A numerical investigation of constraint effects in circumferentially cracked pipes and fracture specimens including ductile tearing

• Extensive numerical analysis of crack-tip constraint in pipelines and fracture specimens.
• Shallow-crack SE(B) and clamped SE(T) specimens have very similar levels of constraint.
• J-resistance curves for SE(B) specimens depend rather strongly on crack size.
• J-resistance curves for clamped SE(T) specimens depend weakly on crack size.
• Shallow crack SE(B) specimens accurately characterize circumferentially cracked pipes.

This work addresses a two-parameter description of crack-tip fields in bend and tensile fracture specimens incorporating the evolution of near-tip stresses following stable crack growth with increased values of the crack driving force as characterized by J. The primary objective of this study is twofold. First, the present investigation broadens current understanding on the role of constraint and test conditions in defect assessment procedures for pipeline girth welds using SE(T) and SE(B) specimens. Second, the work addresses the potential coupled effects of geometry and ductile tearing on crack-tip constraint as characterized by the J−Q theory which enables more accurate correlations of crack growth resistance behavior in conventional fracture specimens. Plane-strain and 3-D finite element computations including stationary and growth analyses are conducted for 3P SE(B) and clamped SE(T) specimens having different notch depth (a) to specimen width (W) ratio in the range 0.1 ≤ a/W ≤ 0.5. Additional 3-D finite element analyses are also performed for circumferentially cracked pipes with a surface flaw having different crack depth (a) over pipe wall thickness (t) ratios and fixed crack length. A computational cell methodology to model Mode I crack extension in ductile materials is utilized to describe the evolution of J with the accompanying evolving near-tip opening stresses. Laboratory testing of an API 5L X70 steel at room temperature using standard, deeply cracked C(T) specimens is used to measure the crack growth resistance curve for the material and to calibrate the key cell parameter defined by the initial void fraction, f0. A key result emerging from this study is that shallow crack SE(B) specimens can accurately and conservately produce crack growth resistance curves that describe well the measuring toughness capacity of circumferentially cracked pipes under remote bending. The present results provide additional understanding of the effects of constraint on crack growth which contributes to further evaluation of crack growth resistance properties in pipeline steels using SE(T) and SE(B) specimens while, at the same time, eliminating some restrictions against the use of shallow cracked bend specimens in defect assessment procedures.