Further Insights on the Relationship Between J and CTOD for SE(B) and SE(T) Specimens Including Ductile Crack Propagation

Abstract

In structural assessment procedures the crack driving force is usually estimated numerically based on the J -Integral definition because its determination is well established in many finite element codes. The nuclear industry has extensive fracture toughness data expressed in terms of J-Integral and huge experience with its applications and limitations. On the other hand, material fracture toughness is typically measured by Crack Tip Opening Displacement (CTOD) parameter using the hinge plastic model or double clip gauge technique. The parameter CTOD has a wide acceptance in the Oil and Gas Industry (OGI). Also, the OGI has a lot of past data expressed in terms of CTOD and the people involved are very familiar with this parameter. Furthermore, the CTOD parameter is based on the physical deformation of the crack faces and can be visualized and understood in an easy way. There is a unique relationship between J and CTOD beyond the validity limits of Linear Elastic Fracture Mechanics (LEFM) for stationary cracks. However, if ductile crack propagation occurs, the crack tip deformation profile and stress-strain fields ahead of the crack tip will change significantly when compared to the static case. Thus, the stable crack propagation may change the well established relationship between J and CTOD for stationary cracks compromising the construction of resistance curves J-Δa from CTOD-Δa data or vice versa. This investigation is a complementary study on the relationship between J-Integral and CTOD under ductile crack propagation of a previous work. The theoretical definition of CTOD using the 90° method and the empirical expression used in the standard ASTM E1820 are analyzed under stable crack growth. Plane-strain finite element computations including stationary and growth analysis are conducted for 3P SE(B) and clamped SE(T) specimens having different notch length to specimen width ratios in the range of 0.1–0.5. For the growth analysis, the models are loaded to levels of J consistent to a crack growth resistance curve representative of a typical pipeline steel. A computational cell methodology to model Mode I crack extension in ductile materials is utilized to describe the evolution of J with a. 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, f 0. The presented results provide additional understanding of the effects of ductile crack growth on the relationship between J-Integral and CTOD for standard and non-standard fracture specimens. Specific procedures for evaluation of CTOD-R curves using SE(T) and SE(B) specimens with direct application to structural integrity assessment and defect analysis in pipelines and risers will be proposed, yielding accurate and robust relations between J-Integral and CTOD.