The main cause for failure of composite laminates is delamination under shear or transverse loading. A common approach to prevent such failure is to introduce a discrete interleaf at the midplane of the laminate in order to increase its fracture toughness and, consequently, its resistance to delamination. Accordingly, a study of Mode I fracture toughness of ultra-high molecular weight polyethylene fibers/polyurethane matrix composite laminates interleaved with thin polyurethane films reinforced with either untreated or functionalized carbon nanotubes is presented here. The results show that-depending on the surface treatment of the carbon nanotubes-the introduction of an interleaf at the midplane of the laminate generates a significant improvement of the Mode I initiation and propagation fracture toughness compared to the non-interleaved laminates and to laminates interleaved with un-reinforced polyurethane films. The Mode I fracture toughness results correlate with the "trouser-leg" fracture surface energy of the films.

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