PSI - Issue 13

Yoshiki Mikami et al. / Procedia Structural Integrity 13 (2018) 1804–1810 Author name / Structural Integrity Procedia 00 (2018) 000–000

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5

Fig. 6 Mapping of the residual stress distribution from (a) the multipass weld joint to (b) the fracture toughness test specimen.

a

b

Magnified view for the notch r oot or crack tip at mid-thickness

Tied nodes

Residual stress mapped

Notch introduced

c

d

Reverse bent

Unloaded

e

f

Un-tied nodes

Precracked

Three-point bent

Fig. 7 Transition of residual stress ( σ y , the component in notch or crack opening direction) distribution along the simulation steps; (a) un-notched specimen after residual stress mapping; (b) notched specimen; (c) reverse bent specimen at maximum loading; (d) unloaded specimen after reverse bending; (e) precracked specimen; (f) three-point CTOD bent specimen. This simulation method allows incorporating the residual stress distribution in the multipass welded joints into the CTOD test simulation. Then, the effect of residual stress distribution on the CTOD test result can be investigated. For comparison, the CTOD test simulation of as-welded and base metal specimens were also performed. The as-welded specimen was simulated along steps 1, 2, 4, and 5. The base metal specimen was simulated without mapping the residual stress distribution, i.e., through steps 2, 4, and 5. 4. Results and discussion The results of the CTOD test simulation for the base metal (BM), as-welded (AS), and residual stress modified, i.e., reverse bent (RB) specimen model cases are presented and discussed. 4.1. Residual stress distribution at notch root The distribution of transverse stress σ y at the notch root is shown in Fig. 8 for cases AW and RB. The simulation model is cut in the cross section passing through the notch root to display the residual stress distribution in the ligament part in front of the notch. In the AW case, tensile–compressive–tensile stress is distributed through the thickness at the notch root, which may retard fatigue precracking—initiation and propagation—at mid-thickness. On the other hand, in the RB case, the stress at the notch root changes to tensile and becomes uniform along the thickness. Therefore, it is expected that a straight fatigue precrack front can be obtained. The mechanism of residual stress modification by reverse bending is the introduction of compressive plastic deformation in the vicinity of the notch root in reverse