Measures to improve the fatigue strength of welded structures

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Measures to improve the fatigue strength of welded structures: 1) Reducing stress concentration. Any means to eliminate or reduce stress concentration at the stress concentration points in welded joints and structures can enhance the fatigue strength of the structure. (1) Adopt a reasonable structural form: ① Give priority to butt joints, and try to avoid lap joints ； For critical structures, change T-joints or corner joints to butt joints so that the welds avoid the corners ； When using T-joints or corner joints, full-penetration butt welds are desired. ②Try to avoid designs with eccentric loading, so as to ensure smooth transfer and even distribution of internal forces in the components, thereby preventing the generation of additional stresses. ③Reduce abrupt changes in cross-section; when there is a large difference in plate thickness or width and welding is required, a gradual transition zone should be designed ； Sharp corners or edges in the structure should be rounded, with a larger radius of curvature being preferable. ④Avoid the spatial intersection of three-way welds; welds should be placed as far as possible away from stress concentration areas, and transverse welds should not be used on components that are primarily under tension ； When it is inevitable, it is essential to ensure the internal and external quality of the weld and to reduce stress concentration at the weld toe. ⑤For butt welds that can only be welded on one side, permanent shims are not allowed to be placed on the back side in critical structures ； Avoid using intermittent welds, as there is high stress concentration at the beginning and end of each weld segment. (2) Correct weld shape and good internal and external quality of the weld ① The excess height of the weld in butt joints should be as small as possible; it is preferable to plane (or grind) it flat after welding so that no excess height remains ； ②For T-joints, fillet welds with a concave surface are preferred; fillet welds with a convex surface should not be used ； ③The weld toe at the junction of the weld seam and the base metal surface should have a smooth transition; if necessary, the weld toe should be ground or remelted using TIG welding in order to reduce stress concentration at that location. Any welding defect involves stress concentration to varying degrees; in particular, sheet-like welding defects such as cracks, lack of penetration, lack of fusion, and undercut have the greatest impact on fatigue strength. Therefore, in structural design, it is necessary to ensure that each weld is easy to weld in order to reduce welding defects, and any defects that exceed the acceptable limits must be removed. 2) Adjusting residual stress: Reducing the residual compressive stress present on the surface of components or at stress concentration areas can improve the fatigue strength of welded structures. For example, by adjusting the welding sequence or applying local heating, it is possible to obtain a residual stress field that helps improve fatigue strength. In addition, surface deformation strengthening can also be employed, using processes such as rolling, hammering, or shot peening to plastically deform the metal surface and thereby harden it, while generating residual compressive stress in the surface layer to improve fatigue strength. For notched members, a single pre-overload tensile test can induce residual compressive stress at the notch tip. Because after elastic unloading, the sign of the residual stress at the notch is always opposite to the sign of the stress at the notch during (elasto-plastic) loading. This method is not suitable for bending overload or multiple tensile loading. It is often used in conjunction with structural acceptance tests; for example, when pressure vessels are subjected to hydrostatic testing, it can provide a pre-overload tensile effect. 3) Improving the microstructure and properties of materials: Firstly, enhancing the fatigue strength of the base metal and weld metal also requires considering the inherent quality of the materials. The metallurgical quality of the material should be improved to reduce the inclusions in it. Important components can be made from materials processed using techniques such as vacuum melting, vacuum degassing, or even electroslag remelting, in order to ensure purity ； Refining grain size in steel at room temperature can improve fatigue life ； The optimal microstructural state can be achieved through heat treatment, which improves strength while also enhancing its plasticity and toughness ； Structures such as tempered martensite, low-carbon martensite, and lower bainite all possess high fatigue resistance. Secondly, strength, plasticity, and toughness should be properly balanced. Strength is a material’s ability to resist fracture, but high-strength materials are sensitive to notches. The main function of plasticity is to absorb deformation energy through plastic deformation, reducing stress peaks and redistributing high stresses. It also helps to blunt the tips of notches and cracks, thereby slowing down or even stopping crack propagation. The plasticity ensures that the strength function is fully utilized. Therefore, for high-strength steel and ultra-high-strength steel, improving their plasticity and toughness to some extent will significantly enhance their fatigue resistance. 4) Special protective measures: Erosion by the atmospheric environment often affects the fatigue strength of materials; therefore, it is beneficial to use appropriate protective coatings. For example, applying a plastic layer containing fillers at stress concentration points is a practical improvement method.