Predicting structural life is one of the difficult problems in engineering. The service life of orthotropic steel bridge deck is much lower than the design expectation. Multiple fatigue fractures not only reduce the bearing capacity of the structure, but also are very difficult to repair, which has become a difficult heart disease in the bridge industry. The reason is that the load effect does not match the fatigue resistance of welded joints.
Fatigue is the result of the interaction between joint damage and repeated load. On the one hand, it is difficult to determine the damage distribution and type of ultra long and high-density U-rib weld of orthotropic steel deck; On the other hand, the location, size and frequency of the moving load acting on it are not clear, so it is more difficult to determine the expected life of this kind of bridge deck.
The fundamental to solve the problem is to realize low damage and high quality welded joints. Limited by the closed structural space of U-rib, the one-sided partial penetration weld is used to connect with the top plate, so that the non penetration part becomes a crack like notch, which greatly reduces the joint resistance.
Details improve behavior, damage control life, and process determines success or failure. The domestic original bilateral submerged arc full penetration welding process has been successfully used in U-rib welding, which greatly improves the fatigue reliability of the joint and becomes a sharp tool to crack the low age of orthotropic steel bridge deck.
current situation: young diseases become a problem
Orthotropic plate steel bridge is an outstanding work of applying welding technology to bridge structure in the 20th century. It has been widely used in bridge engineering all over the world for more than 70 years because of its reasonable structure, economical material use and meeting the complex stress requirements of bridges to the greatest extent. The length and density of welds in steel bridge deck are inferior to all welded structures. However, the design service life of the bridge for up to 100 years is the highest welded structure standard and requirement.
The aging of orthotropic steel deck is mainly controlled by the longitudinal weld zone of super long U-rib. During the operation and service period, multiple longitudinal fatigue cracks with uncontrollable shape, length, location and quantity occur randomly, which not only prematurely reduces the service function of the bridge, but also has a very low success rate and high repair cost. The low age disease of orthotropic steel bridge surface has become an unsolved problem in the bridge industry. According to the statistics of Japan in the 1970s, the service life of through cracks on bridge decks passing through 3000 large vehicles in 12 hours is about 10 years. In comparison, the time of disease in China is shorter.
After the fatigue crack propagates and penetrates the bridge deck, it will accelerate under load. When the length and number increase, the bearing function of the structure decreases significantly. The difficulty of repairing and strengthening the bridge deck can be seen from figures 3 and 4. Welding repair is effective in a short time, but has not been successful in a long time.
welded joint: connection of concentrated structure life
The welding thermal process not only coarsens the micro metal structure and reduces the comprehensive mechanical properties in the joint area, but also forms geometric defects such as internal and external defects, undercut, incomplete penetration and incomplete fusion, but also forms mechanical, chemical, metallurgic and geometric discontinuities in the joint area. Geometric defect is a "crack like notch" in fracture mechanics, which is the same as welding crack. It is the most important factor to cause fatigue crack and control life. The fatigue test results of 128 welded steel beams are summarized in the report no. 147 of the national highway cooperative research program (NCHRP), which shows that the fatigue cracks originate from small defects in the weld. In most of the fatigue life, the fatigue crack is semi elliptical and propagates along the plate thickness. The crack propagation process through the plate thickness accounts for about 80% ~ 95% of the fatigue life, depending on the details. From this point of view, the influence of mechanical discontinuity on welding residual stress is much less than "notch effect".
The fatigue resistance of welded joints is not directly related to the size of welds, nor to the yield strength of structural steel (FY ≤ 960mpa), but is determined by the damage to the joint area during welding (crack like notch).
German Welding master D. radaj used "the best welding structure is the structure without weld", indicating the inevitability of defects in welded joints. Professor Xu Jinquan, author of fatigue mechanics, also expressed the concept of defect control life as, "All things have defects, and they will produce defects by themselves. Defects are born and die constantly and have a cumulative potential. Their potential changes from strong to weak, so the number of all things varies. Look at their images, observe their theories, form their potential, know their numbers, and how they exist. If numbers are contrary to each other, they will be false, theories will be contrary, and images will be false, so it is important to know numbers."
The welded joint between U-rib and bridge deck is inclined T-shaped (70 ° ~ 75 °), which is a T-shaped welded joint. American Bridge Welding Code (AASHTO / AWS d1.5m / d1.5) The T-joint has the following provisions: the fillet joint and T-joint that bear bending parallel to the joint axis must be designed to avoid tensile stress concentration at any weld root. When the T-joint adopts bilateral partial penetration weld, the non penetration part is regarded as a defect, the fatigue resistance is lower than that of the penetration weld, and its reduction coefficient is in the American code for design of building steel structures (ANSI / AISC 360-10) is calculated according to the incomplete penetration length and expressed as rpjp & lt; 1.
For a long time, orthotropic plates all over the world have been welded outside the U-rib to form a partially penetrated single-sided weld, which has become a special case of violation.
confusion in predicting fatigue life
Predicting the structural life is a process of predicting damage propagation, which is a major problem in the engineering field. The bridge design code can only use the expected value for the structural life. The design service life ≥ 100 years is the expected life of the structure, which is limited to the occurrence of visible cracks without repair. Under the condition of clarifying the crack location and the nominal stress amplitude level of material mechanics, the S-N curve method is used to evaluate the structural life The S-N curve is determined by the fatigue test statistics of typical welded joints. The life can be expressed as: n = C · s-m
S is the unidirectional stress amplitude obtained by the method of material mechanics. For complex stress system, it needs to be simplified.
The fatigue life is predicted by the method of fracture mechanics Δ K and initial damage A0 are obtained:
Due to the uncertainty of the location, size and frequency of the acting load on the orthotropic plate, the damage distribution and damage degree of the bridge deck structure during the operation period are also uncertain, and the nominal stress at the damage is difficult to calculate accurately. Many factors can not form a definite relationship, when and where fatigue cracks will occur, and the prediction of life is also difficult to be accurate.
Because the stresses of the first system and the third system of orthotropic bridge deck are orthogonal, they have no effect on fatigue, and the prediction of fatigue life will be different.
The failure criterion of fatigue life is the appearance of visible cracks, and the surface length of penetrating cracks is more than 3 times the plate thickness.
The fatigue life is determined according to the equal fatigue damage principle
The orthotropic bridge deck is multi span continuous in the longitudinal direction, and the multiaxial effect makes the repeated times of the live load action of the third system greatly exceed that of the main structure of the first system. According to the principle of equal fatigue damage, when the main structure fails, the constant amplitude fatigue life of the bridge deck is n Ⅰ, and the crack length a Ⅰ is the same as that of the bridge deck a Ⅲ, the constant amplitude fatigue life of the bridge deck is n Ⅲ & gt; n Ⅰ, which can Avoid the early occurrence of fatigue cracks in bridge deck.
According to the standard fatigue vehicle loading in the domestic highway code, assuming that the vehicle crosses the bridge once, and considering the multi axle effect of the loaded vehicle, the loading times of the third system are k times that of the first system. Because the transverse loading is not sensitive to the longitudinal wheelbase, when the diaphragm spacing is set as 3M, k = 2.5 and the constant amplitude loading times n Ⅲ = 2.5 × two × 106=5 × 106. Therefore, the first system and the third system of orthotropic bridge deck should be designed according to different life to delay the occurrence time of fatigue cracks in bridge deck.
Equal damage and unequal life design method is also one of the fatigue design principles of aircraft structures.
the best countermeasure against fatigue
The high quality of welded joint needs the process to realize. Taking the rolled T-shaped steel as the standard, it is determined as the process goal of U-rib welded joint.
For welded joints, the design and process criteria formulated by the International Welding Association (IIW) are "not perfect, but suitable for use". When applied to u ribs, it can be specifically the process index of "striving for low defects and high quality".
The U-rib non grooved internal and external submerged arc welding process is adopted to realize full penetration and high-quality welded joints that can effectively control welding defects. The U-rib non grooved welding has small filler metal, and the welding deformation and residual stress are correspondingly reduced. The submerged arc welding under flux protection has stable thermal process, good forming, few defects, and the penetration rate can reach 100%, which is more helpful to improve the fatigue resistance The ribbed plate adopts intelligent equipment for welding and industrialized high-efficiency production, and the full-length detection of the weld is realized, so that the product quality is stable and reliable.
confirmatory fatigue test
Through the fatigue tests of different models, it is verified that the U-rib full penetration welded joint has good fatigue resistance and reliable welding process, as shown in FIG. 17, 18 and table 1.
It is the basic concept of fatigue resistance of welded joints and the expression of the concept of welded structural integrity design.
From unilateral to bilateral welding, the performance of U-rib welded joint has changed substantially. The implementation of full penetration bilateral submerged arc welding process has optimized the joint quality and greatly improved the fatigue resistance. The goal of ultra long U-rib welded joint, ultra long service life and low maintenance has been realized. It reflects the contribution of made in China to the development of bridges in the world. At present, this process has been officially used For highway and railway bridges, the low age cracking of the fatigue resistance of orthotropic steel bridge decks allows bridge engineers to obtain more space in optimizing the thickness of bridge decks and the spacing of diaphragms, making lightweight steel structures more internationally competitive.
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