Chapter 4.pdf

After park and sun 1995b with permission from john

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(After Park and Sun, 1995b, with permission from John Wiley and Sons). c 11 (N/m 2 ) c 12 (N/m 2 ) c 13 (N/m 2 ) c 33 (N/m 2 ) c 44 (N/m 2 ) 13.9 10 10 7.78 10 10 7.43 10 10 11.3 10 10 2.56 10 10 e 31 (C/m 2 ) e 33 (C/m 2 ) e 15 (C/m 2 ) 11 (C/Vm) 33 (C/Vm) 6.98 13.84 13.44 6.00 10 -9 5.47 10 -9 Fig. 4.11. Fracture loads under applied electric fields for PZT-4 compact tension specimens. (From Park and Sun, 1995b, with permission from John Wiley and Sons). Fracture Mechanics of Electromagnetic Materials Downloaded from by UNIVERSITI SAINS MALAYSIA LIBRARY on 01/09/13. For personal use only.
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Overview on Fracture of Electromagnetic Materials 93 4.6.3 Bending test To further verify the validity of the mechanical strain energy release rate as a fracture criterion, Park and Sun (1995b) also conducted fracture tests on PZT-4 piezoelectric ceramics using three-point bending specimens with a symmetric crack for mode-I fracture and an asymmetric crack for mixed-mode fracture (Fig. 4.12). The entire setup, including the indenter, was made of Plexiglas to avoid electric discharge. The prepared specimen was placed in the silicone oil tub that was mounted on the MTS machine. The poling direction is parallel to the span of the bending setup. Fracture loads versus applied electric fields were obtained for various crack locations (Fig. 4.13). Fig. 4.12. Experimental setup for three-point bending specimens under combined mechanical and electrical loadings. Fracture Mechanics of Electromagnetic Materials Downloaded from by UNIVERSITI SAINS MALAYSIA LIBRARY on 01/09/13. For personal use only.
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Fracture Mechanics of Electromagnetic Materials 94 Fig. 4.13. Fracture loads under applied electric fields for various crack locations in PZT three-point bending specimens. (From Park and Sun, 1995b, with permission from John Wiley and Sons). It appears that the center-cracked three-point bending specimens exhibit the same fracture behavior as the compact tension specimens. Specifically, the fracture load has an odd dependence on the applied electric field – positive electric field aids crack propagation, while negative electric field impedes crack propagation. The three-point bending specimens with an off-center crack also exhibit the same trend. Later, Soh et al . (2003) used central crack specimens to study the effects of an applied electric field on the fracture toughness of poled piezoelectric ceramics and demonstrated that changing the applied electric field from negative to positive reduced the fracture toughness of poled PZT-5 ceramic, which is consistent with the observations by Park and Sun (1995b). Fracture Mechanics of Electromagnetic Materials Downloaded from by UNIVERSITI SAINS MALAYSIA LIBRARY on 01/09/13. For personal use only.
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