1 fatigue of welded structures and research significance

1.1 fatigue of welded structures

Since the early 20th century, the invention has an electrode applicator hundred years, has become the most widely used welding process method, it is difficult to find another development so fast, and in the application of scale and diversification, compared with the welding process , so that many of the most important contemporary technological issues must be resolved by welding, for example: shipbuilding, railways, automotive, aviation, aerospace, bridges, boilers, large-scale plant and high-rise buildings and so can not do without the support of welding technology. If the welding had not been invented then, many structural and even frankly the whole industry is not generated. Needless to say, currently in production engineering, Welding Wire China is the most important connection method, the weight of welded structures accounted for 50% of steel production, industrial countries, this proportion is close to 70%. However, constantly broken welded accident, of which 90% of fatigue failure.

Fatigue damage has been considered the structure of the ship and marine engineering the destruction of a major form of self-steel ship since its inception, due to structural fatigue cracks in the generation, expansion, leading to vessel damage cases frequently reported. U.S. Coast Guard Ship Structure Committee (Ship Structure Committee, USCoast Guard) has organized a force of six different types of 77 civilian ships and nine warships in the 600 000 structures was investigated and detailed statistical analysis, the results show that about one-ninth of the damage and fatigue. History of several major accidents offshore platform, such as Japan, the United States in 1965 built semi-submersible platform Sedco type way damaged in delivery sank, killing 13 people; No. 1980, Alexan-derKeyland semi-submersible platform capsized in the North Sea , so that a hundred people died in the sea, the survey results of the analysis show that the structure of the fatigue is a major reason for the accident.

Similarly, the fatigue failure is also frequent rail roads and bridges and power stations in the pipeline. In the fifties and sixties, has been the rapid development of road network in Europe, when they use a welding construction steel bridge, the bridge because of fatigue on the highway when the lack of knowledge, there are no provisions in the standard anti-fatigue design, there were many poorly designed welding joints, in today's increasingly busy and increasing transportation load, speed up the process of fatigue damage, there have been many welded steel bridge fatigue cracks.

Welded structure in China due to fatigue and failure of engineering examples are also emerging, such as the late nineties, high-speed passenger bogies in the fatigue fracture of welded joints, and turbine blade root fatigue fracture, all have resulted in the state and enterprises huge economic losses.

1.2 Fatigue Failure of Welded Structures

Fatigue failure of welded structures because of the following aspects: ① Objectively speaking, the static load of welded joints is generally not less than the capacity of the base metal; and pay changes under load, its capacity is far lower than the base metal, And with the type of welded joints and welded structures are closely related forms. This is caused by a number of structural fatigue of welded joints due to premature failure of a major factor; ② the early design of welded structure designed primarily to static strength, not considered anti-fatigue design, fatigue design of welded structures or specifications are not complete , so that there were many design now seems unreasonable welded joints; ③ engineering and technical personnel fatigue performance of welded structures is not enough to understand the characteristics, the design of welded structures tend to copy other metal structures fatigue design guidelines and structure; ④ welded structure becomes more extensive, and in the design and manufacturing process of the blind pursuit of man-made structure of low-cost, lightweight, resulting in the design of welded structures increasing load; ⑤ high-speed heavy welded structure to the direction of the trend of welding structure to withstand dynamic load capacity requirements higher and higher, while the fatigue strength of welded structural aspects of scientific research is lagging behind.

1.3 Methods to improve fatigue performance of welded structures of meaning

Fatigue accidents occur frequently in a certain extent, restricted the welding structure further extensive application of a number of occasions had to abandon the use of welded structures, or even suspect that welded structures under dynamic load can be applied to engineering practice, therefore fatigue resistant welded structures caused by domestic and foreign experts and technicians, especially fatigue, International Institute of Welding Professional Committee's attention. In a large number of fatigue tests and engineering practice, based on the welded structure fatigue design specifications been introduced, such as the British bridge fatigue design specification BS5400, the European Association for the fatigue design of steel specifications, steel bridge design codes in Japan, the United States railway bridge and highway design specifications, International Institute of Welding of cyclic loading fatigue welded steel design specifications IIW.DOC-639-8l and China's steel design specification GB-17-88. The world's major shipbuilding and marine resource development countries are in the ship and marine structure design and construction engineering and inspection into the level specification of the fatigue strength of welded structures made regulations and requirements.

As the weld toe of welded joints of welding defects, stress concentration and residual tensile stresses, the fatigue strength significantly lower than the fatigue strength of base metals. Therefore, the fatigue strength of welded structures depends on the joint fatigue performance, that is, anti-fatigue properties of welded joints, the relationship between the welded structure can be used safely. Therefore, in order to ensure the reliability of welded structures, pay movements in the design to withstand the load of welded structures, the design specification to the fatigue strength of welded joint fatigue strength of the structure as a whole, instead of the fatigue strength of base metals, which obviously caused great waste . Even so, the local stress concentration at the joints under the effect of the overall structure still occur premature fatigue failure. In order to meet the engineering welded well to bear on its proposed dynamic load requirements, measures can be taken two main points. On the one hand, increase the fatigue performance of welded structures to understand, well-designed structure and fittings form, so that the design of welded structure is more reasonable, with higher fatigue strength; while improving weld quality and strict control to prevent and reduce welding defects generation; the other hand, directly facing the poor fatigue properties of welded joints of weaknesses in the welded structures manufacturing process, the process is completed and the use of technology to take effective measures to improve the fatigue strength of joints to increase its exposure to dynamic load capacity, extending its life.

Therefore, to enhance and improve the fatigue strength of welded joints has great potential economic and social benefits, long time, it is home to a hot research topic experts.

2 affect the fatigue strength of welded structures of the main factors

2.1 Static load intensity on the fatigue strength of welded structures

In the iron and steel materials research, people always want to materials with high specific strength, that its lighter weight to shoulder a greater load weight, because the structure of the same weight can have great carrying capacity; or the same carrying capacity can reduce their weight. Therefore, high-strength steel came into being, also has a high fatigue strength, fatigue strength of base metals is always with the static strength increases.

But for welded structures, the situation is different, because the fatigue strength of welded joints and base metal static strength, static strength of weld metal, heat affected zone and weld metal microstructure and properties match the intensity of the relationship is not much, that is, that as long as the details of welded joints, high-strength steel and mild steel fatigue strength is the same, have the same SN curve, this rule for butt joints, corner joints and welding beams and other types of connectors. Maddox of the yield point between the 386-636MPa and in the six carbon-manganese steel electrode welding the weld metal and heat affected zone of fatigue crack growth, the results show that: the mechanical properties of the crack propagation rate has some impact , but not too great. Designed to withstand alternating loads of welding structure, trying to use higher strength steels to meet the project needs to be meaningless. Only in the case of stress ratio greater than 0.5, the static strength conditions play a major role, the base metal of welded joints should be used only high-strength steel.

Cause these results is due to the location along the weld toe at the joint line of the existence of a similar fusion of the slag undercut wedge defect, its thickness 0.075mm-0.5mm, cutting-edge by less than half of 0.015mm. The sharp fatigue crack defect is the place to start, the equivalent stage of fatigue crack formation, thus fitting in a certain stress amplitude the fatigue life, mainly by the fatigue crack growth stage of the decision. These defects appear that all of the same type of steel welded joints have the same fatigue strength, and with the base metal and welding materials have little static strength.

2.2 stress concentration effects on the fatigue strength

2.2.1 Types of joints

Mainly in the form of welded joints: butt joint, cross joint, T-joints and lap joints, the joints as power transmission line interference, and therefore a stress concentration. Butt joints of power line interference smaller, and thus a smaller stress concentration factor, the fatigue strength will be higher than other joints in the form. However, experiments show that the fatigue strength of butt joints in a large range of changes, it is because there are a number of factors affect the fatigue properties of butt joints of the reason. If the sample size, slope form, the welding method, welding rod type, weld position, weld shape, weld after welding process, heat treatment after welding, etc will affect its occurrence. Plate with a permanent type plate at the butt joints as a serious stress concentration, reducing the fatigue strength of joints. This joint fatigue cracks are from the junction of the weld and plate produced, not produced at the weld toe, the fatigue strength - without a plate-like and most of the poor shape of the fatigue strength of butt joints equal.

T-shaped cross joints or welded joints in the structure has been widely used. In this load-bearing joints, due to the transition metal in the weld at the base section has a significant change in the stress concentration factor than the butt joint of the stress concentration factor is high, so cross or T-joints is lower than the fatigue strength of butt connector. No groove on the connecting joints with fillet welds and partial penetration groove weld joints, weld pass when work stress, the fatigue fracture may occur in the two weak links, namely, the basic metal and weld junction or seam weld toe on. For the groove weld penetration of the cruciform joints, fracture generally occurs only in the weld toe, rather than in the weld. Weld does not bear the stress of work and cross the T-shaped joint fatigue strength of the weld depends on the force plate with the main junction of the stress concentration, T-shaped connector with high fatigue strength, fatigue strength and low cross joints. T-shaped or cruciform joints increase fatigue strength of the fundamental measures are groove welding, and machining the weld smooth transition area to make the transition, this improvement measures, fatigue strength can have a more substantial increase.

The fatigue strength of lap joints is very low, which is due to power lines has been severely distorted. So-called "enhanced" cover butt joint is extremely unreasonable, because increased stress concentration effects, the use of cover, the original high fatigue strength butt joint is greatly reduced. For the bearing cover joints, fatigue cracks can occur in the base material, may also occur in the weld, the other to change the width of the cover or the length of the weld will change the stress distribution in the base metals, it will affect the joints fatigue strength, ie with the weld length and width ratio of the increase in cover, joints increase fatigue strength, because the stress distribution in the base metals due to more uniform.

2.2.2 Shape of weld

No matter what kind of connector forms, which are connected by two welds, butt welds and fillet welds. Weld different shapes, the stress concentration factor is not the same, which have greater fatigue strength of dispersion. The shape of the butt weld joint fatigue strength for the greatest impact.

(1) the impact of the transition angle Yamaguchi, who established the basic fatigue strength and the transition between the metal and weld metal corner (outside the obtuse angle) relationship. Test W (weld width) and h (height) changes, but h / W ratio remains unchanged. This means that the angle remains the same, the test results show that the fatigue strength was also unchanged. However, if W remains the same, change parameters h, h is found to increase, decrease joint fatigue strength, which is obviously the result of lower external angle.

(2) the transition radius of the weld Sander et al study results show that the transition radius of the same weld fatigue strength of joints has an important influence, that the transition radius increases (transition angle remains the same), fatigue strength increases. The shape of the fillet weld joint fatigue strength for greater impact.

When calculating the thickness of a single weld thickness B and the ratio a / B <0.6 ~ 0.7, the general fracture in the weld; when a / B> 0.7, generally off in base metals. But the increase in weld size on fatigue strength increased only valid within a certain range. Because the increase in weld size does not change the cross-section that is another weak base metal at the weld toe side of strength, so at best, can not exceed the fatigue strength of the department. Soete, Van Crombrugge using 15mm thick plate with different Stainless Steel Electrode, the axial load fatigue tests found that the fillet weld of 13mm, the fracture occurred in the weld toe or weld base metals. When the weld fillet is smaller than this value, the fatigue fracture occurred in the weld; when fillet size is 18mm when the fracture occurred in the base metals. Accordingly, they raised the limit fillet size: S = 0.85B where S is the weld leg size, B is the thickness. Weld leg size to be seen even when the thickness (15mm), still have to weld the fracture results, the results agree well with theoretical results.

2.2.3 the impact of welding defects

Weld toe area of ??the existence of a large number of different types of defects, these defects result in different types of early cracking and fatigue crack fatigue strength of the base metal fell sharply (down to 80%). Welding defects in general can be grouped into two categories: planar defects (such as cracks, lack of fusion, etc.) and volume-type defects (pores, slag, etc.), their impact is not asked, and weld defects on fatigue strength of joints impact and the type of defect, direction and position.

1) The crack in the weld cracks, such as cold, heat cracks, in addition to the organizational structure associated with the brittle, but is a serious source of stress concentration, it can significantly reduce the fatigue strength of the structure or joints. Early studies have shown that a wide 60mm, 12.7mm thick low carbon steel butt joint specimen, the weld has a long 25mm, 5.2mm depth of cracks (which account for about 10% of the sample cross-sectional area) the alternating load conditions, the cycle life of 2 × 106 to reduce the fatigue strength of approximately 55% to 65%.

2) lack of penetration should be noted, not necessarily the lack of penetration are considered to be flawed, because sometimes some people to ask the Ministry of penetration connectors for the week, a typical example is to take over some of the pressure vessel design. Lack of penetration defects sometimes surface defects (single-sided welds), and sometimes the internal defects (double-sided weld), it can be local in nature, it can be the whole nature. The main impact of reduced cross-sectional area and cause enough stress concentration. To weaken the area of ??10% of the fatigue life and does not contain such defects in the test results compared to the fatigue strength decreased by 25%, which means that its impact is not a serious crack.

3) lack of fusion is difficult because the sample preparation, since the study is extremely rare. But no doubt, lack of fusion defects are flat, and therefore can not be ignored, and the general lack of penetration to the same treatment.

4) characterization undercut undercut undercut the main parameters are the length L, undercut depth h, undercut the width W. The main parameters affecting the fatigue strength is the amount of undercut depth h, is currently available depth h or the ratio of the depth and thickness (h / B) as the assessment of joint fatigue strength parameters.

5) for the volume porosity defects, Harrison on the previous test results were analyzed and summarized, fatigue strength decrease was mainly due to pore size resulting in reduced cross-sectional area between them have a linear relationship. However, some studies have shown that when using machining methods of processing the sample surface, so that pores in the surface, or located just below the surface, the greater the adverse effects of porosity, it will work as a source of stress concentration, and a fatigue crack crack initiation point. This shows that the pore size of the position than its greater impact on the fatigue strength of joints, surface or subsurface pores with the most adversely affected.

6) of the slag IIW study indicate: as the volume-type defects, slag than the fatigue strength of joints affected pores larger.

Welding defects can be seen through the introduction of joint fatigue strength, not only related with the defect size, and once decided on a number of other factors, such as surface defects larger than the impact of internal defects, and the force perpendicular to the planar defects affected than other direction of the large; residual tensile stress at the impact of defects in the region than in the large residual compressive stress zone; defects in the stress concentration area (such as weld toe cracks) than in the uniform stress field in the same defect impact.

2.3 welding residual stress on fatigue strength

Welding residual stress is the unique characteristics of welded structures, therefore, it is for the impact fatigue strength of welded structures is widespread concern that people, for people a lot of experimental research. Test is often used with welding residual stress and heat-treated sample after sample to eliminate residual stress, the fatigue test for comparison. As the welding residual stress is often accompanied by the generation of welding thermal cycle-induced changes in material properties, and heat treatment to eliminate residual stress and also to restore or partially restore the material's performance, but also because of the dispersion of test results, so the test results gave rise to different interpretations of the welding residual stress will have a different evaluation.

Cite some of the early and recent research work carried out, for example, can clearly explain the problem, with a high of more than butt joints for 2 × 106 cycles of test results, different researchers have come to different conclusions. Was found: stress relief heat treatment sample the same fatigue strength than welded specimens fatigue strength increased by 12.5%; while others found that welded and heat-treated specimens fatigue strength is the same, that is little difference; but some found that after heat treatment to eliminate residual stress fatigue strength has also increased, but much lower than the 12.5% ??increase in value and so on. On the polished surface of the butt joint specimen test results, too, that some test that can improve the fatigue strength after heat treatment of 17%, but some test results show no increase in fatigue strength after heat treatment and so on. This question has long puzzled people, until a number of scholars in the former Soviet Union under alternating load a series of tests, and eventually to clarify this issue.

One of the most worth mentioning is Trufyakov characteristics of the stress cycle under different welding residual stress effects on fatigue strength of joint research. Experiment with 14Mn2 ordinary low-alloy structural steel, the specimen has a transverse butt weld and welding both sides of a longitudinal weld. A sample weld residual stress after heat treatment to eliminate the other group without heat treatment. Comparison test fatigue stress cycle in three characteristic coefficient r =- 1, 0, +0.3. In the alternating load (r =- 1), to eliminate residual stress near the fatigue strength of specimens 130MPa, but not eliminate the residual stress of only 75MPa, the pulsating load (r = 0), two sets of fatigue specimens the same intensity, are 185MPa. When r = 0.3, the residual stress after heat treatment to eliminate the fatigue strength of specimens 260MPa, but slightly lower than the samples without heat treatment (270MPa). This phenomenon is mainly due to generation: the r value is high, such as pulse loads (r = 0), fatigue strength is higher, the higher tensile stress, the residual stress to be released quickly, so the residual the impact of stress on the fatigue strength on the decline; when r increased to 0.3, the residual stress in the load, to further reduce, in fact, has no effect on the fatigue strength of. The heat treatment to eliminate residual stress at the same time soften the material, thus making the fatigue strength after heat treatment the worse. This test is better illustrated and welding residual stresses caused by thermal cycling on the fatigue strength of the material impact of changes. From here you can see the welding residual stress on the joint fatigue strength of the stress and fatigue loading cycle characteristics. The cyclical nature of that value is low, the impact is relatively large.

As previously noted, there is achieved due to structural weld residual stress of the material yield point, so the cycle of constant amplitude applied stress joints, the weld is exposed near the actual stress cycle will be down by the material yield point swing , regardless of its original role of cycling characteristics of how. For example, the nominal stress cycle to the + S1-S2, then the stress range should be S1 + S2. However, the actual connection will be from the stress cycle range Sy (yield point of the stress amplitude) to the Sy-(S1 + S2). This is the fatigue strength of welded joints is very important, it led to some design specifications to replace the cycling characteristics of the stress range r.

In addition, in the course of the experiment, the specimen size, loading mode, stress cycle ratio, fatigue strength of load spectrum also has a great impact.

3 to improve the fatigue strength of welded structures process method

Welded joints fatigue crack crack initiation location is generally present in the weld root and weld toe two parts, if parts of the weld root crack initiation of fatigue cracks in danger of being suppressed, the danger point of welded joints are concentrated in the weld toe area. Many methods can be used to improve the fatigue strength of welded joints, â‘  welding to reduce or eliminate deficiencies in particular, open defects; â‘¡ improving the geometry of the weld toe area to reduce stress concentration factor; â‘¢ adjust the welding residual stress field, residual compressive stress field. These improved methods can be divided into two categories, as shown in Table 1.

Welding process optimization method is not only for improving the fatigue strength of welded structures and consider, while the static strength of welded structures, metallurgical properties of welded joints and other areas have great benefits, this information many in this small repeat.

The following methods from the perspective of a three-part process in detail to improve the fatigue strength of welded joints of the main method.

3.1 to improve the weld toe geometry methods to reduce stress concentration

3.1 .1 TIG melt repair

Domestic and international studies show that, TIG melt repair can greatly improve the fatigue strength of welded joints, this method is TIG welded joints of the transition method in parts of remelting time, between the weld and base metals to form smooth transition. Reduce stress concentration, but also reduces the small non-metallic parts of the slag material, thereby improving the fatigue strength of joints.

Melt in the repair process requires the general gun parts from the weld toe at 0.5 ~ 1.5mm, and to maintain the remelting area clean, if accompanied by a slight pre-polished better. Important is that extinction occurs when remelting, how to deal with re-arc method, because it will definitely affect the quality of re-welding Road, is generally recommended to re-arc the best location is in the crater of the weld at the front of 6mm, Recently the International Institute of Welding organization European countries and Japan, some of the Welding Institute, unified by the British Welding Institute has prepared specimens - some methods to improve the effectiveness of joint fatigue strength of the unity of research, confirmed by the treatment process, the connector 2 × 106 cycles under 58% of the nominal fatigue strength increase, if the fatigue strength of 211MPa get converted into the corresponding nominal value of the eigenvalues ??of (K index) to 144MPa. It has been higher than the International Society of welded joints in the details of the highest fatigue strength of FAT values.

3.1 .2 machining

If the machining of the weld surface, the stress concentration will be greatly reduced, the fatigue strength of butt joints have been increased, when there is no weld defects, joint fatigue strength can be higher than the fatigue strength of base metals. But the high cost of machining the surface, so only really useful and really can be processed to the place, it is appropriate to use this process. And do not end with serious defects and seam welding, the weld root defect or stress concentration at the weld surface than the more severe stress concentration, so in this case the machining surface of the weld is meaningless of. If there is lack of penetration defects, because the fatigue crack will not be more than the starting height and weld toe crack, but transferred to the lack of penetration at the weld root. There is lack of penetration in the presence of defects, but tends to reduce machining the fatigue strength of joints.

Sometimes do not make the overall weld metal machining, welding and just toe the use of grinding machining process, this approach can also significantly improve the fatigue strength of joints. Studies have shown that, in this case, the crack initiation point is not in the weld toe, but transferred to the weld defect site.

The former Soviet Union Makorov of high-strength steel (tensile strength σb = 1080 MPa) of transverse butt weld fatigue strength of alternating load tests showed that, in the welded condition 2 × 106 cycles when the fatigue strength of ± 150MPa, if the weld mechanical processing, remove the remaining high, the fatigue strength increased to ± 275MPa, which has the fatigue strength of metals with very basic. However, if the partial toe butt grinding, the fatigue strength of ± 245MPa, which is 83% of the effect of machining and welding state compared to 65% increase fatigue strength, of course, whether the use of machine processing methods, or grinding cut method, if not carefully according to requirements, in order to ensure processing of results, the improvement of fatigue strength is limited.

Welding process optimization method is not only for improving the fatigue strength of welded structures and consider, while the static strength of welded structures, metallurgical properties of welded joints and other areas have great benefits, this information many in this small repeat. 3.1 .4 special welding method

The method is to develop a new type of electrode, its liquid metal and liquid slag has a high ability to dissolve the wet, can improve the transition radius of the weld, reducing the weld toe angle, weld toe to reduce the stress concentration, thereby increasing the fatigue strength of welded joints. And repair the shortcomings of TIG melt similar to its position with a strong selective soldering, welding is particularly suitable for flat position fillet welding of peace, and for vertical welding, horizontal welding and overhead position welding, it significantly reduces the superiority of .

3.2 Adjustment compressive residual stress methods of stress
3.2.1 The pre-set method
If the stress concentration in the samples with applied tensile load until yielding took place in the gap, and with a certain degree of tensile plastic deformation, unloading, set the gap and near the tensile plastic deformation at the compressive stress, The other section of the sample site will have its equilibrium tensile stress below the yield point of generation. Sample by this process, in its subsequent fatigue test, the stress is not applied with the original scope of the pre-overload of different samples, that is significantly smaller, so it can improve the fatigue strength of welded joints. The results show that large welded structures (such as bridges, pressure vessels, etc.) must be put into operation before the need for pre-overload test, which is beneficial to improve fatigue performance.
3.2.2 local heating
Local heating can be adjusted using welding residual stress field, stress concentration is generated in the compressive residual stress, thus improving the fatigue strength of joints is beneficial. This method is currently restricted to non-continuous longitudinal weld or a longitudinal joints stiffened plates.
For single-sided fillet board, the general location of the heating plate from the weld width is about 1 / 3, double-sided fillet board for the location of the heating plates center. This ensures that the compressive stress within the weld, which can improve the fatigue strength of joints. Different researchers applied this method to get results is different for single-sided fillet board, to improve the fatigue strength of 145% -150% of the double-sided fillet board, to improve the fatigue strength of 70% -187%.
Local heating of the fatigue strength of joints position has important implications, as point-like heat in the weld at the two ends of conduct, then the gap in the weld at the end of the residual stress caused by the compression, the results of the fatigue strength increased 53% ; but when the point-like heat in the weld end of the specimen center, the distance from the weld end is the same, although this had the same impact on the microstructure, residual stresses due to tensile residual stress, the The fatigue strength of the joints measured with the same non-treatment sample.
3.2.3 extrusion
Partial extrusion mechanism and point-like heating in the same way, that is by compressive residual stress are increased fatigue strength joints. But its role in point is different, the location should be in the need to squeeze the residual compressive stress of the position. High-strength steel specimens using extrusion effect is more significant than the low-carbon steel.
3.2.4 Gurnnerts method
Ohta success using this method to prevent the docking of the internal fatigue crack pipe. The specific method is to pipe external induction heating method, which is cooled with circulating water. Therefore, the compression stress generated within the pipeline, thus effectively preventing the fatigue crack in the pipe internally generated. Butt weld pipe treatment of fatigue crack growth rate significantly decreased to the same parent material and the crack growth rate.