1.3 Research significance of improving the fatigue performance of welded structures
The frequent occurrence of fatigue accidents restricts the further extensive application of welded structures to a certain extent, so that some occasions have to give up the use of welded structures, and even doubt whether the welded structure can be applied to the engineering practice of bearing dynamic loads, so the fatigue resistance of welded structures It has aroused widespread concern from relevant experts and engineering technicians at home and abroad, especially the Fatigue Professional Committee of the International Welding Society. On the basis of a large number of fatigue tests and engineering practices, the anti-fatigue design specifications for welded structures are continuously introduced, such as the British Bridge Fatigue Design Specification BS5400, the European Steel Structure Association's Fatigue Design Specification, the Japanese Steel Bridge Design Code, the American Railway Bridge and the Highway. Design specification, Fatigue design specification for cyclically loaded welded steel structures of International Welding Society IIW.DOC-639-8l and China's steel structure design specification GB-17-88. The world's major shipbuilding and marine resource development countries have made provisions and requirements on the fatigue strength of welded structures in the design, construction and inspection of marine and offshore engineering structures.
Due to the welding defects, stress concentration and residual tensile stress at the weld toe of the welded joint, the fatigue strength is significantly lower than the fatigue strength of the base metal. Therefore, the fatigue strength of the welded structure depends on the fatigue performance of the joint, that is, the fatigue resistance of the welded joint, and whether the welded structure can be safely used. Therefore, in order to ensure the reliability of the welded structure, when designing the welded structure subjected to the variable load, the design specification stipulates the fatigue strength of the welded joint as the fatigue strength of the overall structure, instead of the fatigue strength of the base metal, which obviously causes great waste. . Even so, premature fatigue failure of the overall structure can occur at local stress concentrations at the joint. In order to make the welded structure well meet the requirements of the engineering to bear the dynamic load, there are two main measures that can be taken. On the one hand, increase the understanding of the fatigue resistance of welded structures, carefully design the structural form and joint form, make the designed welded structure more reasonable, have higher fatigue strength; at the same time improve and strictly control the welding quality, prevent and reduce welding defects On the other hand, directly facing the weakness of the welded joints with poor fatigue performance, effective process measures are taken during the manufacturing process, after completion and during the use of the welded structure to improve the fatigue strength of the joint and increase its dynamic load. Ability to extend its life.
Therefore, improving and improving the fatigue strength of welded joints has great potential economic and social benefits. For a long time, it has been a hot topic for experts at home and abroad.
2 Main factors affecting the fatigue strength of welded structures
2.1 Influence of static load strength on fatigue strength of welded structures
In the study of steel materials, people always want the material to have a higher specific strength, that is, to bear a larger load weight with a lighter weight, because the same weight of the structure can have a great load carrying capacity; or the same The carrying capacity can reduce its own weight. Therefore, high-strength steel comes into being, and it also has high fatigue strength. The fatigue strength of basic metals always increases with the increase of static load strength.
However, for the welded structure, the situation is different, because the fatigue strength of the welded joint does not have much relationship with the static strength of the base metal, the static strength of the weld metal, the microstructure of the heat affected zone and the weld metal strength, that is, It is said that as long as the details of the welded joint are the same, the fatigue strength of the high-strength steel and the low-carbon steel is the same, and has the same SN curve. This rule is suitable for various joint types such as butt joints, angle joints and welded beams. Maddox studied the fatigue crack growth of carbon-manganese steel with a yield point between 386 and 636 MPa and weld metal and heat-affected zone with six kinds of welding rods. The results show that the mechanical properties of the material have a certain influence on the crack growth rate. But the impact is not big. When designing welded structures that are subjected to alternating loads, it is meaningless to attempt to meet engineering needs by selecting higher strength steel grades. Only when the stress ratio is greater than +0.5, when the static strength condition plays a major role, the high-strength steel should be used for the welded joint base metal.
The reason for the above results is that there is a slag wedge defect with a similar undercut along the weld line at the weld toe portion, the thickness of which is 0.075 mm-0.5 mm, and the tip half length is less than 0.015 mm. The sharp defect is the place where the fatigue crack starts, which is equivalent to the fatigue crack formation stage. Therefore, the fatigue life of the joint under a certain stress amplitude is mainly determined by the expansion stage of the fatigue crack. The appearance of these defects makes the same type of welded joints of all steels have the same fatigue strength, and has little to do with the static strength of the base metal and the welding material.
2.2 Effect of stress concentration on fatigue strength
2.2.1 Effect of joint type
The form of the welded joint mainly includes: butt joint, cross joint, T joint and lap joint, and stress concentration occurs due to interference of the force transmission line at the joint portion.
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