13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- and (8). Figure 7 presents the evolution of the strain energy U with respect to the crack mouth opening displacement measured at the center of the crack (Z = 0) for all four plate specimens. The evaluation of the energy release rate requires the determination of the dimensionless η-value. This study determines the η-value using FE models with stationary cracks via equating Eqs. (4) and (6). The strain energy value U in Eq. (4) is computed using the strain energy derived from the area under the moment-rotation curve of the crack plane computed from the large-deformation finite element analyses, as shown in Eq. (7). Figure 8 illustrates the variation of the η-value as the applied load increases for the four plate specimens. The η-value remains approximately at the same magnitude at different load levels in all four specimens. The current study utilizes, therefore, a constant η-value of 1.1 for all plate specimens. Figure 8. The dimensionless η-values for the plate specimens. Figure 9. Comparison of the energy release rate for the surface cracked plate specimen SP1. Figures 9 and 10 compare the energy release rate computed from the finite element model and those measured from the experiments for two selected specimens SP1 and SP4. The average J-values computed from the finite element analysis in Figures 9 and 10 correspond to the plate models with a stationary crack. The comparison in Figures 9 and 10 therefore limits the comparison for the
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