PSI - Issue 54

T. Koščo et al. / Procedia Structural Integrity 54 (2024) 514 – 520 Koščo T., Chmelko V. / Structural Integrity Procedia 00 (2019) 000 – 000

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non-homogenous material field was done by Avril and Sutton (2008). This approach divided the WM, HAZ and BM into seven different zones with material properties considered as uniform. Despite, surprisingly good results, this method does not gain a smooth function of material properties but averages them across the zones. Since then, VFM plays an important role in estimation of material properties across the welded joint. Throughout the years, simplified approaches, that averages the stress in the cross section have been used (Milosevic, 2021). Many comparisons between VFM and Uniform Stress Method (USM) was presented ( Saranath, 2015 ). Another slight disadvantage of VFM lies in the investigation of the material model. The result consists only of the parameters of material model. In case of materials with significant yield, material models such as bilinear model can lead to significant inaccuracies at the beginning of the plastic region. On the other hand, USM leads to an exact tensile diagram. Another possibility lies in the so-called Finite Element Model Updating (FEMU) approach. This method also leads to similar problems as VFM, but it is also significantly more computationally intensive approach.

Nomenclature DIC

Digital Image Correlation

VFM Virtual Fields Method FEMU Finite Element Model Updating SSSIG Sum of Square of Subset Intensity Gradients HAZ Heat Affected Zone BM Base Metal WM Weld Metal

Based on this knowledge, authors wanted to estimate exact tensile curves in all possible locations in the cross section of the weld. Therefore, approach with multiple uniform stress specimens in different locations, described in next section, was used. 1.1. Measurement equipment All measurement were made by using Dantec dynamics Q400 DIC system with two 5.0 MPx monochromatic cameras Baumer VCXU-50 in master-slave configuration. Results were evaluated by using Istra4D 4.4.7 DIC software. Loading of specimens was caried out by using MTS Bionix 370.02. 2. X52 welded joint The measurement was dealing with a pipeline weld joint specimen with a wall thickness of 20mm (Fig. 1a) made up of X52 pipeline steel. The weld specimen was divided into 5 specimens (designated 1-5). The specimens were 7mm wide and between 2.5mm and 3mm thick. The final specimens are shown in Fig. 1b. The working part of the specimen contain the weld, the heat affected zones and the base material as well (Fig. 1). Thus, it should be possible to evaluate the mechanical properties through all areas of the weld. The resulting mechanical property map will not be continuous but composed of five maps of tensile diagrams at different locations along the thickness of the weld. Tensile diagrams in locations between the specimens must be interpolated. The orientation of the specimen in the weld was chosen to minimise the effect of the change in weld width (Fig. 2). All the specimens had a speckle pattern in the working section with SSSIG (Pan, 2008) parameter about ca. 7-9*10 5 . A Gaussian filter was used in the evaluation to remove possible aliasing effects. The subset size was 29 pixels, and the grid size was 20 pixels. Abrupt transitions in material properties naturally cause abrupt changes in the strain field. Thus, it turned out that the filter would play a role in this case. Due to the nature of the strain field, polynomial local regression filter on region of 11x11 grid points was used (11 to 13 grid points across the width of the sample). Due to the specimen size and the possible position of the cameras, the entire specimen could not be monitored by DIC. However, assuming that the weld is symmetrical, it is sufficient in this case, to observe only one half (of the sprayed portion of the specimen). The measured part is visible in Fig. 1c. True strain data was exported along the entire length of the line leading from the base material to the weld metal. Thus, it can be assumed that all essential areas of the weld will be captured in the data. True stress strain maps are visualised in Fig. 3-7.