The force line interference of the butt joint is small, so the stress concentration factor is small, and the fatigue strength will be higher than other joint forms. However, experiments have shown that the fatigue strength of the butt joint varies over a wide range because of a number of factors that affect the fatigue performance of the butt joint. Such as sample size, groove form, welding method, welding rod type, welding position, weld shape, weld after welding, heat treatment after welding, etc. The butt joint with the permanent pad reduces the fatigue strength of the joint due to the severe stress concentration at the pad. The fatigue cracks of such joints are generated from the joint of the weld and the backing plate, and are not produced at the weld toe, and the fatigue strength is generally equal to the fatigue strength of the butt joint of the least advantageous profile without the backing plate.
Cross joints or T-joints have found wide application in welded construction. In this bearing joint, the stress concentration factor is higher than that of the butt joint due to the obvious cross-section change at the transition from the weld to the base metal, so the fatigue strength of the cross or T-joint is lower than that of the butt joint. Connector. For joints with fillet welds and fillet joints with partial penetration welds, the fatigue fracture of welds may occur on two weak links, ie basic metal and weld. At the junction of the toe end or on the weld. For cross joints that are welded through the groove, the fracture generally only occurs at the weld toe, not at the weld. The fatigue strength of the T-shaped and cross-connected joints whose welds are not subjected to working stress mainly depends on the stress concentration at the junction of the weld and the main force-receiving plate. The T-joint has high fatigue strength, while the fatigue strength of the cross joint is low. The fundamental measure to improve the fatigue strength of T-shaped or cross-joints is to open the groove and process the transition of the weld to make a smooth transition. Through this improvement, the fatigue strength can be greatly improved.
The fatigue strength of the lap joint is very low, because the force line is severely distorted. The butt joints using so-called "reinforced" covers are extremely unreasonable. Due to the increased stress concentration, the butt joints with higher fatigue strength are greatly weakened after the cover is used. For bearing sleeve joints, fatigue cracks can occur in the base metal or in the weld. Changing the width of the cover or the length of the weld also changes the distribution of stress in the base metal, thus affecting the joint. The fatigue strength, that is, as the ratio of the length of the weld to the width of the cover increases, the fatigue strength of the joint increases because the distribution of stress in the base metal tends to be uniform.
2.2.2 Effect of weld shape
Regardless of the type of joint, they are joined by two welds, butt welds and fillet welds. The shape of the weld is different, and the stress concentration factor is also different, so that the fatigue strength has a large dispersibility. The shape of the butt weld has the greatest effect on the fatigue strength of the joint.
(1) Effect of transition angle Yamaguchi et al. established the relationship between fatigue strength and the transition angle between the base metal and the weld metal (outer obtuse angle). During the test, W (weld width) and h (height) changed, but the h/W ratio remained unchanged. This means that the angle remains the same and the test results show that the fatigue strength remains unchanged. However, if W remains unchanged and the parameter h is changed, it is found that h increases and the fatigue strength of the joint decreases, which is obviously the result of a decrease in the external angle.
(2) Influence of weld transition radius Sander et al.'s research results show that the weld transition radius also has an important effect on joint fatigue strength, that is, the transition radius increases (the transition angle remains unchanged) and the fatigue strength increases. The shape of the fillet weld also has a large effect on the fatigue strength of the joint.
When the ratio of the calculated thickness a of the single weld to the thickness B is a/B<0.6-0.7, it is generally broken at the weld; when a/B>0.7, it is generally broken by the base metal. However, increasing the weld size is only effective in increasing the fatigue strength within a certain range. Since the increase in the size of the weld does not change the strength of the other weak section, that is, the strength of the base metal at the end of the weld, it must not exceed the fatigue strength at that point. Soete, Van Crombrugge uses 15mm thick plates to weld with different fillet welds. The test under axial fatigue load found that when the weld bead is 13mm, the fracture occurs in the basic metal or weld at the weld toe. When the weld bead of the weld is less than this value, the fatigue fracture occurs on the weld; when the weld bead size is 18 mm, the fracture occurs in the base metal. Accordingly, they proposed the limit solder fillet size: S = 0.85B where S is the solder fillet size and B is the plate thickness. It can be seen that even if the size of the solder fillet reaches the thickness of the plate (15mm), the fracture result at the weld can still be obtained. This result is in good agreement with the theoretical results.
2.2.3 Effect of welding defects
There are a large number of different types of defects in the weld toe. These different types of defects lead to early cracking of fatigue cracks and a sharp drop in the fatigue strength of the base metal (down to 80%). Welding defects can be roughly divided into two types: planar defects (such as cracks, unfused, etc.) and volumetric defects (pores, slag inclusions, etc.), their degree of influence is not asked, and welding defects on joint fatigue strength The impact is related to the type, direction and location of the defect.
1) Cracks in crack welding, such as cold and hot cracks, are accompanied by a brittle structure, which is a serious source of stress concentration, which can greatly reduce the fatigue strength of structures or joints. Early studies have shown that in low carbon steel butt joint specimens with a width of 60 mm and a thickness of 12.7 mm, when there are cracks with a length of 25 mm and a depth of 5.2 mm in the weld (they account for about 10% of the cross-sectional area of ​​the specimen) Under the alternating load condition, the fatigue strength of the 2×106 cycle life is reduced by about 55%~65%.
2) Incomplete penetration should be stated, not necessarily incomplete penetration is considered to be a defect, because sometimes some joints are required to be partially welded through, a typical example is the design of some pressure vessel nozzles. Incomplete penetration defects are sometimes surface defects (single-sided welds), sometimes internal defects (double-sided welds), which may be either local or integral. Its main influence is to weaken the cross-sectional area and cause stress concentration. The fatigue life at 10% of the weakened area is reduced by 25% compared to the test result without such defects, which means that the effect is not as severe as the crack.
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