Application of a Physically-Based Creep Continuum Damage Mechanics Constitutive Model to the Serviceability Assessment of a Large Bore Branch Connection

Premature creep cracking in fabricated large bore branch connections in Grade 91 steel (9Cr-1Mo-VNbN) piping continues to be a commonly observed failure mechanism in high energy applications. Failures have been observed in components fabricated to the requirements of both ASME Section I and B31.1 codes. This paper presents the application of a physically-based creep continuum damage constitutive model developed for Grade 91 steel to the assessment of a large bore fabricated branch connection. For a specific component geometry and operating conditions, model predictions for the expected location and timing of crack initiation as well as for the crack growth behavior have been made. In addition, as validation, trends in the simulated behavior are compared to information from case studies of large bore branch cracking and failure in service. The physically-based continuum damage model is shown to accurately predict both the location and timing of local crack initiation as well as the observed crack growth behavior.