| Based on a series of advantages, such as light weight and high strength, abundant resources and be apt to recycle, magnesium alloy and aluminum alloy are widely applied to welded structures field. It is reported that about 70%-90% failure accidents involving welded structures due to fatigue fracture in welded joints, so the fatigue property of welded joints is a key character that affects welded structures. Consequently, study on fatigue evaluation of magnesium alloy and aluminum alloy welded joints has important theoretical and practical significance for security of welded structures.In this paper, the fatigue strength of AZ31B magnesium alloy common welded joints was evaluated by hot spot stress approach. Both hot spot stress and volumetric approaches were applied to fatigue evaluation of 5A06 aluminum alloy welded joints, and results of two evaluation approaches were compared. Finally, the fatigue properties of AZ31B magnesium alloy and 5A06 aluminum alloy welded joints by different evaluation approaches were compared and analyzed.It is indicated that the nominal stress S-N curve slope m of butt joint, transverse cruciform joint, longitudinal cruciform joint and side link joint of AZ31B magnesium alloy fitted directly to the test data is 4.49,3.39,3.37 and 2.61 respectively, the characteristic fatigue resistance at 2x106 cycles of which is 34.75 MPa,19.31 MPa,25.30 MPa and 23.51 MPa; the hot spot stress S-N curve slope m is 4.47,3.45,3.39 and 2.64 respectively, the characteristic fatigue resistance at 2x1O6 cycles of which is 42.07 MPa,29.27 MPa,37.83 MPa and 36.01 MPa. The hot spot stress S-N curve slope m is 3.22 when four kinds of welded joints are expressed by one S-N curve, the expression is =11.15-3.221gA△σand characteristic fatigue resistance at 2x106 cycles is 32.05 MPa, higher than 31.60 MPa obtained from using 3.0 recommended by International Institute of Welding (IIW).The nominal stress S-N curve slope m of butt joint, transverse cruciform joint and longitudinal cruciform joint of 5A06 aluminum alloy welded joints fitted directly to the test data is 6.78,6.96 and 3.77 respectively, the characteristic fatigue resistance at 2x106 cycles of which is 59.45MPa, 50.90MPa and 33.08MPa; the hot spot stress S-N curve slope m is 6.54,6.95 and 3.79 respectively, the characteristic fatigue resistance at 2x106 cycles of which is 67.87 MPa,61.83 MPa and 47.91 MPa. The hot spot stress S-N curve slope m is 5.98 when three kinds of welded joints are expressed by one S-N curve, the expression is lgN=16.67-5.98lg△σand the characteristic fatigue resistance at 2x106 cycles is 54.19 MPa, higher than 40.09 MPa obtained from using 3.0 recommended by IIW.The volumetric approach is applied to evaluate the fatigue resistance of transverse cruciform joint and longitudinal cruciform joint of 5A06 aluminum alloy, the S-N curve slope is 5.87 and 4.77 respectively, the fatigue resistance at 2x1O6 cycles is 103.96 MPa and 91.80 MPa, average is 97.88 MPa. Fatigue notch reduction coefficient scope△Kf is from 1.754 to 1.951 for transverse cruciform joint, from 1.819 to 2.310 for longitudinal cruciform joint. In comparison with nominal stress approach, the data dispersibility evaluated by hot spot stress and volumetric approaches is reduced greatly, and correlation coefficient R improves 15.95%and 16.29%respectively, which indicates the hot spot stress and volumetric approaches are effective fatigue evaluation methods.The nominal stress and hot spot stress fatigue resistances of three kinds of AZ31B magnesium alloy welded joints are higher than corresponding 5A06 aluminum alloy welded joints. Except for side link joint of AZ31B magnesium alloy, the nominal stress and hot spot stress S-N curve slope of butt joint, transverse cruciform joint and longitudinal cruciform joint fitted directly to the test data of AZ31B magnesium alloy and 5A06 aluminum alloy is greater than 3.0 recommended by IIW, the fatigue resistance at 2x 106 cycles higher than FAT obtained from using 3.0. Moreover, the fatigue property of 5A06 aluminum alloy accords with the standard recommended by IIW basically, and the above research provides theoretical foundation for magnesium alloy criterion.In conclusion, the nominal stress fatigue resistance of four kinds of AZ31B magnesium alloy and three kinds of 5A06 aluminum alloy welded joints has to be expressed by different S-N curves according to the types of welded joints, while the hot spot stress and volumetric fatigue resistance of which can be expressed by only single S-N curve. It indicates that the Fatigue Level of welded joints can be reduced greatly evaluated by hot spot stress and volumetric approaches, which simplify the fatigue evaluation of magnesium alloy and aluminum alloy welded joints, and contribute to design, manufacture and safe employ of welded structures.
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