Effect of Fiber Coating and Interfacial Debonding on Crack Growth in Fiber-Reinforced Composites

The problem of a crack impinging upon an interface between dissimilar materials is investigated using a Consistent Shear-Lag (COSL) model. The purpose is to determine whether the crack will penetrate the interface or be deflected into it. This is important in fiber-reinforced composites since fracture resistance can be enhanced when deflection is favored over penetration. To determine the mode of crack growth, the energy release rates associated with each mode (penetration and deflection) are calculated and the ratio of energy release rates is compared to the corresponding ratio of toughnesses. Results are given for fiber-reinforced composite materials. Since many practical composites are produced using coated fibers, the energy release rate ratio is given for various types of coatings and varying coating thicknesses. It is possible to infer that coating thickness has a small effect, but that the best toughness can be achieved from the thinnest possible coating. Since realistic composite materials often contain inherent flaws such as debonding at the fiber-matrix interface, the previous formulation is modified to include a crack impinging upon an initially debonded interface. It is shown that proximity of the crack tip to the debond is very important in predicting the mode of crack growth. © 1992