Effect of Shear Damage on the Fracture Behavior of Carbon–Carbon Composites

Abstract

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.

Intrinsic parameters in the fracture of carbon/carbon composites

“…For example, distinct crack advance and bridging may coexist with shear matrix cracking in an otherwise Class III composite. This has been proven in the work of Goto et al [9] who directly identified crack extension in a C/C composite using a travelling microscope and of Pappas and Kostopoulos [14] who ultrasonically and optically confirmed the presence of crack growth and bridging in the same 2D C/C woven composite as the one used in this study. At the same time it should not be overlooked that the crack extension calculated for the CT specimens in this study is based on the notion of an ''effective crack length'' where the total damage forming and evolving in the material during testing is assumed to be equivalent to a distinct crack propagating through the matrix.…”

“…A characteristic example are C/C composites where multiple shear cracks appear in the matrix during composite fracture and result in a complex bridging zone of dimensions that can hardly be measured. On the other hand, there are indications that shear cracking coexists with distinct matrix cracks forming and growing in such materials [9]. As a result, it is currently unverified whether C/C composites are purely Class III materials and whether their fracture behaviour is compatible with LSB or SSB (small scale bridging) conditions.…”

“…the local opening of the crack flanks. The bridging law concept is in a sense a measure of the closure traction normalised to the local geometrical and dimensional characteristics, and as such it has been proposed to serve as a basic material law [6][7][8][9][10].…”

Carbon–Carbon Composites

“…Based on their study, the candidates for the damage mechanism were fiber pull-out and shear damages. However, many other experiments showed that the mechanism of the high notched strength was not the shear damages but the zigzag cracking or the fiber bundles splitting on the net cross section [6,12,14]. Moreover, the conflicts widely existed in the 2D-C/Cs, while few reports were documented on the notch effects for the 3D-C/Cs.…”

“…Recently, the notch effects of 2D-C/Cs have been systematically studied [10][11][12][13]. Kostopoulos and Pappas [11] hypothesized the existence of a damage zone in the hole tip and they considered that the stress in damage zone was equal to the unnotched strength.…”