Temperature Dependent Fracture Toughness of KNN-Based Lead-Free Piezoelectric Ceramics
25 Pages Posted: 15 Apr 2019 Publication Status: Accepted
Abstract
The fracture toughness of lead-free KNN-based piezoelectric ceramics with the composition of 0.92KNN-0.02Bi0.5Li0.5TiO3-0.06BaZrO3 was investigated by near-tip crack opening displacement (COD), surface crack in flexure (SCF), as well as single edge V-notch beam (SEVNB) techniques. Results reveal that the intrinsic fracture toughness (KI0), evaluated by the COD technique, is the lowest with a value of 0.70 MPa∙m0.5. The long (through-thickness) crack fracture toughness (KIvnb), obtained by the SEVNB technique, is the highest, with a value of 0.95 MPa∙m0.5. Intermediate short surface crack fracture toughness (KIsc) of 0.86 MPa∙m0.5 was determined by the SCF technique. These results were rationalized by the toughening behavior of the material combined with the crack geometry-dependent stress intensity evolution during crack propagation. With increasing temperature, KIvnb and KIsc decrease. At 350 °C, they are nearly identical, suggesting an absence of toughening. Nonlinear stress-strain analysis of the material was also employed during compression and four-point bending in order to quantify crack tip shielding by domain switching and in order to quantify the actual stress at the point of instable crack propagation.
Keywords: potassium-sodium niobate, lead-free piezoceramics, fracture toughness, domain switching, toughening
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