| Continuous fiber-reinforced composite materials have the highest specific strengths and specific moduli among all engineering materials. However, it is well known that the longitudinal compressive strength is poor relative to the tensile strength in continuous fiber-reinforced composites. The prediction of the compressive strength has been addressed in the past by models incorporating the fiber as a beam-on-an-elastic-foundation. Such theories, based on linear elasticity, grossly over-predict the compressive strength and their poor validity has been explained by invoking factors such as the initial waviness of fibers in real composites, and non-linearity of the matrix. Both effects will tend to lower compressive strength. However, there has been no theory that can quantitatively explain the lower compressive strength in real composites.; The present study treats the compressive strength prediction of continuous fiber-reinforced composites using a different approach. Here, the matrix surrounding the fiber is not treated as being homogeneous, but is divided into a discrete interphase region adjacent to the fiber and a bulk matrix phase beyond the interphase. In real composites, experimental observations have indicated that the matrix in-situ does not have the same microstructure as in the bulk. This is due to different cross-link densities in thermosetting matrices, such as epoxies, or different crystallinity content in thermoplastic matrices such as polyetheretherketone, near the fiber surface.; In the present investigation, the interphase is incorporated in a three-phase model (fiber, matrix, interphase) for predicting the compressive strength of composites. It is shown that fiber initial waviness and matrix non-linearity cannot account for the magnitude of the reduction in the compressive strength seen in real composites. The three-phase model, verified in this study both analytically and numerically using a two-dimensional plane strain finite-element analysis, is shown to provide a more accurate prediction of the compressive strength of typical high-performance composites, such as carbon fiber-reinforced epoxy. The results of this study are validated by the extensive experimental database available in the literature. |
