A Methodology to Assess Craze Initiation in PVDF Pressure Liners

Crazing has been observed in flexible pipe PVDF pressure sheaths in operation. Whilst no loss of containment has been identified, Baker Hughes and Equinor have collaborated with Enodo and NTNU to investigate the root cause of the mechanism and develop a predictive framework. We developed a methodology involving a combination of finite element (FE) simulations and high-pressure tests to establish a craze capacity curve for the material. A custom polymer material model was implemented in Abaqus and calibrated using various tension and compression tests covering a wide range of stress triaxiality ratios and strain rates. Crazes were generated in circular discs using a custom-built rig for high-pressure testing. Water was used to pressurize one side of the discs up to 2000 bar. The stress and strain state in the discs were varied by changing the disc thickness, disc radius, backing plate geometry, and the applied pressure. We simulated the high-pressure tests and compared the deformed shape of the discs between simulations and tests. Numerous simulations and corresponding tests formed the basis for a material capacity curve predicting the onset of craze formation. The material capacity curve is a function of the applied fluid pressure and the principal strain difference.