PSI - Issue 13

Primož Štefane et al. / Procedia Structural Integrity 13 (2018) 1895 – 1900 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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3. Finite element modelling of SE(B) specimens with half OM and half UM weld configurations

The finite element analysis (FEA) has been focused on the SE(B) specimens with half OM and half UM weld configurations. Abaqus CAE 2018 software was used for finite element model (FEM) generation. Two kinds of FEM were created for each SE(B) specimen; i) elastic-incremental plastic model incorporating non-linear geometrical effects for calculation of stress fields and J-integral and ii) fully elastic model for calculation of T-stress. SE(B) specimens were modelled as volumes containing a central symmetrical plane, normal to the direction of specimen thickness. Welded joint was modelled as a combination of volumetric regions corresponding to distinctive material regions in the actual welded joint, including coarse and fine grained heat affected zones. Boundaries of modelled regions were determined as straight edges fitted by least squares method to boundaries visible in the weld macrograph. The geometry was discretized with C3D20R 20-node isoparametric brick elements (Fig. 5.a). The finite element mesh had a common configuration with 9 focused rings of elements surrounding the crack front. The first ring consisted of 15-node wedge C3D15 elements, with a collapsed side and mid-side nodes moved to ¼ point nearest to the crack tip, in order to simulate singularity at the crack front. The load was applied as displacement of a rigid roller pin. The obtained results are shown in figures 4.a and 4.b.

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Figure 4 Comparison of FEA and experimental results in form of a) P-CMOD and b) J-P/P Y recorded history.

4. Discussion and conclusions

Tensile testing results clearly indicate relatively high strength heterogeneity across distinctive material regions in fabricated welded joints. This is caused by various microstructures which occurred due to complex thermal history. In general, the highest values of strength mismatch were observed in coarse and fine grained HAZ, resulting in a suppressing effect on the development of a plastic zone under applied load. This has an impact on the fracture behavior of a welded joint with crack. Welded joint with half OM and half UM configuration and a notch located in UM weld material exhibits improved fracture toughness due to ductile UM weld material surrounding the crack tip. The high strength OM weld material in front of the crack extension path, provides the highest load bearing capacity observed in figure 3a. Due to higher yield strength material in front of the crack extension path, the J integral reduces at the vicinity of UM-OM interface as it was proven by Predan et al. (2007). This leads to crack path deviation through the HAZ and consequentially to discrepancy of J-R curves at approx. Δa=0.8 mm for UM-OM SE(B) specimens. Opposite is observed in case of welded joints in half OM and half UM configuration with a notch located in the OM material. Due to lower yield strength material in front of the crack extension path, the increase of J-integral at the vicinity of the OM-UM interface was observed.