Strain model and experimental study of elastic viscoplastic for crack-tip zone of X80 steel pipe

The mechanical state of the process zone at the crack-tip is one of the main factors that affect crack initiation and growth in the long-term safe operation of oil and gas pipelines. Many tests showed that the stress and strain fields in the fracture-tip process zone can be used to define the crack propagation behavior. The goal of this paper is to improve the Hutchinson, Rice, and Rosengren (HRR) model by using cubic asymptotic polynomials to describe the stress-strain field at the crack-tip, as well as to validate the model by a thorough comparison with full-scale burst test results that account for the fracture zone's mechanical properties. The asymptotic polynomial elastic viscoplastic distribution model of stress-strain displacement field at the crack-tip is developed, and the HRR method is modified reasonably. The crack propagation test of the X80 steel pipeline in the burst test site is carried out. Experimentally, the distribution of stress and strain field in the crack-tip process zone of the X80 testing pipeline is obtained, then compared with the theoretical solution. The final results showed consistency, and the elastic viscoplastic model matched well with the running fracture test data for the X80 gas pipeline. Besides, the stress and strain fields distribution in the crack-tip process zone is verified through the full-scale test of the X80 steel pipeline, which provides a theoretical basis for the integrity evaluation of the steel pipeline.