Stresses around fiber ends at free and embedded ply edges

Using the finite element method, the stress state around fiber ends at a free surface in a unidirectional fiber composite was studied. The material is subjected to a thermal load consistent with a cryogenic application as well as ply level transverse tensile loading associated with thermal stresses in a laminate. At the free end the fiber–matrix interface sees tensile radial stresses as well as shear stresses. This makes the fiber end a likely site for interface debonding (crack initiation). These initial cracks can then link up and grow to become full through-thickness matrix cracks. The effect that an epoxy coating over the fiber ends has on the local stress distribution was also considered. These capped models simulate fiber ends embedded within a laminate. The epoxy cap has the effect of suppressing the interface stresses at the fiber end. The significance of these results in relation to cryogenic cycling tests using coupon specimens with free and embedded edges is discussed.