Elastic properties were predicted for AS4, IM7, T300, and T650-35 graphite fibers. An inverse method was employed using lamina and epoxy properties taken from literature. Fiber properties predicted using finite element analysis based on a hexagonal microstructure, finite element analysis based on a random microstructure, and the Mori-Tanaka averaging scheme were compared. It was observed that the Mori-Tanaka averaging scheme and finite element analysis based on a hexagonal microstructure predicted nearly identical fiber properties. In contrast, randomness in the arrangement of fibers results in significantly different predictions for the longitudinal shear modulus of the fiber. The three microstructural models were also used to predict properties for isotropic E-glass 21xK43. It was observed that the three models predicted nearly the same properties for the glass fibers.
Failure initiation locations were determined in the tows and matrix pockets of a plain weave textile composite for a wide variety of loading conditions in an attempt to identify characteristic failure initiation sites. Failure was found to initiate in a limited number of locations in the composite for a wide variety of loading conditions. It was also found that the different degrees of weave undulation studied shared similar characteristic failure initiation sites. Additionally, it was observed that some idealized features of the textile geometry led to fictitious stress concentrations that could artificially bias the predicted failure initiation locations. Steps were taken to reduce the influence of such stress concentrations, including modifying the textile geometry and excluding certain problematic regions of the textile from consideration when searching for failure initiation locations.
An investigation is performed into the interaction between thermally and mechanically induced stresses and their relation to failure in a unidirectional composite under transverse tension and longitudinal shear. Multiple realizations of periodic unit cells with randomly positioned fibers are used. It is found that for microstructures with close fiber spacing, thermally induced stresses from cooling after cure tend to relieve the critical mechanically induced stress in the matrix for transverse tensile load, while they tend to worsen the critical stress for microstructures with space between fibers. Matrix strength properties are obtained through the solution of an inverse problem and are then used to perform progressive failure analyses at various temperatures. These analyses indicate that for random microstructures with closely spaced fibers, cooling after cure tends to increase the transverse tensile strength of the unidirectional composite, while cooling tends to weaken models using a hexagonal microstructure.
A finite element-based model was developed to predict progressive damage evolution within a plain weave textile composite subjected to various combinations of in-plane tension and shear. Cracking in the tows, matrix, and interfaces was accounted for through cohesive zone modeling. Shear damage in the tows was accounted for through a continuum damage model. The damage behavior in the tows was stochastic in nature with properties determined from prior investigations of composite microstructures that included randomness in fiber positions. The predicted progressive damage evolution was found to qualitatively match well with experimental observations performed on similar material systems. The effect of temperature change, which modifies the thermally induced stresses in the tows as well as the apparent strength of the tows (due to changes in thermally induced microstresses at the fiber–matrix scale) was examined. Finally, the progressive failure responses under different loadings were compared to identify common characteristic behaviors. The effect of these characteristic behaviors on the textile’s effective response was investigated along with approaches to incorporate the behaviors into a structural scale progressive failure model.
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