PSI - Issue 17

Lise Sandnes et al. / Procedia Structural Integrity 17 (2019) 632–642 L. Sandnes et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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This rough estimate shows that the EZ, according to the reduced strength zone definition in Fig. 5, consists of 71% FM and 29% soft HAZ material. 4.3. Input data used in the FE simulations Implementation of the mechanical model in ABAQUS requires relevant true stress-strain curves for both the T6 heat treated BM, the soft HAZ material and the FM. The necessary input data for the BM have been derived from the tensile results obtained in the present investigation, whereas the corresponding data for the soft HAZ material and the FM are taken from two independent sources (Myhr et al. , 2009, SINTEF, 2018). Fig. 6 contains graphical representations of the assembled true stress-strain curves, which are well-described by the following variant of Ludwik’s law: = + (3) where is the true stress, is the stress at the on-set yielding, is the true plastic strain, while K and n are fitting parameters. Table 4 summarizes the main input data, including the relevant values for , K and n for the BM, the soft HAZ material and the FM. Table 4. Summary of the input data used to calculate the true stress- strain curves for the BM, the soft HAZ material and the FM from Ludwik’s law. (MPa) K (MPa)) n Source BM 182 360.9 0.72 Present investigation HAZ 100 194.5 0.33 (Myhr et al. , 2009) FM 186.5 453.3 0.62 (SINTEF, 2018) To obtain the true stress-strain cure for the EZ, we first invoke the “rule of mixture s ” (Callister and Rethwisch, 2007), which in the past also has been successfully applied to predict the mechanical properties of dissimilar welds (Abdullah et al. , 2001). Then the next step is to assume that the EZ can be treated as a composite material, where the true stress acting on an element during loading, at any strain, is given by the weighted average of the true stress acting on each of its components and : = + (1 − ) (4) where and refer to the true stress in the FM and the soft HAZ material, respectively, as calculated from Equation 3. It follows from Equation 4 that the true stress-strain curve for the EZ is determined by the actual value of and can therefore vary within wide limits, depending on the operational conditions applied. Fig. 6 includes a plot of the predicted stress-strain curve for the EZ at = 0.71 , which refers back to the present experimental set-up and a groove width k of 7.5 mm. 4.4. Validation of the FE model Fig. 7 shows a comparison between measured (solid line) and simulated (dashed line) engineering stress-strain curves for the 2 mm AA6060-T6 HYB butt weld following implementation of the mechanical model in ABAQUS. As can be seen from the figure, the FE model gives a fair representation of the mechanical response during tensile testing, showing that the simulation set-up is sound and the applied input data reasonable in the content of the model being developed. This justifies further use of the FE model to re-assess the previously reported tensile test results in Fig. 4 and evaluate how the yield strength values achieved depend on the applied specimen geometry.