Correlation of Fracture Behavior in Circumferentially Cracked Pipes and Fracture Specimens Including Ductile Tearing

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 the J-integral. The primary objective of this study is to assess the coupled effects of geometry and ductile tearing on crack-tip constraint, as characterized by the J – Q theory, to correlate fracture behavior in circumferentially cracked reeled pipes and common fracture specimens. 3-D finite element computations including stationary and growth analyses were conducted for 3P SE(B) and clamped SE(T) specimens having different notch depth (a) to specimen width (W) ratio. Additional 3-D finite element analyses were also performed for circumferentially cracked pipes with a surface flaw having different crack depth (a) over pipe wall thickness (t) ratios. A cell methodology to model Mode I crack extension in ductile materials was utilized to describe the evolution of J with the evolving near-tip opening stresses. Laboratory testing of an API 5L X70 steel using deeply cracked C(T) specimens was used to measure the crack growth resistance curve for the material and to calibrate the cell parameter defined by the initial void fraction, f₀. The present results provide further understanding of crack growth resistance measurements in pipeline steels using SE(T) and SE(B) specimens while eliminating some restrictions against the use of shallow cracked bend specimens in defect assessment procedures.