1 Fatigue problems of welded structures and their significance
1.1 Fatigue problems of welded structures
Since the invention of the coated electrode in the early 20th century, welding has become the most widely used process for a hundred years. It is difficult to find another development that is so fast, and compared with welding in terms of application scale and diversification. The process, so many of the most important technical problems of the contemporary must be solved by welding, such as: shipbuilding, railway, automotive, aviation, aerospace, bridges, boilers, large factories and high-rise buildings are inseparable from the support of welding technology. If the welding is not invented, many structures are even frankly said that the entire industry will not produce. Undoubtedly, welding is currently the most important connection method in engineering production. The weight of the welded structure has accounted for more than 50% of the total steel production. The proportion of industrial developed countries is close to 70%. However, welding structures often suffer from breakage accidents, of which 90% are fatigue failures.
Fatigue damage has always been considered as a major form of damage to ship and marine engineering structures. Since the birth of steel seagoing vessels, there have been many reports of the occurrence of fatigue cracks in the structure and the resulting destruction of ships. The US Coast Guard Ship Structure Committee (USCoast Guard) has organized investigations and statistical analysis of more than 600,000 structural details of seven different types of 77 civilian ships and nine warships. About one-ninth of the damage is related to fatigue. Historically, several major accidents on the offshore platform, such as the Sedco-type semi-submersible platform built by Japan in the United States in 1965, were damaged and sank in the delivery process, causing 13 deaths; in 1980, the Alexan-derKeyland semi-submersible platform sank in the North Sea. More than 100 people were buried in the seabed. The results of the investigation and analysis indicate that structural fatigue is one of the important causes of accidents.
Similarly, fatigue failures occur frequently on pipelines in railways, highways, and power stations. In the 1950s and 1960s, the European road network developed rapidly. At that time, most of them used welding technology to build steel bridges. Because of the lack of understanding of road bridge fatigue at that time, there was no anti-fatigue design in the specification, and many unreasonable weldings appeared. Joints, under today's increasingly busy and aggravated traffic loads, accelerate the fatigue damage process, and many welded steel bridges have fatigue cracks.
Engineering examples of failure of welded structures in China due to fatigue problems have also emerged. For example, in the late 1990s, fatigue fractures of welded joints in high-speed passenger car bogies and fatigue fractures at the roots of turbine blades have caused problems for countries and enterprises. Huge economic loss.
1.2 Causes of fatigue failure of welded structures
The causes of fatigue failure of welded structures are mainly as follows: 1 Objectively speaking, the static load bearing capacity of welded joints is generally not lower than that of the parent metal; while the subjected to variable load, the bearing capacity is much lower than that of the parent metal. It is also closely related to the type of welded joint and the form of welded structure. This is a major factor causing premature failure of some structures due to fatigue of welded joints; 2 Early welding structure design is based on static load strength design, no anti-fatigue design is considered, or the fatigue design specification of welded structure is not perfect. So that there are many welded joints that seem to have unreasonable design; 3 engineering design technicians have insufficient understanding of the fatigue resistance characteristics of welded structures, and the designed welded structures often copy the fatigue design criteria and structural forms of other metal structures; 4 The welding structure is increasingly wide, and in the design and manufacturing process, the blind pursuit of low cost and light weight of the structure leads to the increasing design load of the welded structure. 5 The welding structure has a tendency to develop in the direction of high speed and heavy load, and the welding The requirements for the structure to withstand the dynamic load capacity are getting higher and higher, and the research level of the fatigue strength of the welded structure is relatively lagging behind.
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