Measures to improve the fatigue strength of welded structures
1) Reduce the stress concentration of the fatigue crack source in the welded joint and the structure of the stress concentration point, eliminate or reduce all means of stress concentration, can improve the fatigue strength of the structure.
(1) Adopt a reasonable structural form
① Give priority to butt joints, try not to use lap joints; The T-shaped joint or Angle joint is changed into a butt joint, so that the weld avoids the corner part; When using T-joints or Angle joints, it is desirable to use fully permeable butt welds.
② Try to avoid the design of eccentric load, so that the internal force of the member is smooth and evenly distributed, and does not cause additional stress.
③ To reduce the sudden change of section, when the plate thickness or plate width is very different and the docking is needed, a smooth transition area should be designed; The sharp corner or corner on the structure should be made into an arc shape, and the larger the radius of curvature, the better.
④ Avoid the spatial convergence of three-way welds, try not to set the welds in the stress concentration area, and try not to set the transverse welds on the main tension members; When unavoidable, it is necessary to ensure the internal and external quality of the weld and reduce the stress concentration at the toe.
⑤The butt weld can only be welded on one side, and the permanent backing plate is not allowed to be placed on the back of the important structure; Avoid using intermittent welds because there is a high stress concentration at the beginning end of each weld.
(2) Correct weld shape and good weld inside and outside quality
(1) The residual height of the butt joint weld should be as small as possible, and it is best to plane (or grind) flat after welding without leaving any residual height;
② It is best to use fillet welds with concave surfaces for T-shaped joints, without fillet welds with convexity;
③ The toe at the junction of the weld and the base metal surface should be smoothly transited, and the toe should be ground or argon arc remelted if necessary to reduce the stress concentration there.
All welding defects have different degrees of stress concentration, especially flake welding defects, such as cracks, non-penetration, non-fusion and edge biting, etc., have the greatest impact on fatigue strength. Therefore, in the structural design, it is necessary to ensure that each weld is easy to weld, in order to reduce welding defects, and the defects that exceed the standard must be removed.
2) Adjust the residual stress
The residual compressive stress on the surface of the member or the stress concentration can improve the fatigue strength of the welded structure. For example, by adjusting the welding sequence and local heating, it is possible to obtain a residual stress field that is conducive to improving the fatigue strength. In addition, surface deformation strengthening, such as rolling, hammering or shot peening, can also be adopted to make the metal surface plastic deformation and hardening, and produce residual compressive stress in the surface layer to achieve the purpose of improving fatigue strength.
The residual compressive stress at the top of the notch can be obtained by using one-time pre-overload stretching for the notched member. This is because the sign of the notch residual stress after elastic unloading is always the opposite of the sign of the notch stress during (elastoplastic) loading. This method is not suitable for bending overload or multiple tensile loading. It is often combined with structural acceptance tests, such as pressure vessels for hydraulic tests, can play a pre-overload tensile role.
3) Improve the structure and properties of the material
First of all, improving the fatigue strength of base metal and weld metal should also be considered from the intrinsic quality of the material. The metallurgical quality of the material should be improved to reduce the inclusion in it. Important components can be made of materials from smelting processes such as vacuum melting, vacuum degassing, and even electroslag remelting to ensure purity; The fatigue life of grain steel can be improved by refining at room temperature. The best microstructure can be obtained by heat treatment, and the plasticity and toughness can be improved while the strength is increased. Tempered martensite, low carbon martensite and lower bainite have higher fatigue resistance. Secondly, strength, plasticity and toughness should be reasonably matched. Strength is the ability of a material to resist breaking, but high-strength materials are sensitive to notches. The main function of plasticity is that through plastic deformation, deformation work can be absorbed, stress peak can be reduced, high stress can be redistributed, and the notch and crack tip can be passivated, and the crack expansion can be alleviated or even stopped. Plasticity can ensure that the strength of the full play. Therefore, for high-strength steel and ultra-high-strength steel, trying to improve a little plasticity and toughness will significantly improve its fatigue resistance.
4) Special protection measures
Atmospheric medium erosion often has an impact on the fatigue strength of materials, so it is advantageous to use a certain protective coating. For example, coating a plastic layer containing fillers at stress concentrations is a practical improvement method.
