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CFCC Composites Code - Axial Bundle Damage

Axial bundle damage develops in a series of stages. The first stage is the post-fabrication stage in which the composite has a distribution of existing matrix flaws. The second stage, after the application of a tensile load, is the onset of local damage at the "most critical" flaw. Next, as the load increases, damage accumulates as the initial crack propagates and secondary cracks develop and propagate. The next stage may be one of two possibilities, either matrix crack saturation or fiber crack initiation. Matrix crack saturation occurs at the stress level at which matrix cracks are so close together and fiber/matrix debonding is so extensive that the matrix can no longer carry any portion of the applied load (i.e., the load is carried solely by the fibers). Fiber crack initiation may occur before or after matrix crack saturation when the local fiber stress (due to applied load and fiber stress concentration at matrix crack sites) exceeds the fiber strength. Fiber cracks then continue until the applied load is too large to be carried by the remaining fibers and the composite ultimately fails.

The approach taken to simulate this process employs a combination of micro-mechanical and statistical models. The micro-mechanical models (Aveston, Cooper and Kelly, 1971; Aveston and Kelly, 1973) evaluate fiber and matrix stresses in the presence of transverse matrix cracks (i.e., perpendicular to the fibers) and fiber/matrix debonding. The statistical models account for the statistical strength distributions of both fiber and matrix. This combined approach is employed in order to assess the damage progression in both the fibers and the matrix and is integrated into the damage model algorithm given below.

The algorithm utilized in the model proceeds as follows:

1. Input of bundle description (i.e., fiber, matrix, volume fraction, etc.) and calculation of initial properties;
2. Input of statistical paramters describing fiber, matrix and fiber/matrix interface;
3. Application of incremental load;
4. Calculation of damage parameters (i.e., matrix and/or fiber crack densities and fiber/matrix debonding) at the specified stress level;
5. Calculation of instantaneous bundle properties based on damage parameters.
6. Continue iterative process from step 3.

© 1995, M.S.C.