PSI - Issue 54

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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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Poissons ratio curve must be evaluated from the true strain values, especially in the plastic region. In elastic region the differences are negligible. The evaluation is valid until the necking initiation. After the necking initiation, stress state in the necking region is no longer uniaxial. Therefore, Poissons ratio evaluated from the necking region can increase over 0.5 or start decreasing. Both of these cases are not reflecting the reality. In this type of measurement this effect occurs only in the Base metal region of every specimen. Evaluating of the Poissons ratio only until around 20% of strain prevents this phenomenon. 4. Conclusion Presented paper deals with the full field strain measurement by using digital image correlation method to investigate the full field of tensile properties in the cross section of the X52 pipeline steel welded joint. Over the years, many approaches were proposed for this purpose. Presented approach uses multiple uniformly stressed specimens to gain multiple maps of partial tensile diagrams. Tensile properties in locations between the specimens can be interpolated to estimate full field of tensile properties. The main advantage of this approach lies in the possibility to gain not only material model parameters (case of VFM or FEMU), but partial tensile diagrams itself. Naturally noisy DIC results must be filtered. Displacement fields must be smoothed by least possible amount of filtering, in order to gain smooth but not disturbed strain fields. This is especially important in areas around the weld where significant strain changes may occur. Another important quantity to evaluate is the Poissons ratio. In this case, noise in DIC data plays a crucial role. Therefore, average strain values from larger part of the strain field were used in the evaluation. This led to the evaluation of the Poissons ratio in fewer locations. Noise influenced values from the elastic region were averaged and plotted as a constant. On the other hand, in the plastic region, signal to noise ratio is larger so the resulting curve is naturally smoother. Values of Poissons ratio in the necking region are not recommended to use, because of the stress multiaxiality that lead to non-realistic increase or decrease of the values. Acknowledgements This work was supported by Slovak Educational Grant Agency under the Contract No. KEGA 038STU-4/2022. References Avril,S., Pierron,F., Sutton, M. A., Yan, J., 2008. Heterogeneous elasto-plastic identification on welds using the virtual fields method. Proceedings of the XIth International Congress and Exposition June 2-5, 2008, Orlando, Florida USA . Grédiac, M., Toussaint, E., Pierron, F., 2002. Special virtual fields for the direct determination of material parameters with the virtual fields method.1. Principle and definition. IntJSolidsStruct, 39(10). Milosevic, N., Younise, B., Sedmak, A., Travica, M., Mitrovic, A., 2021. Evaluation of true stress-strain diagrams for welded joints by application of Digital Image Correlation, Engineering Failure Analysis 128. Pan, B., Xie, H., Wang, Z., Qian, K., Wang, Z., 2008. Study on subset size selection in digital image correlation for speckle patterns. Opt. Express 16. Pierron, F., Grédiac, M., 2020. Towards Material Testing 2.0. A review of test design for identification of constitutive parameters from full-field measurements. Strain 57 Pierron, F., Grédiac, M., 2012. The Virtual Fields Method, Extracting Constitutive Mechanical Parameters from Full-field Deformation Measurements, Springer NY, NewYork. Saranath, K.M., Ramji, M., 2015. Local zone wise elastic and plastic properties of electron beam welded Ti – 6Al – 4V alloy using digital image correlation technique: A comparative study between uniform stress and virtual fields method, Optics and Lasers in Engineering 68, 222-214. Sutton, M. A., Yan, J., Avril,S., Pierron,F., Adeeb, S.M., 2008. Identification of Heterogeneous Constitutive Parameters in a Welded Specimen: Uniform Stress and Virtual Fields Methods for Material Property Estimation, Experimental Mechanics 48. Sutton, M. A., Orteu, J.J., Schreier, H.W., 2009. Image correlation for Shape, Motion and Deformatiom Measurements, Springer NY, NewYork.