13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Figure 4. The flow chart for the VUMAT To study the damage process, the damage model is introduced to the fiber yarns with VUMAT by Fortran, and the flow chart is shown in Fig. 4. Here the maximum stress criterion and linear stiffness degradation are chosen as the damage initiation criterion and damage evolution law of the fiber yarns. 3.3. Yarn/matrix interface The interface zone plays a key role in the mechanical behavior of composite materials. Its properties can be influenced by different fiber coatings such as PyC, BN or SiC and its thickness. And the most important mechanism for improving the toughness of CMCs is the crack deflection along the interface after the initiation of the matrix cracking. In this meso-level FEM model, the inner-yarn interface is not considered, and fiber bundles is simplified as transversely isotropy. In this work, bilinear models both for normal and tangential separation is used to simulate the response of the yarn/matrix interfacial zone. The curve of normal and tangential tractions with respect to n andt are shown in Fig.5, where max and max are the interface normal and tangential strength, are 200Mpa and 150Mpa respectively;max is the interface characteristic length parameter; n andt denote the non-dimensional normal and tangential displacement respectively. Figure 5. The curve of normal(left) and tangential(right) tractions with respect to n and t In the FE model, CZM was realized by either cohesive element or cohesive behavior and maximum stress criterion was used to predict the damage initiation in Abaqus/explicit, and detailed constitutive model for the CZM can be found in the help documentation[24]. 4. The result analysis
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