Research progress and latest technology to improve the fatigue performance of welded joints (4)
3) Unfused Because the sample is difficult to prepare, research has been extremely rare so far. However, there is no doubt that unfused is a planar defect and therefore cannot be ignored. It is generally treated in the same way as unwelded.
4) Undercut The main parameters characterizing the undercut are the undercut length L, the undercut depth h, and the undercut width W. The main parameter affecting the fatigue strength is the undercut depth h. Currently, the depth h or the ratio of the depth to the plate thickness (h/B) can be used as a parameter to evaluate the fatigue strength of the joint.
5) The pores are volume defects. Harrison analyzed and summarized the previous test results. The decrease in fatigue strength is mainly due to the reduction of the cross-sectional area of ​​the pores, and there is a certain linear relationship between them. However, some studies have shown that when the surface of the sample is machined by machining, so that the pores are on the surface, or just below the surface, the adverse effects of the pores increase, and it acts as a source of stress concentration and becomes a fatigue crack. Cracking point. This indicates that the position of the pores has a greater influence on the joint fatigue strength than the size, and the surface or subsurface porosity has the most adverse effect.
6) The relevant research report of slag inclusion IIW indicates that as a volume type defect, the slag inclusion has a greater influence on the fatigue strength of the joint than the stomata.
Through the above description, it can be seen that the influence of welding defects on the fatigue strength of the joint is not only related to the size of the defect, but also depends on many other factors, such as the surface defect is more affected than the internal defect, and the surface defect perpendicular to the direction of the force is more affected than the other. The direction is large; the influence of defects located in the residual tensile stress zone is larger than that in the residual compressive stress zone; the defects located in the stress concentration zone (such as weld toe cracks) have greater influence than the same defects in the uniform stress field.
2.3 Influence of welding residual stress on fatigue strength
Welding residual stress is a characteristic of welded structures. Therefore, its influence on the fatigue strength of welded structures is a common concern. For this reason, a lot of experimental research work has been carried out. The test is often carried out by using a sample with welding residual stress and a sample after heat treatment to eliminate residual stress. Since the generation of welding residual stress is often accompanied by changes in material properties caused by welding thermal cycling, the heat treatment restores or partially restores the properties of the material while eliminating residual stress, and also due to the dispersion of the test results, the test results Different interpretations have been made and the effects on the residual stress of the weld have been evaluated differently.
Taking the research work carried out by some people in the early and recent days as an example, this problem can be clearly explained. Different researchers have reached different conclusions on the results of the 2×106 cycle test with the high-level butt joint. It has been found that the fatigue strength of the heat-treating stress-relieving specimen is 12.5% ​​higher than that of the welded specimen. The fatigue strength of the welded and heat-treated specimens is consistent, that is, the difference is not large; It was found that although the fatigue strength was increased by heat treatment to eliminate residual stress, the added value was much lower than 12.5% ​​and so on. The test results of the butt joint specimens polished on the surface are also the same, that is, some tests consider that the fatigue strength can be increased by 17% after heat treatment, but some test results show that the fatigue strength after heat treatment is not improved. This problem has long been confusing, until some scholars in the former Soviet Union conducted a series of experiments under alternating load, and gradually clarified this problem.
The most worthwhile mention is Trufyakov's study of the influence of welding residual stress on joint fatigue strength under different stress cycle characteristics. The test uses 14Mn2 ordinary low-alloy structural steel. There is a transverse butt weld on the sample, and one longitudinal weld bead is welded on both sides. One set of samples was heat treated to eliminate residual stress after welding, and the other set was not heat treated. The fatigue strength comparison test uses three stress cycle characteristic coefficients r = -1, 0, +0.3. Under alternating load (r=-1), the fatigue strength of the sample with residual stress is close to 130 MPa, while the residual stress without elimination is only 75 MPa. Under pulsating load (r=0), the fatigue of the two groups of samples The strength is the same, both are 185 MPa. When r = 0.3, the fatigue strength of the sample which was subjected to heat treatment to eliminate residual stress was 260 MPa, which was slightly lower than that of the unheated sample (270 MPa). The main reason for this phenomenon is: when the r value is high, for example, under the pulsating load (r = 0), the fatigue strength is higher, and under the higher tensile stress, the residual stress is released faster, so the residual The effect of stress on fatigue strength is weakened; when r is increased to 0.3, the residual stress is further reduced under load and actually has no effect on fatigue strength. The heat treatment softens the material while eliminating the residual stress, so that the fatigue strength decreases after the heat treatment. This test better illustrates the effect of material stress on the fatigue strength caused by residual stress and welding thermal cycle. It can also be seen from this 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 cycle characteristic value is low, the influence is relatively large.
It has been pointed out earlier that due to the residual stress in the structural weld that reaches the yield point of the material, the actual stress cycle experienced in the vicinity of the weld will be oscillated downward from the yield point of the material in the joint that normally applies the stress cycle. Regardless of the cyclical characteristics of its original role. For example, if the nominal stress cycle is +S1 to -S2, the stress range should be S1+S2. However, the actual stress cycle range in the joint will be from Sy (the stress amplitude at the yield point) to Sy-(S1+S2). This is very important when studying the fatigue strength of welded joints, which led to some design specifications replacing the cyclic characteristics r with stress ranges.
In addition, during the test, the size, loading mode, stress cycle ratio and load spectrum of the test piece also have a great influence on the fatigue strength.
3 Process method for improving the fatigue strength of welded structures
Fatigue cracks of welded joints generally occur at the root and weld toe. If the risk of crack initiation of the weld root is suppressed, the dangerous point of the welded joint is concentrated on the weld toe. Many methods can be used to improve the fatigue strength of welded joints, 1 to reduce or eliminate welding defects, especially opening defects; 2 to improve the geometry of the weld toe area to reduce the stress concentration factor; 3 to adjust the welding residual stress field, resulting in residual compressive stress field. These improvements can be divided into two broad categories, as shown in Table 1.
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