Test-based equivalent-material method for collapse qualification of helically wound and layered cylindrical structures - with application to flexible-riser carcass

Reliable qualification of collapse resistance is critical across pressurised cylindrical shells ranging from offshore risers and subsea cables to aerospace tanks and even biological ducts. Existing approaches either rely on costly hyperbaric chambers or oversimplified geometric surrogates that neglect the formation-induced residual stresses and imperfections. Here, we propose a back-inferred equivalent-material method (EMM), which transforms a simple benchtop flat-plate compression test into a nonlinear constitutive law that embeds cold work, residual stress, friction, and small-scale compliance implicitly. Embedding this equivalent law into a homogeneous finite-element model reproduces hydrostatic collapse without the need for full-scale testing. We demonstrate the method on flexible-pipe carcass layers and steel-strip reinforced thermoplastic pipes, showing close agreement with explicit-geometry simulations and hyperbaric measurements. More broadly, the geometry-agnostic formulation enables rapid, low-cost, and physically faithful qualification across diverse layered shells, offering a general route to replace hyperbaric testing in industries spanning energy, aerospace, and biomedical engineering.