Damage Model Prediction of Crack Initiation and Propagation in Five Fracture Geometries for X80 Pipeline Steel

Abstract

Test specimens used to calibrate damage-mechanics models are designed to produce a range of stress triaxialities and Lode angles to accurately capture the fracture envelope of the metal. Many of these specimens have lower constraint than deeply notched fracture specimens that undergo stable tearing and have high constraint at the crack tip. Often just one or two fracture geometries are used to calibrate the model. In this work, the ability of a damage model to capture variability associated with constraint at a crack tip, particularly for crack initiation, is assessed. A recent round-robin (Wilkowski et al., 2019, “1st Round-Robin for Exploring the Effects of Constraint on Fracture Initiation Toughness for Surface-Cracked Pipe/Fittings,” Panel Session held in Conjunction With ASME PVP2019, San Antonio, TX) study on the initiation toughness of X80 provided data for five fracture specimens in which crack tip constraint varied. As a damage model for X80 was not available, the well-calibrated modified Mohr–Coulomb (MMC) damage models from literature for X65 and X70 were used as a starting point for the model. Experimental data from the single compact tension C(T) specimen were used to slightly modify the X65 and X70 models to capture the X80 fracture response. The MMC damage model was applied in finite element analysis (FEA) to simulate both the crack initiation and propagation responses of single-edge-notched-tension (SENT) and surface-cracked pipe specimens. Except for a low J-integral at initiation predicted for the C(T) specimen, the remaining predicted responses for force, pressure, and initiation were in good agreement with the experimental data provided in the round-robin.