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The notch insensitivity of carbon-carbon composites (C/Cs) has been believed to result primarily from shear damage near sources of stress concentration. To evaluate this hypothesis, notch sensitivity has been examined for C/Cs with crossply laminates (CP-C/Cs) and quasi-isotropic (QI-C/Cs) laminates. The main difference in both laminates involves their shear behavior: the QI-C/Cs have an almost-linear stress-strain curve and high strength, whereas the CP-C/Cs exhibit strong nonlinearity and low strength. Thus, the effect of shear damage can be extracted by comparison of both materials. Experimental results from the present study have shown that the fracture behaviors of both C/Cs are quite similar. Finiteelement analyses also have revealed that the stress redistribution caused by shear nonlinear deformation is too small to explain its toughening behavior, even in the CP-C/C. From these results, it is concluded that the notch-insensitive behavior of the C/Cs cannot be explained by the already-proposed shear-damage mechanism. To this end, a discussion has been conducted on a new possible toughening mechanism that is capable of generating the R-curve and notch insensitivity.

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