DAMAGE TOLERANCE OF OPEN-HOLE CFRP LAMINATES LOADED IN COMPRESSION

The static compressive strength is measured for a wide range of T800/924C carbon fibre-epoxy multidirectional laminates, containing a circular hole; the proportion of O-degrees plies is varied from 17% to 67%. It is observed that the remote failure stress is generally well above the value one might predict from the elastic stress-concentration factor K(t), suggesting that the composite material is not ideally brittle and some stress relief occurs around the hole. X-ray radiography reveals that damage in the form of 0-degrees fibre microbuckling, delamination and matrix cracking initiates at the edge of the hole at approximately 80% of the failure load and extends stably under increasing load before becoming unstable at a critical length of 2-3 mm. This damage reduces the stress concentration at the edge of the hole by degrading the local material properties and delays final fracture to higher applied loads. The damage growth and failure are analyzed by a new fracture model (Soutis and Fleck, 1990, J. Compos. Mater. 24(5), 536-558) in which damage around the hole is represented by a through-thickness crack. Using the independently measured laminate parameters of compressive unnotched strength and fracture toughness, the model successfully predicts the notched strength of various multidirectional lay-ups and a wide range of hole sizes.