VISCOELASTIC EFFECTS ON INTRALAMINAR FRACTURE-TOUGHNESS OF EPOXY CARBON-FIBER LAMINATES

The Double Torsion technique has been applied to characterize the fracture resistance of a unidirectional and a woven epoxy/carbon-fibre composite material. The crack speed has been varied by about four orders of magnitude for testing temperatures ranging from below the glass transition of the rubber modifier of the epoxy matrix to the glass transition of the matrix itself. A noteworthy variation was found in the fracture toughness and surface morphology of the unidirectional composite. Decreasing fracture toughness values were observed by increasing crack speed at 23-degrees-C, which is somewhat unusual for viscoelastic materials. Results have been discussed in the framework of viscoelastic fracture mechanics, and the fracture behaviour at different rates and temperatures has been shown to be related to different viscoelastic mechanisms.