(1) reduce the stress concentration fatigue crack source in the welded joints and the structure of the stress concentration point, eliminate or reduce stress concentration of all means to improve the fatigue strength of the structure.
(1) the use of reasonable structural form
(1) Priority is given to butt joints, try not to use lap joints; important structures to change T-joints or corner joints into butt joints, so that the weld avoids the corner parts; the use of T-joints or corner joints, it is hoped that the use of full penetration butt welds.
② Try to avoid the design of partial load, so that the transfer of internal force of the member is smooth and evenly distributed, without causing additional stress.
③ Reduce the section mutation, when the plate thickness or plate width difference and need to butt joints, should be designed to smooth the transition zone; the structure of the sharp corners or corners should be made into a rounded arc, the larger its radius of curvature, the better.
④ Avoid the spatial convergence of three-way weld, weld as far as possible not to be set in the stress concentration area, as far as possible not to be set in the main tensile members of the transverse weld; unavoidable, be sure to ensure that the inside and outside of the quality of this weld, to reduce the concentration of stress at the weld.
⑤ Butt welds that can only be applied on one side, and permanent pads are not allowed to be placed on the back side on important structures; avoid using interrupted welds because of the high stress concentration at the beginning and end of each weld section.
(2) Correct weld shape and good quality of weld inside and outside.
(1) The residual height of the butt joint weld should be as small as possible, and it is better to plane (or grind) flat without residual height after welding;
② T-type joints should preferably use fillet welds with concave surfaces, not with convex fillet welds;
③ weld and the parent material surface junction of the weld toe should be smooth transition, if necessary, the toe of the weld grinding or argon arc remelting, in order to reduce the concentration of stress in the place.
Any welding defects have different degrees of stress concentration, especially flaky welding defects, such as cracks, unwelded, unfused and biting edges, etc. have the greatest impact on fatigue strength. Therefore, the structure should be designed to ensure that each weld is easy to apply to minimize welding defects, while any defects found to be excessive must be removed.
2) Adjustment of residual stress
The residual compressive stress that exists on the surface of the member or at the stress concentration can improve the fatigue strength of the welded structure. For example, by adjusting the welding sequence, local heating, etc. are likely to obtain a residual stress field conducive to improving fatigue strength. In addition, you can also take the surface deformation strengthening, such as rolling, hammering or shot blasting and other processes to make the metal surface plastic deformation and hardening, and in the surface layer of the residual compressive stress, in order to achieve the purpose of improving the fatigue strength.
For notched members, take a one-time pre overload stretching, can make the top of the notch to get the residual compressive stress. Because after elastic unloading, the sign of the notched residual stress is always opposite to the sign of the notched stress during (elastic-plastic) loading. This method is not suitable for bending overloading or multiple tensile loading. It is often combined with structural acceptance tests, such as pressure vessels to do hydraulic testing, can play a pre-overload tensile role.
(3) improve the organization and properties of materials
First of all, to improve the fatigue strength of the 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 inclusions in it. Important components can be used in vacuum melting, vacuum degassing, and even electroslag remelting and other smelting processes of the material to ensure purity; refinement of grain steel at room temperature can improve fatigue life; heat treatment can be obtained through the optimal state of the organization, in order to improve the strength at the same time, but also to improve its plasticity and toughness; tempered martensite, low carbon martensite and lower bainite and other organizations have a higher fatigue resistance. Secondly, strength, plasticity and toughness should be reasonable cooperation. Strength is the ability of the material to resist fracture, but high-strength materials are sensitive to notch. The main role of plasticity is through plastic deformation, can absorb the deformation work, cut the stress peak, so that the high stress redistribution, but also to make the gap and the crack tip can be blunted, the crack expansion is eased or even stopped. Plasticity ensures that the role of strength is given full play. So for high-strength steel and ultra-high-strength steel, try to improve a little plasticity and toughness, will significantly improve its fatigue resistance.
(4) Special protective measures
Atmospheric media erosion often has an impact on the fatigue strength of the material, therefore, the use of certain protective coatings is beneficial. For example, in the stress concentration coated with a plastic layer containing filler is a practical method of improvement.
