Application of Fraunhofer Diffraction in Measuring Butt Joint Root Opening Width

Relevance of the study is due to the significant impact of the root opening width in a welding joint on the quality, structure, and mechanical properties of the joint. Non-compliance of the root opening width with regulatory values leads to defects in the welded joint, such as burn-through and lack of fusion. Traditional methods for measuring root opening width, including welding gauges, do not provide the necessary accuracy for small root openings (less than 1 mm), widely used in GTAW, LBW and EBW. The paper addresses the problem of accurate non-contact measurement of small root openings in welded joints. As a solution, it is proposed to use the phenomenon of Fraunhofer diffraction on the root opening. To experimentally verify the possibility of applying the method, an optical setup with an NG-2P laser was assembled. Measurements were carried out on butt joint square weld samples with thicknesses of 3 mm (type C2 GOST 14771–76) and 6 mm (type C7 GOST 14771–76). The joint root opening was varied from 0,1 to 0,58 mm. It is shown that the method provides satisfactory accuracy: the relative error in determining the root opening width compared to control measurements using a microscope did not exceed 26,5%, and in 80% of cases, the microscopy results fell within the confidence interval of the diffraction measurements. The main limitations of the method’s application are the distortion of the diffraction pattern for thick (over 6 mm) metal due to beam reflection from the root opening wall surfaces, and difficulties in measuring root opening widths less than 0,1 mm, related to the positioning accuracy of the measuring setup elements. It was established that the displacement of joint edges within the limits regulated by GOST 14771-76 does not affect the accuracy of measuring the gap width by the studied method. A conclusion is drawn about the practical possibility of applying the method in automatic control systems for technological processes and in scientific research on the influence of root opening width and regime parameters on weld formation.
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References

[1] Wang Z., Chen H., Zhong Q. et al. Recognition of penetration state in GTAW based on vision transformer using weld pool image. Int. J. Adv. Manuf. Technol., 2022, vol. 119, no. 7–8, pp. 5439–5452, doi: http://doi.org/10.1007/s00170-021-08538-6

[2] Ferdinandov N., Gospodinov D., Ilieva M. et al. Effect of the root gap on the structure and properties of high strength steel S700MC welds. KEM, 2021, vol. 890, pp. 201–208, doi: http://doi.org/10.4028/www.scientific.net/KEM.890.201

[3] Jang G.B., Kim H.K., Kang S.S. The effects of root opening on mechanical properties, deformation and residual stress of weldments. Weld. J., 2001, no. 3, pp. 80–89.

[4] GOST 14771–76. Dugovaya svarka v zashchitnom gaze. Soedineniya svarnye. Osnovnye tipy, konstruktivnye elementy i razmery [Gas-shielded arc welding. Welded joints. Main types, design elements and dimensions]. Moscow, Izd-vo standartov Publ., 1980. 56 p. (In Russ.).

[5] GOST 8713–79. Svarka pod flyusom. Soedineniya svarnye. Osnovnye tipy, konstruktivnye elementy i razmery [Flux welding. Welded joints. Main types design elements and dimensions]. Moscow, Izd-vo standartov Publ., 1982. 63 p. (In Russ.).

[6] Öberg A.E., Wikstrand S., Mattsson V. Impact of gaps on resource efficiency in heavy welding industry. 6th Swedish Production Symposium, 2014. URL: https://publications.lib.chalmers.se/records/fulltext/240909/local_240909.pdf (accessed: 15.10.2025).

[7] Öberg A.E., ?strand E. Improved productivity by reduced variation in gas metal arc welding (GMAW). Int. J. Adv. Manuf. Technol., 2017, vol. 92, no. 1–4, pp. 1027–1038, doi: https://doi.org/10.1007/s00170-017-0214-4

[8] Svarochnyy shablon WG1. rktm.ee: website URL: http://rktm.ee/rus/welding-gauge.html (accessed: 15.07.2025).

[9] Jenkins F.A., White H.E. Fundamentals of optics. McGraw-Hill, 2001. 768 p.

[10] Sivukhin D.V. Obshchiy kurs fiziki. T. 4. Optika [General course of physics. Vol. 4. Optics]. Moscow, Nauka, 1980. 751 p. (In Russ.).

[11] Gmurman V.E. Teoriya veroyatnostey i matematicheskaya statistika [Probability theory and mathematical statistics]. Moscow, Vysshaya shkola Publ., 1972. 368 p. (In Russ.).