Compression fatigue damage in notched CFRP laminates

This paper reviews the results of recent experimental studies (Soutis 1991, 1994, Soutis & Fleck 1990, Soutis et al. 1991a, b, c, 1993) concerned with fibre microbuckling in T800/924C carbon fibre-epoxy notched laminates subjected to static and fatigue compressive loading. The static compressive strength is measured for a wide range of multidirectional laminates. 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, due to local damage around the hole in the form of 0 degrees fibre microbudding, delamination and matrix cracking; this type of damage initiates at the edge of the hole at approximately 80% of the failure load, reduces the stress concentration at the hole edge and delays final failure at higher applied loads. In compression-compression fatigue, provided that the cyclic stress is below 85% of the notched static compressive strength, drilled holes have no effect on the long life fatigue behaviour, because longitudinal splitting and delamination remove the stress concentration due to the hole. Above this limit, however, fibre microbuckling extends in the transverse direction and causes fracture of the plate when it reaches a length of about 3 mm.