A fracture mechanics assessment of surface cracks existing in protective layers of multi-layer composite pipes

The influence of a cracked protective layer on the integrity of composite multi-layer pipes is analyzed in this paper. The multi-layer pipe is modeled as a particular case of a bi-material body consisting of two different homogeneous materials. The problem is studied under assumptions corresponding to small scale yielding conditions and ideal adhesion between protective and main layers is assumed. The case of a crack with its tip at the interface between protective and main layers is analyzed in detail and the influence of the material composition on the corresponding critical pressure for a brittle-like failure is discussed. An approximate approach based on estimation of crack mouth opening displacement for thin protective layers is suggested. The results have been applied to study the scratch resistance of multi-layered plastic pipes.