Influence of Constraint on J-resistance Curves for an X100 Pipe Steel

There continues to be interest in utilization of high strength steels of X100 (Grade 690) for strain-based designed large diameter, high pressure on-shore pipelines where ground movements are of great concern. While mechanical properties requirements for yield to ultimate tensile ratio, uniform elongation and work hardening are important, fracture toughness evaluation is an integral part of the strain based design analysis. We developed test procedures for measuring J-resistance (J-R) curves using both single-edge bend (SE(B)) and single-edge tension (SE(T)) specimens with shallow and deep cracks. The test procedures and analysis are based on ASTM E1820-11 and a recommended practice for SE(T) developed by CanmetMATERIALS using MTS TestSuite software. Automation of the test procedures has allowed display and export testing parameters, raw data and results, including J-R curves during and after testing.The developed test procedures have been used to measure J-R curves at room temperature for both shallow (a/w=0.20) and deep (a/w=0.50) cracks with different SE(B) and SE(T) specimens from an X100 pipe steel. The results show that for both the shallow and deep crack cases, the J-R curves are slightly higher for SE(T) than for SE(B). SE(T) specimens with a fine EDM notch tend to produce slightly higher J-R curves than those with a fatigue precrack. The differences in J-R curves between SE(T) and SE(B) specimens are established in detailed finite element analysis and shown to be related to the constraint effect which is characterized by the Q-parameter following the J-Q theory. The results obtained in the present study provide more realistic fracture toughness data where two-parameter-based structural integrity assessment is necessary for strain-based design analysis.