PSI - Issue 38

Tuomas Skriko et al. / Procedia Structural Integrity 38 (2022) 393–400 Skriko et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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In the FE analyses, the ENS models were created based on the geometry measurements, i.e. an idealized local weld toe geometries fitted from the true measurement data were applied employing 2D plane strain finite element models with 6-node modified quadratic plane strain triangle elements, of which size was set to 0.05 mm in accordance with conducted mesh convergence study. An example of a certain laser-dressed fillet weld toe geometry with idealized shape is showed in Fig. 4.

Fig. 4. Idealized local geometry of a laser-dressed fillet weld based on true measurement data and the corresponding FE model.

4. Results and discussion Fatigue test results of laser-dressed non-load-carrying cruciform joints based on different assessment methods are shown in Fig. 5 and a summary of the corresponding numerical values are presented in Table 4. In the analyses and calculations of the 4R method, a maximum tensile residual stress ( σ res ) of σ res = 550 MPa was applied based on the X ray measurements. On the grounds of the results obtained in this study, it can be noted that the effect of R on fatigue strength is lesser and FAT values of laser-dressed fillet welds are lower compared to fatigue strength results of corresponding material and joint type in TIG-dressed condition (Skriko et al., 2017), which can be due to different residual stress state produced by laser and TIG remelting. For the applied structure with described welding and post-weld treatment procedures, the laser dressing caused tensile residual stresses, whereas TIG dressing created compressive residual stresses, in general. However, further and more comprehensive studies must be made to define the reasons behind this finding. For the nominal stress, structural hot-spot stress and ENS approaches, the slope parameter ( m ) of S - N curve is close to m = 3, which is the standard value for weld joints in as-welded condition, and thus, differs from the proposed m = 4 for TIG-dressed weld joints (Yildirim, 2015). On the other hand, the IIW guideline instructs to apply m = 3 to TIG dressing (Haagensen and Maddox, 2013). In addition, the number of test specimens was relatively low in this study and hence, more experimental tests and analyses are needed in order to ensure the proper slope parameter for laser dressed weldments. In terms of the 4R method, the results indicate that it is an applicable tool for analyzing the fatigue strength of laser dressed fillet weld joints. The scatter of the 4R results is notably smaller compared to other fatigue assessment methods. The 4R method is based on the combined effect of material properties ( R m ), applied loading ( R ), residual stresses ( σ res ) and weld quality ( r ) (Nykänen and Björk, 2016) and former studies have shown that it can take into account the positive effect of compressive residual stresses and low stress ratio on fatigue durability of weldments, especially in the case of UHSS (Ahola et al., 2019, Ahola, 2020, Ahola et al., 2020, Björk et al., 2018). However, in this study, due to relatively high measured and applied residual stress value, the effect of R on calculated results is minor. On the other hand, this is in line with the well-known general statement that with high residual stresses, the global stress ratio and thus, mean stress does not have a significant role regarding fatigue strength of welded joints and structures (Macdonald, 2011).