PSI - Issue 42

T. Koščo et al. / Procedia Structural Integrity 42 (2022) 1600 – 1606 Koščo, T., Chmelko, V. / Structural Integrity Procedia 00 (2019) 000 – 000

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approaches is a comparison with samples with defined radii. Estimated value for this case is approximately 130 pixels per radius. First principal strain (longitudinal direction) is 0.82*10 -3 and second principal strain is -0.21*10 -3 . Principal strains have been recalculated to the principal stresses in linear elastic region by using Young modulus of 200 000 MPa and Poisson ration of 0.3. Then first principal stress (longitudinal direction) is 166.4 MPa and second 7.9 MPa. Stress concentration factor calculated from longitudinal stress is 1.478. 6. Conclusion Possibility of use of digital image correlation method for measurement in the notches has been investigated in this paper. Measurement on two specimen geometries with known analytical solution were carried out. Measurement on the open hole tensile loaded specimen with partial side view resulted relatively large filter dependency for both filtering methods. Relatively relevant results were obtained by using the local regression. Absolute error was between 2*10 -5 and 3.1*10 -5 what corresponds to 4 or 6.4 MPa respectively (for steel specimen). Second configuration, symmetric view resulted significantly better results with almost no filter dependency of the result. In case of smoothing spline filter, absolute strain error was under 1*10 -5 what corresponds to 2 MPa. In case of local regression filter, absolute strain error was between 1*10 -5 and 2*10 -5 what corresponds to 2 or 4 MPa respectively. After considering these results, authors consider the measurement on the cylindrical surface of the specimen as the most suitable. On the side notched specimen geometry, sensitivity analysis has been performed. Authors consider notches larger than 150 pixels per radius as sufficiently large for satisfactory results with standard deviation less then 4*10 -5 with described speckle pattern properties. Smaller standard deviation can be obtained by averaging results from multiple image sets. Based on knowledge gained from previously mentioned results, measurement on welded specimen has been performed. For a notch with estimated 130 pixels per notch radius. Notch stress concentration factor has been calculated based on measured strain field in the notch. Results of the strain peak did not report significant dependency on smoothing parameters. Based on previous measurements, authors consider the DIC method as suitable for measurement on notched specimens as the notch is relatively large on the acquired image. Any notch size can be measured by customization of the measurement equipment, such as cameras or optics. Acknowledgements This work was supported by the Slovak Research and Development Agency under the contract No. APVV-17-0666 and by the Research & Development Operational Programme funded by the ERDF ITMS: 26240220084 Science city Bratislava. References Sutton, M. A., Orteu, J.J., Schreier, H.W., 2009. Image correlation for Shape, Motion and Deformatiom Measurements, Springer NY, NewYork. Wang, L., Liu, G., Xing, T., Zhu, H., Ma, S., 2021. Boundary Deformation Measurement by Mesh-Based Digital Image Correlation Method. Applied Sciences 11. Wang, P., Pierron, F., Thomsen, O.T., 2013. Identification of Material Parameters of PVC Foams using Digital Image Correlation and the Virtual Fields Method. Exp Mech 53, 1001 – 1015. Ma, C., Zeng, Z., Zhang, H., Rui, X., 2021. Variable subset DIC algorithm for measuring discontinuous displacement based on pixel-level ZNCC value distribution map. Measurement 180. Sun, Y., Pang, J. H. L., Wong, C. K., Su, F., 2005. Finite element formulation for a digital image correlation method. Appl. Opt. 44, 7357-7363. 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. Pan, B., Lu, Z., Xie, H., 2010. Mean intensity gradient: An effective global parameter for quality assessment of the speckle patterns used in digital image correlation. Optics and Lasers in Engineering, 48,469 – 477. Hua, T., Xie, H., Wang, S., Hu, Z., Chen, P., Zhang, Q., 2011. Evaluation of the quality of a speckle pattern in the digital image correlation method by mean subset fluctuation. Optics & Laser Technology 43, 9 – 13. Sun, Y., Pang, J. H. L., 2007. Study of optimal subset size in digital image correlation of speckle pattern images. Opt. Lasers Eng. 45, 967 – 974. Howland, R., 1930. On the Stresses in the Neighborhood of a Circular Hole in a Strip under Tension. Philosophical Transactions of the Royal Society A. Mathematical, Physical and Engineering Sciences 229, 670-680. Chmelko, V., Harakal’, M. , Žlábek, P. , Margetin, M., Ď urka, R., 2022. Simulation of Stress Concentrations in Notches. Metals 12, 43.