Next Generation Ductile Fracture Arrest Analyses for High Energy Pipelines Based on Detail Coupling of CFD and FEA Approach

Axial ductile fracture propagation and arrest in high energy pipelines has been studied since the early 1970’s with the development of the empirical Battelle Two-Curve (BTC) model. Numerous empirical corrections on the backfill, gas decompression models, and fracture toughness have been proposed over the past decades. While this approach has worked in most cases, the dynamic interaction between the decompression of the fluid in the vicinity of the crack tip and the behaviour of the pipe material as it opens to form flaps behind the crack has been very difficult to solve from a more fundamental approach. The effects of the pressure distribution on the flap inner surface making up the crack-driving force which drives the crack propagation speed has been suggested in the past, but due to intensive computational effort required, it was never realized. The present paper attempts to tackle this problem by employing an iterative solution procedure where the gas pressure field in the vicinity of the crack tip is accurately solved for by computational fluid dynamics (CFD) for a given flap geometry determined from a dynamic FEA model to render a new flap geometry. In this model a cohesive-zone element at the crack tip is employed as a representation of the material toughness parameter. The final outcome is the determination of the cohesive energy in the FEA (as a representation of the material toughness parameter) to match the measured fracture propagation speed for the specific case. A case study was taken from full-scale rupture test data from one of the pipe joints from the Japanese Gas Association (JGA) unbackfilled pipe burst test data conducted in 2004 on the 762 mm O.D., 17.5 mm wall thickness, Gr. 555 MPa (API 5L X80) pipe.

Effect of ao/W Ratio on Fracture Toughness Parameter of Extra Deep Drawn Steel Sheets: Experimental and Finite Element Studies

Fracture toughness parameter is significantly affected by specimen dimensions i.e. specimen thickness (B), width (W) and unbroken ligament length (W-ao) in elastic-plastic region. Present study is about the third dimension of test specimen (W-ao). In order to investigate effect of ao/W ratio on fracture toughness parameter, fracture test and finite-element - cohesive zone model (CZM) simulation tool are used. Fracture tests are carried out on extra deep drawn (EDD) steel sheets using compact tension (CT) type specimens with different ao/W ratio (0.5, 0.525, 0.55 and 0.575). After successive experimental attempts, load drop technique is used as a fracture criterion. Critical CTOD is used as a fracture toughness parameter. An alternative constant traction separation law is used to account for maximum load and large load line displacements. Experimental findings as well as finite element studies show that the critical CTOD decreases with ao/W ratio. It has been observed that as ao/W ratio increases, the location of plastic hinge shifts towards the crack tip (i.e. size of tensile plastic zone reduces), which reduces fracture toughness. That is, the material is less resistant to crack growth for deeper cracks.