Pin-loaded SENT specimen for constraint-matched fracture testing of radially propagating longitudinal cracks in thin-walled pipelines

Abstract

There is considerable interest in the fracture community for representative measurements of the resistance of pipe structures to fatigue crack propagation and fracture. However, to date there has been a lack of standard test methods available for the accurate fracture testing of radially propagating longitudinal flaws in thin-walled piping. A pin-loaded single edge-notched tensile (SENT) specimen was developed, denoted the CSR SENT specimen. This test geometry has been shown to reproduce the plastic constraint for longitudinal cracks in thin-walled piping, resulting in transferable fracture test data to that application beyond the conventional limits of single-parameter J-dominated fracture mechanics. The CSR SENT was simulated with finite element analysis (FEA) to yield the parameters necessary for the experimental determination of the J-integral using standard fracture test methods. The FEA results showed that the stress intensity geometry factor, F, for the CSR SENT matched literature solutions for most crack lengths, but that the CMOD-Eta factor – ηCMOD, used to calculate the plastic component of the J-integral from the applied load and crack mouth opening displacement (CMOD) – differed from literature solutions and is specimen size-dependent. The error associated with the calculation of the J-integral is estimated to be < 5% with the obtained ηCMOD, while the use of literature solutions for ηCMOD result in significant non-conservative overestimates of the experimental J-integral. It is recommended that fracture testing be limited to a specific specimen geometry associated with a FEA-derived ηCMOD solution. The crack growth correction to the J-integral measurement was also obtained, and is recommended to be derived from the ηpin (also referred to as ηLLD, derived from the load-displacement of the pin boundary), which can be used alongside the ηCMOD for experimental calculation of the J-integral in standard single-specimen fracture testing. Friction at the pin-specimen interface results in significant non-conservative bias error in the J-integral, and a rolling pin-clevis contact is recommended with a pin diameter twice that of the specimen width to mitigate pin contact plasticity and damage.