13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 2. Two views of the three-dimensional failure surface from Eq. (13) with the test data shown. The crack is parallel to the poling direction. This curve is plotted with 20.3 = Ic G N/m in Fig. 3. The test values cG , recalculated with ˆ 0= II K , together with the values of ψ are plotted as points in Fig. 3. This may be justified since the values of II Kˆ are small with respect to IKˆ and IV Kˆ except for one specimen. The root mean square error in Eq. (12) was calculated as 1.10 N/m for the three-dimensional surface and 1.05 N/m for the two-dimensional curve. Finally, the crack propagation angle θ measured from a line extending ahead of the crack was obtained for each test. The angle varied between 0 1 and 0 10 . Points for 0 0 10 9 ≤θ≤ showed the greatest scatter from the theoretical curve, although even for 0 5 and 0 6.5 there was some scatter. Generally, smaller propagation angles led to less scatter in the test results as compared to the theoretical curve. There seemed to be no apparent correlation between large values of II Kˆ or Figure 3. Two-dimensional failure curve from Eq. (14) with test data shown. The crack is parallel to the poling direction.
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