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A Brief Introduction of Four Main Factors Affecting the Fatigue Strength of Welded Joints
Release time:
2021-09-18
Author:
Source:
Factors that affect the fatigue strength of the base metal (e. g., stress concentration, cross-sectional dimensions, surface conditions, loading conditions, media, etc.) also have an impact on the fatigue strength of the welded structure. In addition, some characteristics of the welded structure itself, such as changes in the performance of the joint near the seam area, welding residual stress, etc., may also affect the fatigue strength of the welded structure. To understand the specific effects of these factors is beneficial to improve the fatigue strength of welded structures. The impact of these factors is discussed separately below. 1. The influence of stress concentration in welded structure, due to different stress concentration in the joint part, they have strong fatigue effect on the joint
Factors that affect the fatigue strength of the base metal (e. g., stress concentration, cross-sectional dimensions, surface conditions, loading conditions, media, etc.) also have an impact on the fatigue strength of the welded structure.In addition, some characteristics of the welded structure itself, such as changes in the performance of the joint near the seam area, welding residual stress, etc., may also affect the fatigue strength of the welded structure.To understand the specific effects of these factors is beneficial to improve the fatigue strength of welded structures.The impact of these factors is discussed separately below.
In the welded structure, due to the different stress concentration in the joint, they have different degrees of adverse effects on the fatigue strength of the joint.
① Butt weld: the stress concentration is smaller than that of other types of joints, but the residual height and θ↑-stress concentration ↑-fatigue strength of the joint ↓. If the weld surface is machined, the stress concentration will be greatly reduced and the fatigue strength of the butt joint will be improved accordingly.
② T-joint and cross joint: stress concentration factor> stress concentration factor of butt joint. Therefore, the fatigue strength of the T-joint and the cross joint is much lower than that of the butt joint.
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The fundamental measures to improve the fatigue strength of T-joints and cross joints are to open the groove welding and process the weld transition zone to make the transition smooth;
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T-joints and machined joints have higher fatigue strength: while cross joints and unmachined joints have lower fatigue strength. This is because an eccentric moment on the asymmetric T-joint reduces the stress in the transition zone, and its stress concentration is lower than that of the symmetrical cross joint.
The fatigue strength of lap joints with only side welds is low (only 34% of the base metal).
④ The use of butt joints with so-called "reinforced" cover plates is extremely unreasonable: the test results show that in this case, the butt joints with higher fatigue strength are greatly weakened.
① Low carbon steel and low alloy steel near the seam of the metal mechanical properties of the change of the joint fatigue strength of the smaller.
When high strength steel is welded, the influence of metal performance change in the near seam area depends on the matching of the joint:
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For welded joints with high assembly, I .e. soft and hard, the non-uniformity of mechanical properties has basically no effect on the fatigue strength of the joint, and the fatigue strength of the joint depends on the softer base metal.
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When there are serious stress concentration factors in the soft interlayer of the high-assembly hard-clamp soft joint, the fatigue strength of the joint is greatly reduced, and its value depends on the mechanical properties of the soft zone itself.
The effect of welding residual stress on structural fatigue strength is a widespread concern. For this problem people have done a lot of experimental research work. The test is often compared with the fatigue test of the specimen with the welding stress after heat treatment to eliminate the internal stress. Because the generation of welding residual stress is often accompanied by the change of material properties caused by the welding thermal cycle, and the heat treatment also recovers or partially recovers the performance of the material while eliminating the internal stress. Therefore, the results of the test have been interpreted differently; the effect of internal stress has also been evaluated differently.
The fatigue strength of the specimens after the elimination of internal stress is higher than that of the end heat treatment, and the effect of internal stress is greater at higher stress concentration.
The effect of welding defects on fatigue strength is related to the type, size, direction and position of the defects.
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Influence of flake defects (e. g., crack, incomplete fusion, final penetration)> influence of fillet defects (e. g., pores, etc.);
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Effect of surface defects> internal defects;
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Influence of flake defects perpendicular to the acting direction> influence of other directions;
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The influence of defects located in the residual tensile stress field> the influence of defects in the residual compressive stress area;
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The influence of defects located in the stress concentration area (such as the toe crack of the weld) is the same as the influence of defects in the uniform stress field.
This article is reproduced from the welding and cutting online, infringement will be deleted.
Key words:
Joint, influence, stress, fatigue strength, welding, defect, concentration, structure, residual
