EXPERIMENTAL AND NUMERICAL-STUDIES IN MODEL COMPOSITES .1. EXPERIMENTAL RESULTS

Results on strength, apparent toughness, fatigue crack growth and fibe r debonding on specially made composite materials are reported. The co mpact tension composite specimen used consisted of an epoxy matrix and layers of long aligned glass or kevlar fibers that were equally space d. The experimental data on crack initiation strength showed that for a range of fiber spacing lambda, the composite's strength sigma(A), sc aled with the fiber spacing in the form of sigma(A) root lambda=kappa. The apparent toughness of the composite specimens increased with a de crease in fiber spacing. Two sets of fatigue crack propagation experim ents were performed. The first one was on specimens with the same fibe r spacing and under different applied loads. The second set was on spe cimens with different fiber diameter and the same loading conditions. While crack arrest was observed in the first set, crack arrest was see n in the second set for the relatively large diameter fibers and speci men fracture for the relatively thin fibers. A method, based on fractu re mechanics principles and crack opening displacements, for evaluatin g bridging tractions is outlined. Using this method, simulations for t he bridging tractions and stress intensity factor were carried out usi ng a linear crack opening profile. The total stress intensity factor w as found to decrease with crack length. The debonding in the bridging zone, on specimens with different fiber spacing, was evaluated using a one dimensional debonding analysis. The model was calibrated with the debonding on the first fiber and consequently used to describe debond ing on the bridging zone of specimens with different fiber spacing. In spite of the assumptions adopted in the present studies, the model se ems to describe debonding well.