13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Figure 5. Material Failure Curve and Crack Driving Force curves for the Fraunhofer specimens (A508 material) at -85°C 4. Discussion of Results The results presented in Fig. 5 reveal that the two parameter J-A2 method does have the potential to i) predict failure, and ii) quantify constraint. The 3PB and biaxial cruciform specimens reveal significant loss of constraint when compared to the high constraint C(T) specimen. The biaxial cruciform exhibits higher fracture toughness (lower constraint) than the 3PB specimen. This may be attributed to the fact that the crack depth of the biaxial cruciform (a/W = 0.08) is more than twice as shallow as the 3PB specimen (a/W = 0.18), which may have overshadowed the biaxial effect. This supports the findings of Lidbury et al. [19] of a shallow crack effect. Fig. 5 also shows the experimental fracture toughness data [10, 11] added to the Crack Driving Force curves in the form of the triangular symbols. The experimental fracture toughness data has been added at the |A2| values corresponding to the predicted critical |A2| to be compared with the Crack Driving Force curves. The average values of the experimental fracture toughness for all three specimens lies near the intersection of the MFC and the specimen CDFs. The constraint effect of each of the specimens can also be quantified and compared using the Crack Driving Force curves. The Crack Driving Force curves in Fig. 5 for the biaxial loading lies to the right of the 3PB specimen, meaning some loss of constraint and increased fracture toughness based on the loading and geometry. For the purposes of comparing the fracture toughness values, a similar FEA and J-A2 analysis of a uniaxial cruciform was conducted as part of the current investigation. The supports in the FEA model on the cruciform arms parallel to the crack, i.e., the horizontal arms shown in Fig. 2, were removed to simulate uniaxial loading. The same procedure was followed to develop the Crack Driving Force curve for the theoretical uniaxial cruciform specimen, which is shown as the red curve in Fig. 6. Clearly, the uniaxial cruciform specimen exhibits significant loss of constraint (increased fracture toughness) compared to the biaxial cruciform specimen.
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