The causes of fatigue failure of welded structures mainly include the following aspects:
①? Objectively speaking, the static load bearing capacity of welded joints is generally not lower than that of base metal; However, under the action of alternating dynamic load, its bearing capacity is much lower than that of base metal, and it is closely related to the type of welded joint and the form of welded structure. This is the main factor leading to premature failure of some structures due to fatigue of welded joints.
②? Early welding structure design was mainly based on static load strength design, without considering fatigue design, or the fatigue design specification of welding structure was not perfect, so that many unreasonable welded joints appeared now.
③? Engineering designers and technicians do not know enough about the anti-fatigue characteristics of welded structures, and the designed welded structures often copy the fatigue design criteria and structural forms of other metal structures;
④? Welding structures are more and more widely used. In the process of design and manufacture, people blindly pursue the low cost and light weight of structures, which leads to the increasing design load of welding structures. ?
⑤? Welded structures tend to develop in the direction of high speed and heavy load, which requires higher and higher dynamic bearing capacity of welded structures, while the scientific research level of fatigue strength of welded structures is relatively backward.
2 Main factors affecting the fatigue strength of welded structures
2. 1 Influence of static load strength on fatigue strength of welded structure
In the research of iron and steel materials, people always hope that the material has higher specific strength, that is, it can bear greater load weight with lighter self-weight, because the structure with the same weight can have great bearing capacity; Or the same bearing capacity can reduce its own weight. Therefore, high strength steel came into being, and it also has high fatigue strength. The fatigue strength of base metal always increases with the increase of static load strength.
But for welded structures, the situation is different, because the fatigue strength of welded joints has little to do with the static strength of base metal, the static strength of weld metal, the microstructure and properties of heat affected zone and the strength of weld metal. That is to say, as long as the details of welded joints are the same, the fatigue strength of high-strength steel and low-carbon steel are the same, with the same S-N curve, which is suitable for various joint types such as butt joints, fillet joints and welded beams. Maddox studied the fatigue crack growth of carbon manganese steel with yield points between 386-636MPa and 636 MPa, and six kinds of weld metals welded in covered electrode and heat affected zone. The results show that the mechanical properties of materials have certain influence on the crack growth rate, but the influence is not significant. When designing welded structures subjected to alternating loads, it is meaningless to try to meet the engineering needs by selecting higher strength steel grades. Only when the stress ratio is greater than +0.5 and the static strength condition plays a major role, high strength steel should be used as the base material of welded joints.
The reason for the above results is that there is a slag wedge defect similar to undercut along the fusion line at the weld toe of the joint, with a thickness of 0.075mm-0.5mm and a tip radius 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, so the fatigue life of the joint under a certain stress amplitude is mainly determined by the fatigue crack propagation stage. The appearance of these defects makes the fatigue strength of all welded joints of the same type of steel the same, which has little to do with the static strength of base metal and welding materials.
2.2 Stress concentration on fatigue strength
2.2. 1 Influence of connector type
The main forms of welded joints are butt joint, cross joint, T-joint and lap joint. Due to the interference of force transmission line, stress concentration will occur at the joint.
The force-line interference of butt joint is small, so the stress concentration coefficient is small, and its fatigue strength will be higher than other joint forms. However, the test shows that the fatigue strength of butt joint varies widely because of a series of factors affecting the fatigue performance of butt joint. Such as sample size, groove form, welding method, covered electrode type, welding position, weld shape, weld treatment after welding, post-welding heat treatment, etc. will have an impact on it. Due to the serious stress concentration at the pad, the fatigue strength of the butt joint of the permanent pad is reduced. The fatigue cracks of this kind of joint all occur at the joint between the weld and the backing plate, not at the weld toe, and its fatigue strength is generally equivalent to that of the butt joint without backing plate with the worst shape.
Cross joints or T-joints have been widely used in welded structures. In this kind of pressure-bearing joint, the cross section at the transition between weld and base metal changes obviously, and the stress concentration coefficient is higher than that of butt joint, so the fatigue strength of cross joint or T-joint is lower than that of butt joint. For the joints with no fillet weld and the groove joints with partial penetration weld, when the weld transmits working stress, its fatigue fracture may occur in two weak links, namely, the joint between the base metal and the weld toe or weld. For the cross joint with groove penetration, the fracture usually occurs only at the weld toe, not at the weld. The fatigue strength of T-joints and cross-joints where the weld does not bear working stress mainly depends on the stress concentration at the joint between the weld and the main bearing plate. The fatigue strength of T-joints is higher, while that of cross joints is lower. The fundamental measure to improve the fatigue strength of T-joint or cross joint is groove welding, and the weld transition is processed to make it smooth. Through this improvement measure, the fatigue strength can be greatly improved.
The fatigue strength of lap joint is very low, which is caused by severe distortion of force line. It is extremely unreasonable to butt joint with the so-called "reinforced" cover plate. Due to the increase of stress concentration, the butt joint with high fatigue strength was greatly weakened after using the cover plate. For the bearing cover plate joint, fatigue cracks may appear in the base metal or weld. In addition, changing the cover width or weld length will also change the stress distribution in the base metal, thus affecting the fatigue strength of the joint, that is, with the increase of the ratio of weld length to cover width, the fatigue strength of the joint will increase, because the stress distribution in the base metal tends to be uniform.
Influence of weld shape
No matter what joint form, it is connected by butt weld and fillet weld. The stress concentration coefficient of different weld shapes is different, so the fatigue strength has great dispersion.
The shape of butt weld has the greatest influence on the fatigue strength of joint. ?
? Influence of transition angle (1) Yamaguchi et al. established the relationship between fatigue strength and transition angle (outer obtuse angle) between base metal and weld metal. W (weld width) and H (height) changed during the test, but the h/W ratio remained unchanged. This means that the included angle remains unchanged, and the test results show that the fatigue strength also remains unchanged. However, if W remains unchanged and the parameter H changes, it is found that the fatigue strength of the joint decreases with the increase of H, which is obviously the result of the decrease of the outer included angle.
? (2) The influence of weld transition radius? The research results of Sander et al. show that the weld transition radius also has an important influence on the fatigue strength of the joint, that is, the fatigue strength increases with the increase of the transition radius (the transition angle is unchanged).
The shape of fillet weld also has a great influence on the fatigue strength of joint.
When calculating the ratio a/b of the thickness a to the thickness b of a single weld; 0.7, generally broken in the parent material. However, increasing weld size is only effective to improve fatigue strength within a certain range. Because the increase of weld size can not change the strength of the base metal at the weld toe of another weak section, at best, it can not exceed the fatigue strength. Soete, Van Crombrugge used 15mm thick steel plate to weld through different fillet welds. Under axial fatigue load, it is found that when the weld toe is 13mm, the fracture occurs in the base metal or weld at the weld toe. When the weld leg is less than this value, fatigue fracture occurs on the weld; When the weld leg size is 18mm, the fracture occurs in the base metal. Based on this, they put forward the limit leg size: S=0.85B, where s is the leg size and b is the plate thickness. It can be seen that even if the leg size reaches the plate thickness (15mm), the fracture results of the weld can still be obtained, which is in good agreement with the theoretical results.
Influence of welding defects
There are a large number of different types of defects in the welding toe, which leads to the early cracking of fatigue cracks and the sharp decline of the fatigue strength of the base metal (down to 80%). Generally speaking, welding defects can be divided into two types: surface defects (such as cracks, incomplete fusion, etc. ) and volume defects (such as porosity and slag inclusion, etc. ), and their influence is not asked. At the same time, the influence of welding defects on the fatigue strength of joints is related to the type, direction and location of defects.
1) ? Cracks? Cracks in welding, such as cold cracks and hot cracks, are serious stress concentration sources besides brittle structures, which can greatly reduce the fatigue strength of structures or joints. Previous studies have shown that under the condition of alternating load, when there are cracks with a length of 25mm and a depth of 5.2mm in the weld (they account for about 10% of the cross-sectional area of the sample), the fatigue strength of the low-carbon steel butt joint sample with a width of 60mm and a thickness of 12.7mm decreases by 55%~65%.
2) ? Incomplete penetration? It should be noted that incomplete penetration is not necessarily considered as a defect, because sometimes some joints are artificially required to be circumferential penetration, and a typical example is the design of some pressure vessel nozzles. Incomplete penetration defects are sometimes surface defects (single-sided welding) and sometimes internal defects (double-sided welding), which can be local or global in nature. Their main effect is to weaken the cross-sectional area and cause stress concentration. Compared with the test results without this defect, the fatigue strength of the weakened area of 10% in fatigue life is reduced by 25%, which means that its influence is not as serious as that of cracks.
3) ? Unfire? Due to the difficulty of sample preparation, the related research is extremely rare so far. But there is no doubt that incomplete fusion belongs to plane defects and cannot be ignored. Generally, it is treated as incomplete fusion.
4) ? The main parameters that characterize undercut are undercut length L, undercut depth H and undercut width W. The main parameter that affects fatigue strength is undercut depth H. At present, depth H or depth-to-thickness ratio (h/B) can be used as a parameter to evaluate the fatigue strength of joints.
5) ? Stomatal? For the volume loss, Harrison analyzed and summarized the previous experimental results. The decrease of fatigue strength is mainly caused by the decrease of pore cross-sectional area, and there is a certain linear relationship between them. However, some studies show that when the surface of the sample is machined by mechanical processing, when the pores are on or just below the surface, the adverse effects of the pores will increase, which will serve as a stress concentration source and become the starting point of fatigue cracks. This shows that the position of the gas hole has greater influence on the fatigue strength of the joint than its size, and the surface or subsurface gas hole has the most unfavorable influence.
6) ? Slag inclusion? The relevant research report of IIW points out that slag inclusion, as a volume defect, has greater influence on the fatigue strength of joints than porosity. ?
It can be seen from the above introduction that the influence of welding defects on the fatigue strength of joints is not only related to the size of defects, but also depends on many other factors, such as surface defects have greater influence than internal defects, and surface defects perpendicular to the force direction have greater influence than other directions; The influence of defects in residual tensile stress area is greater than that in residual compressive stress area; Defects located in stress concentration area (such as weld toe cracks) have greater influence than the same defects in uniform stress field.
2.3 Influence of welding residual stress on fatigue strength
Welding residual stress is a unique feature of welded structure, so its influence on fatigue strength of welded structure is a widely concerned issue, and a lot of experimental research work has been carried out for this purpose. In experiments, samples with welding residual stress are often compared with those after heat treatment to eliminate the residual stress. Because the generation of welding residual stress is often accompanied by the change of material properties caused by welding thermal cycle, heat treatment can not only eliminate residual stress, but also restore or partially restore material properties. At the same time, due to the dispersion of the test results, there are different interpretations of the test results and different evaluations of the influence of welding residual stress.
Taking some people's early and recent research work as an example, this problem can be explained clearly. Different researchers have drawn different conclusions about the results of 2× 106 cycle test of ultra-high butt joint. It is found that the fatigue strength of the stress-relieved samples after heat treatment is 65438 02.5% higher than that of the samples in the same welding state. Others found that the fatigue strength of welded and heat-treated samples is the same, that is, there is little difference; However, it is also found that the fatigue strength is improved after heat treatment to eliminate residual stress, but the added value is much lower than 12.5% and so on. The same is true for the test results of butt-joint samples with polished surface, that is, some tests think that the fatigue strength can be improved by 17% after heat treatment, but some test results show that the fatigue strength is not improved after heat treatment. This problem has been puzzling for a long time, and it was not until some scholars in the former Soviet Union conducted a series of experiments under alternating load that the problem was gradually clarified.
One of the most noteworthy is Trufyakov's research on the influence of welding residual stress on the fatigue strength of joints under different stress cycle characteristics. The test adopts 14Mn2 ordinary low alloy structural steel, and there is a transverse butt weld on the sample, and a longitudinal weld is surfacing on both sides. One group of samples was heat treated to eliminate the residual stress after welding, and the other group was not heat treated. Three stress cycle characteristic coefficients, r =- 1, 0 and +0.3, are used in the fatigue strength comparison test. ? Under alternating load (r=- 1), the fatigue strength of the specimen without residual stress is close to 130MPa, while the fatigue strength of the specimen without residual stress is only 75MPa. Under pulsating load (r=0), the fatigue strength of the two groups is the same, which is 185MPa. However, when r=0.3, the fatigue strength of the sample with residual stress removed by heat treatment is 260MPa, which is slightly lower than that of the sample without heat treatment (270MPa). The main reasons for this phenomenon are: when the R value is high, for example, under pulsating load (r=0), the fatigue strength is high, and under the action of high tensile stress, the residual stress is released quickly, so the influence of residual stress on fatigue strength is weakened; When r increases to 0.3, the residual stress under load is further reduced, which has no effect on fatigue strength. However, heat treatment not only eliminates the residual stress, but also softens the material, so the fatigue strength decreases after heat treatment. This test well explains the influence of residual stress and material change caused by welding thermal cycle on fatigue strength. It can also be seen that the influence of welding residual stress on joint fatigue strength is related to the stress cycle characteristics of fatigue load. That is, when the cyclic eigenvalue is low, the influence is greater.
As mentioned above, because there is residual stress reaching the yield point of the material in the structural weld, the actual stress cycle near the weld will swing downward from the yield point of the material in the joint with equal amplitude stress cycle, regardless of the cyclic characteristics of its original action. For example, if the nominal stress period is +S 1 to -S2, the stress range should be S 1+S2. However, the actual stress cycle in the joint will range from Sy (stress amplitude of yield point) to Sy-(S 1+S2). This is very important in studying the fatigue strength of welded joints, which leads some design codes to replace the cyclic characteristic R with the stress range.
In addition, the sample size, loading mode, stress cycle ratio and load spectrum also have great influence on the fatigue strength during the test.