Influence of the fiber length on the stress transfer from glass and carbon fibers into a thermoplastic matrix in the pull-out test

The stress field arising in the single fiber pull-out test is analysed within the framework of the linear theory of elasticity by means of the finite element method, utilizing an idealized model. The finite element analysis is compared with the frequently employed shear lag theory. It is shown that the stress distribution in the vicinity of the fiber is extremely inhomogeneous with strong concentrations in the boundary layer, at the matrix surface as well as at the fiber end, which are not predicted by the shear lag theory. The influence of the fiber length to diameter ratio on the stress field in the fiber and in the interface is analysed for an E-glass fiber and a high-modulus carbon fiber embedded in a polycarbonate matrix by varying the length to diameter ratios between 2.5 and 15.