When a notched 3D carbon carbon specimen is subjected to tension,the failure mode will change with the change of off-axis angle.If the off-axis angle is sufficiently large,the notched specimen will break along the fiber bundle nearest to the notch tip or perpendicular to the fiber bundle depending on which layer was conesrnod,oaly one fiber bundle is sufficient to cause total rupture,catastrophic regardless of the failure modes.Finite element analysis was performed to find the real failure mechanism for the case of tension with large off-axis angle.The analysis leads.to the conclusion that the failure of that case is dominated by the shear strength of the fiber bundle.The shear strengths of fiber bundle and matrix were measured with specially designed setups.Tofind the stressas in the fiber bundle immediately in front of the notch tip,the FEM plane stress analysis was conducted with 3D carbon carbon taken as homogeneous anisotropic material to find the displacements around the notch.With these displacements as boundary condition,the three dimensional FEM analysis of tip stresses were completed with the local region taken as hetarogeneous material.Similar to the average stress criterion,a portion of fiber bundle vas taken as element and the stress at the center of 8-node isoparametric element was used to predict the notched strength of 3D C/C.Assuming the probability of at least one breakage of fiber bundles is egual to the failure probability of whole 3D C/C,Zweben's statistical strength criterion was employed to predict the strength.To compare with the experimental results,26 straight specimens were tested.The comparison justifies the modelling used in this paper.
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