4 Measures to Improve Fatigue Strength of Welded Structures

Reducing the stress concentration point of the stress concentration fatigue crack source on the welded joint and structure, and eliminating or reducing the stress concentration by all means can improve the fatigue strength of the structure.

(1) Adopt reasonable structure

① Butt joint shall be preferred, and lap joint shall not be used as far as possible;

For important structures, the T-shaped joint or corner joint shall be changed into butt joint, so that the weld can avoid the corner;

When T-joints or corner joints are used, full penetration butt welds are preferred.

② The design of eccentric loading shall be avoided as far as possible to ensure smooth transmission and uniform distribution of internal forces of members without causing additional stress.

③ Reduce the abrupt change of the section. When butt joint is required due to the great difference of plate thickness or width, a gentle transition zone shall be designed;

The sharp corners or corners of the structure shall be made into circular arcs, and the larger the curvature radius, the better.

④ Avoid the space intersection of three-way welds.

The welds shall not be set in the stress concentration area as far as possible, and the transverse welds shall not be set on the main tension members as far as possible;

If unavoidable, the internal and external quality of the weld must be guaranteed to reduce the stress concentration at the weld toe.

⑤ For butt welds that can only be welded on one side, it is not allowed to place permanent backing plates on the back of important structures;

Intermittent welds shall be avoided because there is high stress concentration at the beginning and end of each weld.

(2) Correct weld shape and good internal and external weld quality

① The weld reinforcement of butt joint shall be as small as possible, and it is better to plane (or grind) the weld after welding without leaving any reinforcement;

② It is better to adopt fillet weld with concave surface for T-shaped joint, instead of convex fillet weld;

③ The weld toe at the junction of the weld and the base metal surface shall be transited smoothly.

If necessary, the weld toe shall be ground or remelted by argon arc to reduce the stress concentration.

Any welding defect has different degree of stress concentration, especially sheet welding defects, such as crack, incomplete penetration, incomplete fusion and undercut, which have the greatest impact on fatigue strength.

Therefore, in the structural design, it is necessary to ensure that each weld is easy to weld, so as to reduce the welding defects.

At the same time, the defects that exceed the standard must be removed.

2) Adjust residual stress

The residual compressive stress on the component surface or at the stress concentration can improve the fatigue strength of welded structures.

For example, it is possible to obtain the residual stress field that is conducive to improving the fatigue strength by adjusting the welding sequence and local heating.

In addition, surface deformation strengthening, such as rolling, hammering or shot peening, can be used to harden the metal surface due to plastic deformation, and generate residual compressive stress in the surface layer to improve the fatigue strength.

For the components with notches, the residual compressive stress can be obtained at the top of the notch by adopting one-time pre overload tension.

Because after elastic unloading, the sign of notch residual stress is always opposite to that of notch stress under (elastic-plastic) loading.

This method should not use bending overload or multiple tensile loading.

It is often combined with the structural acceptance test. For example, when the pressure vessel is subjected to the hydrostatic test, it can play a pre overload tensile role.

3) Improve the structure and properties of materials

First of all, improving the fatigue strength of base metal and weld metal should also be considered from the internal quality of materials.

The metallurgical quality of materials should be improved to reduce inclusions.

Important components can be made of materials by vacuum smelting, vacuum degassing, or even electroslag remelting to ensure purity;

Refining grain steel at room temperature can increase fatigue life;

The best microstructure can be obtained by heat treatment, which can improve the plasticity and toughness as well as the strength;

Tempered martensite, low carbon martensite and lower bainite have high fatigue resistance.

Secondly, strength, plasticity and toughness should be properly matched.

Strength is the ability of materials to resist fracture, but high-strength materials are sensitive to notch.

The main function of plasticity is to absorb deformation work, reduce stress peak value, redistribute high stress, passivate notch and crack tip, and ease or even stop crack growth through plastic deformation.

The plastic property ensures the full play of strength.

Therefore, for high strength steel and ultra-high strength steel, trying to improve a little plasticity and toughness will significantly improve their fatigue resistance.

4) Special protective measures

The corrosion of atmospheric medium often affects the fatigue strength of materials.

Therefore, it is beneficial to use a certain protective coating.

For example, it is a practical improvement method to coat the plastic layer containing filler at the stress concentration.

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