group of material parameters and an electric field function emerge, and form the key ingredients of switch-toughening. The model is used to delineate several observations, including: poling-induced anisotropy of the fracture toughness, asymmetric variation of the fracture toughness under positive and negative electric fields of a poled specimen; upside-down butterfly loop for the fracture toughness response under cyclic electric loading. (C) 1998 Elsevier Science Ltd. All rights reserved.