The factors that affect the fatigue strength of the base metal (such as stress concentration, sectional size, surface state, loading condition, medium, etc.) also affect the fatigue strength of the welded structure. In addition, some characteristics of the welded structure itself, such as the change of the joint near the joint area, welding residual stress, may also affect the fatigue strength of the welded structure. It is beneficial to improve the fatigue strength of welded structures to understand the specific effects of these factors. The influences of these factors are discussed below.
1. The influence of stress concentration
In welded structures, different stress concentrations in the joint have different adverse effects on the fatigue strength.
(1) Butt weld: the stress concentration is smaller than other forms of joints, but the residual height and θ↑ — stress concentration ↑ — fatigue strength of joints ↓. If the weld surface is machined, the stress concentration will be greatly reduced and the fatigue strength of butt joint will be correspondingly increased.
(2) T-shaped and cross joint: stress concentration factor > butt joint stress concentration factor. Therefore, the fatigue strength of t-shaped and cross joints is much lower than that of butt joints.
The basic measures to improve the fatigue strength of t-shaped and cross joints are groove opening welding and machining weld transition zone to make it smooth and smooth.
T-joints and machined joints have higher fatigue strength, while cross joints and unmachined joints have lower fatigue strength. This is because the asymmetrical T-joint has an eccentric moment that 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 the lowest (only 34% of that of base metal).
④ It is extremely unreasonable to use the butt joint of the so-called “strengthened” cover plate: the test results show that, in this case, the butt joint with high fatigue strength is greatly weakened.
2. Influence of metal properties change near the seam area
① The change of metal mechanical properties near the joint of low carbon steel and low alloy steel has little effect on the fatigue strength of the joint.
(2) During welding of high strength steel, the influence of the change of metal properties near the joint depends on the matching of the joint:
For the welded joints with high composition, that is, soft clamped hard, the fatigue strength of the joints is determined by the soft base metal.
When there are serious stress concentration factors in soft interlayer of hard clip soft joint with high composition, the fatigue strength of the joint is greatly reduced, and its value depends on the mechanical properties of the soft zone itself.
3. Influence of residual stress
The influence of welding residual stress on fatigue strength of structure is widely concerned. A lot of experimental work has been done on this question. The specimens with welding stress are often compared with the specimens with heat treatment to eliminate the internal stress. Because the welding residual stress is always accompanied by the change of material properties caused by the welding thermal cycle, the heat treatment can eliminate the internal stress and partially restore the material properties. Therefore, different interpretations of the results of the experiment arise; The influence of internal stress has also been evaluated differently.
After the elimination of internal stress, the fatigue strength of the sample is higher than that of the final heat treatment, and the effect of internal stress is greater when the stress concentration is higher.
4. The impact of defects
The influence of welding defects on fatigue strength is related to the type, size, direction and position of the defects.
Influence of flaky defects (such as cracks, incomplete fusion and final penetration) > influence of rounded defects (such as pores, etc.);
Influence of surface defects > internal defects;
The effect of flaky defect perpendicular to the direction of force was higher than that of other directions.
Influence of defects in residual tensile stress field > influence of defects in residual compressive stress area;
Influence of defects located in stress concentration zone (such as weld toe crack) > influence of defects in uniform stress field.铜焊丝,Copper Welding Rods,铝焊丝,aluminium welding wire,镍焊条,Nickel electrode,药皮焊条,Flux Coated Brazing Welding Rods