Shear-lag versus finite element models for stress transfer in fiber-reinforced composites

The stress transfer from broken to unbroken fibers in fiber-reinforced polymer-matrix (PMC) and aluminum-matrix (AMC) composites is studied using a detailed 3D finite element model (FEM) and using the standard shear-lag model (SLM). The stress transfer predicted by the SLM is in good agreement with the FEM results for the PMC at high fiber volume fractions but not at low volume fractions. For the AMC with an elastic/plastic matrix, the FEM results depend on whether the fiber is broken initially (pre-break) or after loading (post-break), while the SLM is independent of the fiber break load. Compared to the pre-break FEM, the stress transfer in the SLM is too low at high volume fractions and fairly accurate at low volume fractions while the reverse trend obtains when the SLM is compared to the post-break FEM. These results suggest that the SLM is generally accurate for high fiber/matrix stiffness ratios and high fiber volume fractions, and/or when global matrix yielding proceeds fiber breaking such that the tangent stiffness of the matrix is small. At low volume fractions, matrix load carrying capacity accounts for much of the discrepancy between the SLM and the FEM, which can be rectified by more sophisticated SLMs. A simple elastic model demonstrates that neglect of the finite fiber dimensions and fiber shear deformation in the SLM can also affect the stress transfer. It is concluded that although the standard SLM cannot generally be used for simulations of composite deformation and failure, it is reasonably accurate for an important range of composite constituent material properties.