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The mechanically contaminated layer theory of sand-filled fibreglass-reinforced polyester pipes: An explanation of the initial process for the mechanical and hydrolytical behaviour of massive pipeline failures

This investigation describes the fracture mechanism which explains a significant part of massive failures due to the existence of a sand layer placed near the neutral axis in the core making the composite very sensitive to impacts in fibreglass-reinforced polyester pipes. These failures create interface delamination, and consequently fluid can leak into supporting sand backfill thereby initiating the fracture process that can, at times include resin hydrolysis. In order to assess the delamination magnitude, an analytic method is developed and a squared root law between delamination and energy applied proposed. Vertical blunt ram tests on samples extracted from complete pipes have been carried out to verify this theory, reaching a goodness of fit up to 92%. It is concluded that low-energy impacts, around 90–160 J in 800–1000 diameter PN 16–20 continuous filament winding pipes, can seriously compromise their structural integrity with no external trace.