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Fracture mechanics approach to forecast delamination lifetime

Interlaminar fracture (delamination) is one of the major concerns in the design of laminated composite structures, adhesive joints, coatings, sealants and other multilayered material systems. Service lifetime of a laminated structure is limited by the time an interlaminar flaw propagates to a size perceived critical to the stiffness and/or the strength of the structure. The time required to cause certain magnitude of delamination, under stresses below the initiation stress, could be forecasted if the constitutive equation for the rate of delamination is known. This paper describes an approach to develop the constitutive equation for delamination under mode I conditions. The approach rests on principles of linear elastic fracture mechanics (LEFM) and uses elevated temperature to accelerate interlaminar fracture at constant loads. The experiments used double cantilever beam test specimens fabricated as a model system from poly(methyl methacrylate) (PMMA) beams and epoxy adhesive whose stiffness was equivalent to that of a typical carbon/epoxy laminated composite. Mechanistic observations indicated that the fracture front displayed similar mechanism at all test conditions. A modified form of Paris power law is suggested to forecast service lifetime in terms of temperature, service load and the initial flaw size.