PSI - Issue 37

Cheng Qian et al. / Procedia Structural Integrity 37 (2022) 926–933 Cheng Qian et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction The fracture toughness resistance R -curve (e.g. J - R curve and  - R curve) is important in facilitating strain-based design and integrity assessment of oil and gas pipelines where influence of ground movements and presence of planar weld flaws must be accounted for. ASTM E1820-18a, ASTM (2018) provides the R -curve testing procedures and guidelines for SE(B) and C(T) specimens, while Shen et al. (2008 and 2009) from CanmetMATERIALS developed similar testing procedures for SE(T) specimen. In both test procedures, the J equations were developed based on finite element analysis (FEA) assuming plane-strain crack-tip constraint and the  equations were determined by converting J -values using a linear relationship. Tang et al. (2010) from ExxonMobil Upstream Research Company proposed another procedure for measuring  directly from a double-clip-gauge by linearly extrapolating the upper and lower clip gauge displacements to the original crack-tip position. Due to constraint effect, specimen geometric configuration, loading mode, and material mismatch can have significant effects on the crack-tip stress distributions and R -curves. Constraint parameter, generally denoted by Y , is a quantitative characterization of the crack-tip constraint state. Existing constraint parameters include the stress-based ones, such as Q in O ’Dowd et al. (1991, 1992 and 1994) and Zhu et al. (2001 and 2007), A 2 in Yang et al. (1993) and Chao et al. (1997 and 2004), h in Hancock et al. (1993) and T z in Guo et al. (1993 and 1995), as well as the plastic zone-based ones, such as C p in He et al. (2019) and Simha (2020), A p in Yang et al. (2014), and V p in Tonge et al. (2020). In this study, the material considered was an X60 pipe steel and duplicate SE(B) and clamped SE(T) specimens with two initial crack sizes were experimentally tested at room temperature to establish J - R and  - R curves. FEA was performed to calculate all the abovementioned constraint parameters. Linear correlations between critical J - and  - values and the eleven calculated constraint parameters were subsequently evaluated with the objective of determining their adequacy in correlating fracture toughness with different levels of constraint.

2. Experimental procedures 2.1. Mechanical properties

The mechanical properties of an X60 pipe steel were determined for two identical rectangular tensile specimens which were machined in pipe’s longitudinal direction and tested at room temperature. Figure 1 shows the stress-strain curves and the fitted average curve in the form of Ramberg-Osgood relation.

Fig. 1. Stress-strain curves of X60 pipe steel.