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Factors affecting metal fatigue strength, such as stress concentration, section size, medium, etc., also affect the welding fatigue strength. In addition, some characteristics of the welded structure itself, such as changes in properties near the joint and welding residual stress, may also affect the fatigue strength of the welded structure. Finding out the specific effects of these factors is conducive to improving the fatigue strength of the welded structure, The influence of these factors is discussed below.
1.Effect of stress concentration
In welded structures, due to the different stress concentrations at the joints, they have different effects on the fatigue strength of the joints.
(1)Butt weld: the stress concentration is less than that of other joints. Machining the weld surface will reduce the stress concentration and improve the fatigue strength of butt joints.
(2)T-joint and cross joint: the stress concentration factor is greater than that of butt joint, so the fatigue strength is lower than that of butt joint.
Grooving welding and machining the weld transition zone to make it smooth transition can improve the fatigue strength of T-joint and cross joint.
(3)Only the lap joint of side weld has the lowest fatigue strength (only 34% of the base metal).
2.Influence of metal properties in near seam region
(1)The change of metal mechanical properties near the seam in low carbon steel has little effect on the fatigue strength of the joint.
(2)When welding high-strength steel, for the welded joint with high composition, the non-uniformity of mechanical properties has no effect on the fatigue strength, which depends on the softer metal.
When there is a stress concentration factor in the high assembly hard clip soft joint, the fatigue strength will decrease, which depends on the mechanical properties of the soft zone itself.
3.Effect of residual stress
The influence of welding residual stress on structural fatigue strength is a widely concerned problem. People have done a lot of experimental research on this problem. The test often compares the sample with welding stress with the sample with internal stress eliminated after heat treatment. The welding residual stress changes with the material properties caused by welding thermal cycle. Heat treatment can eliminate internal stress and restore some material properties, There are different explanations for the test results and different evaluations for the influence of internal stress.
The fatigue strength of the sample after internal stress elimination is higher than that of the final heat treatment. The higher the stress concentration, the greater the influence of internal stress.
4、Effect of defects
(1)The influence of fatigue strength is related to the type, size and direction of welding defects.
(2)The influence of sheet defects is greater than that of defects with rounded corners
(3)The influence of surface defects is greater than that of internal defects
(4)The influence of flake defects perpendicular to the force direction is greater than that in other directions
(5)The influence of defects in the residual tensile stress zone is greater than that in the residual compressive stress zone
(6)The effect of defects in the stress concentration area is greater than that of the same defects in the uniform stress field.
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