Unified correlation of constraints with crack arrest toughness for high-grade pipeline steel

Highlights: • Sensitivity analysis for crack arrest toughness of different specimens to in-plane and out-of-plane constraints. • A new unified parameter η for characterizing both in-plane and out-of-plane constraints is proposed. • A linear relationship between the dimensionless fracture toughness CTOA_C/CTOA_ref and η^1/2 was found. There is much controversy surrounding using laboratory specimens to predict the fracture behavior of high-grade pipeline steel due to their different constraints. Since there is a lack of unified parameters to quantitatively characterize the constraint effects, a model for the transformation of crack arrest toughness between different specimens and pipes has not been formed. In order to solve this problem, the evolution law of the plastic zone of the crack tip with the crack tip opening degree under different in-plane and out-of-plane constraints is explored using the finite element simulation technology. On this basis, a constraint parameter η which is based on the area A_PEEQ surrounded by the isoline of equivalent plastic strain ε_p at the crack tip is proposed. The feasibility of this parameter to characterize both in-plane and out-of-plane constraints is discussed, and a quantitative correlation model between it and the crack arrest toughness, critical crack tip opening angle (CTOA_C), is established. The results show that the proposed parameter can be used to characterize the overall constraints caused by different specimens with different loading forms and geometries. The normalized arrest toughness, CTOA_C/CTOA_ref, has a linear relationship with η^1/2, and the correlation line is only related to the material. The quantitative correlation model has universal applicability for different specimens and is expected to predict the transformation of crack arrest toughness between laboratory specimens and pipelines.