PSI - Issue 2_B

J.-J. Han et al. / Procedia Structural Integrity 2 (2016) 1724–1737

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J-J Han et al. / Structural Integrity Procedia 00 (2016) 000–000

Fig. 6. Normalised initiation toughness of notched C(T) specimens for API X70 and API X65 pipeline steels, Inconel alloy 617, and SA508 Gr.3 low alloy steel

a factor of 3 greater for notched specimens relative to reserve factors for specimens with sharp cracks (Figure 7.c)

4.2. Constraint e ff ect on fracture properties

The benefit of using the higher fracture toughness measured on shallow cracked single edge tension (SE(T)) speci mens in FFS assessments is addressed here. Ductile fracture simulations performed by Han et al. (2015) are extended here to include shallow cracked SE(T) geometries. Again, J-R curves were constructed and a comparison of the stress fields ahead of the notch tip is reported, to specifically investigate the constraint e ff ect in ductile fracture. Figure 8 compares the stress triaxiality fields in the vicinity of sharp cracks and blunt notched C(T) specimens with that of a shallow crack SE(T) specimen. It is observed that in a SE(T) specimen the stress triaxiality is smaller and therefore the fracture-toughness value is expected to be higher than that in a high-constraint configuration such as the standard C(T) specimen. Fracture resistance curves for both sharp crack SE(T) and C(T) specimens are shown in fig. 9. From the constructed J-R curves, the fracture toughness, J IC , defined as the J -integral value at a crack extension of 0.2 mm was determined according to ESI (1992). Table 2 shows the values of J IC for the di ff erent specimens and the value J S E ( T ) IC / J C ( T ) IC , quantifying the enhanced toughness of the shallow cracked SE(T) sample with respect to deeply cracked specimen.

Table 2. Predictions of initiation toughness: SE(T) VS. C(T). Values in kJ / m 2 .

J S E ( T ) IC 655.89 392.00 546.77 603.48

C ( T ) IC

S E ( T ) IC

C ( T ) IC

Material

J

J

/ J

API X65 API X70

475.85 292.74 475.62 529.87

1.378 1.339 1.149 1.138

INCONEL 617

SA 508

Analysis of Figs. 8 and 9 reveals an evident loss of constraint in the SE(T) specimen resulting in an increased capacity for sustaining load. For example, in most of the materials address in this work, the apparent fracture tough ness for deeply cracked notch C(T) specimens with ρ ≥ 0.2 mm, is greater than that of the shallow cracked SE(T) specimen ( J C ( T ) ,ρ ≥ 0 . 2 mm IC ≥ J S E ( T ) IC ), showing that a standard C(T) specimen with more than 0.2mm notch radius is less