A 3-DIMENSIONAL MICROMECHANICAL MODEL OF THE COMPRESSIVE BEHAVIOR OF UNIDIRECTIONAL FRP COMPOSITES

A 3D model of fiber microbuckling assuming a square packed fiber array has been developed and analyzed under the assumption of fiber in-plan e microbuckling. In this model, the fiber equilibrium equation is firs t developed as a function of uniaxial loading, material properties, an d fiber/matrix interfacial stresses. By utilizing 3D boundary element modeling, the 3D stress distribution along the fiber/matrix interface is determined. The interfacial stresses obtained from the 3D boundary element analysis are then incorporated into the 3D equilibrium equatio n of fiber microbuckling to provide a closed-form analytical solution of the 3D compressive fiber strength in unidirectional composites. Res ults show that the in-plane shear stress component is predominant, whi le one of the two out-of-plane stress components, which cannot be capt ured by a 2D model, is not negligible. Furthermore it is found that th e in-plane interfacial shear stress is strongly dependent upon fiber s pacing. These results indicate that the results for a 2D model are qui te different from those of the 3D model. Therefore, the 3D model of co mpressive behavior of unidirectional composites is necessary in order to properly model the real interfacial stress distribution in a unidir ectional composite subjected to axial compressive load. Because of the strong dependence of interfacial shear stress upon fiber spacing, mor e accurate calculation of in-plane interfacial shear stresses and theo retical fiber microbuckling strength will result from a 2D model if fi ber spacing in the model is set to match that of a 3D model for a give n 3D model-based fiber volume fraction.