PSI - Issue 16

Vitalii Knysh et al. / Procedia Structural Integrity 16 (2019) 73–80 Vitalii Knysh et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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3.3. Fatigue behavior of welded joints

Before fatigue testing of welded T-joint specimens, the corrosion products were physically removed from the surfaces in the grip areas. The results of fatigue testing of specimens of all series are given in Fig. 6. Application of HFMI technology significantly increases the fatigue behaviour of welded joints, which were not exposed to the corrosive environment (Fig. 6, curves 1, 2): cyclic fatigue life increases more than 20 times and the fatigue limit at 2·10 6 cycles increases by 47% (from 180 up to 265 MPa). Exposition of as-welded T-joints at high humidity for 1200 hours (specimens of (3) series) led to decreasing of fatigue limit at 2·10 6 cycles by 13% (from 180 up to 157 MPa) (curve 3, Fig. 6). In addition, exposition in the corrosive environment also promoted decreasing of fatigue limit of welded joints treated by HFMI (specimens of (4) series), by 13% (from 265 to 230 MPa) (curve 4, Fig. 6). At the same time, obtained results indicate on rationality of application of HFMI technology for welded T-joints at the stage of metal structure fabrication, as after HFMI-treatment the fatigue limit at 2·10 6 cycles o f such welded joints increased by 32% (from 157 up to 230 MPa) and fatigue life increased 5…7 times. After exposure to high humidity, fatigue fracture of as-welded T-joints occurred along the fusion line, and that welded joints treated by HFMI ran in the HAZ, or in BM at 1 5…60 mm distance from the weld. The results of fatigue testing of the welded T-joints after pre-loading and exposure to corrosive environment without further HFMI treatment (specimens of (5) series) and after subsequent HFMI treatment (specimens of (6) series) are given in Fig. 6, curves 5, 6. It was experimentally established that pre-loading decreased the fatigue limit of the welded T- joints at 2·10 6 cycles by 14% (from 157 to 135 MPa), and fatigue life – by 1.4…1.8 times (curve 5, Fig. 6), compared to the welded joints without pre-loading. Fatigue life of welded T-joints after pre-loading and exposure to corrosive environment and after their subsequent treated by HFMI (curve 6, Fig. 6) is in the range of scatter of experimental data of specimens of (4) series (curve 4, Fig. 6). This is mainly related to the fact that fracture in the majority of welded joints of (6) series, as in those of (4) series, runs through the base metal at 15…60 mm distance from the weld. Fatigue limit at 2·10 6 cycles of these specimens increased by 7% (from 230 up to 246 MPa), compared to welded joints treated by HFMI without pre-loading. The obtained experimental data shows that HFMI treatment of the welded T-joints of structures even after their long-term service under atmospheric conditions, increase the fatigue limit at 2·10 6 cycles by 82% (from 135 up to 246 MPa) and fatigue life rises more than 10 times. Thus, results of experimental studies show high effectiveness of HFMI treatment of the welded T-joints of st ructures, which are exposed to alternating loads in service in atmospheres of category С1 - С4, according to ISO 12944-2 (temperate climate zone). a b

Fig. 6. S-N curves of welded T-joints of 15KhSND steel as-welded (1) and treated by HFMI (2) in air; as-welded (3) and treated by HFMI (4) after corrosion tests; after pre- loading 2·10 6 cycles and corrosion testing without HFMI treatment (5) and with further HFMI treatment (6).