スポット溶接した冷延鋼板の疲労強度

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Pulsating tension fatigue tests were made with spot welded and plain specimens of cold rolled steel sheets l.0 mm thick. The breadth of spot welded specimens was 25mm, and the joining types were (a) 1-spot welded single shear splice, (b) 2-spot welded single shear splice and (c) 1-spot welded double shear splice. The fatigue test results of the spot welded specimens were expressed in terms of nominal stress amplitude, which was deduced from the stress distribution of the critical section of each specimen under tension-shear load. Then, the "fatigue strength reduction factor" of a spot welded area, i. e. the ratio between the endurance limits of plain and spot welded specimens, was studied. Reference was also made to the previous results of plane bending fatigue tests with 2-spot welded specimens of single shear splice. The results obtained are as follows : (1) Fatigue fracture initiated in the base metal area adjacent to the nugget and developed transversely for all three welded specimens. (2) The nominal stresses, σ_n, of the spot welded specimens (a), (b) and (c) were estimated to be 4P/bt, 2.5 P/bt and P/bt, respectively, where P is the tension-shear load, b the specimen breadth and t the sheet thickness. They correspond to the average stresses on the interfaces of the critical sections. (3) The fatigue test data of the welded specimens, when arranged on the relation between load amplitude Pa and cycles td fracture N, showed that the load amplitude equivalent to the endurance limit increased by the factor of about two in order of (a), (b) and (c). (4) In terms of nominal stress amplitude σ_<na> the S-N relations of the spot welded fatigue specimens (a), (b) and (c) converged almost on the same level. (5) The slopes of S-N relation under pulsating tension load of the spot welded specimens were considerably steeper than that of the plain specimen, owing to the effect of the stress gradient in the critical area. They were nearly parallel to the slope of S-N relation obtained from the previous reversed plane bending tests on the spot welded specimens. (6) In both present pulsating tension and previous reversed bending tests, the " fatigue strength reduction factor " of the spot welded specimen expressed in terms of the nominal stress, attained to almost the same value (about 1.4), irrespective of the difference in joining type of specimens or loading.
1975-04-15

スポット溶接した冷延鋼板の疲労強度

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