Failure Modes of a Unidirectional Ultra-High-Modulus Carbon-Fiber Carbon-Matrix Composite

The objective of this study was to observe the effects of various microstructural features on the in situ, room‐temperature tensile fracture behavior of an ultra‐high‐modulus, unidirectional carbon/carbon (C/C) composite as a function of processing heat‐treatment temperature (HTT) over the range of 1100° to 2750°C. An in situ SEM flexural stage was used to observe the interactions between the advancing crack tip and the microstructural features in the frontal process zone. Following the lowest HTT of 1100°C, failure is dominated by the well‐bonded brittle matrix; a tortuous crack path in the E130 fibers appears to contribute to a relatively high utilization of fiber strength in spite of this brittle‐matrix failure. Approximate calculations of the interfacial shear stress that might be generated by matrix shrinkage during pyrolysis of the polymer to carbon were compared to approximations of crack‐tip interfacial shear stresses (IFSS) using the Cook‐Gordon approach. The results suggest that the strong bonding in the 1100°C HTT composite cannot be accounted for by friction alone, and, therefore, chemical bonding or some type of fiber‐matrix mechanical interlocking must be involved. Higher HTTs lead to progressive weakening of the fiber‐matrix interface, and, with heat treatment to 2150°C, multiple matrix cracking (MMC) is observed. Using the crack‐spacing model of Aveston, Cooper, and Kelly (ACK), an IFSS of 1 MPa is estimated for the MMC case. Attempts to calculate the matrix failure strain using the ACK formulation led to a large overprediction of the failure strain, although a number of the parameters used in the calculation are known only very approximately. Heat treatments to 2400° and 2750°C led to longitudinal intramatrix cohesive failure; at 2750°C, this damage is extensive and results in composites with strength utilizations approaching those of dry fiber bundles.