Thread Content
The factors that affect the fatigue strength of base metals (such as stress concentration, cross-sectional dimensions, surface condition, loading conditions, medium, etc.) also have an impact on the fatigue strength of welded structures. In addition, certain characteristics of the welded structure itself, such as changes in the properties of the area near the joint, and welding residual stresses, can also affect the fatigue strength of the welded structure. Understanding the specific effects of these factors is beneficial for improving the fatigue strength of welded structures. Below, we will discuss the effects of these factors respectively. 1. Effect of stress concentration: In welded structures, different stress concentrations at the joints have varying degrees of adverse effects on the fatigue strength of those joints. ① Butt welds: Stress concentration is lower compared to other types of joints, but the excess height and θ increase, which leads to increased stress concentration and thus a decrease in the fatigue strength of the joint. If the weld surface is machined, the degree of stress concentration will **decrease**, and the fatigue strength of the butt joint will increase accordingly. ② T-joints and cross joints: the stress concentration factor is greater than that of butt joints. Therefore, the fatigue strength of T-joints and cross joints is much lower than that of butt joints. The fundamental measure to improve the fatigue strength of T-joints and cross joints is to weld with bevels and to shape the weld transition zone to ensure a smooth transition ； T-joints and mechanically processed joints possess higher fatigue strength, whereas cross joints and unprocessed joints have lower fatigue strength. This is because an eccentric moment exists at the asymmetric T-joint, which reduces the stress in the transition zone; its stress concentration is lower than that of a symmetric cross joint. ③ Joints with only side welds have the lowest fatigue strength (reaching only 34% of that of the base metal). ④ The use of butt joints with so-called “reinforced” covers is highly unreasonable: test results show that in this case, butt joints with relatively high fatigue strength are **significantly weakened**. 2. Influence of changes in the mechanical properties of the zone near the weld seam: ① Changes in the mechanical properties of the metal in the zone near the weld seam in low-carbon steel and low-alloy steel have little effect on the fatigue strength of the joint. ② When welding high-strength steel, the effect of changes in the mechanical properties of the zone near the weld seam depends on the compatibility of the joints: in the case of joints with a high mismatch, i.e., where a softer material is welded to a harder one, the unevenness in mechanical properties has little impact on the fatigue strength of the joint; in such cases, the fatigue strength is determined by the softer base metal. In high-assembled hard-clad soft-joint assemblies where there are severe stress concentration factors in the soft cladding layer, the fatigue strength of the joint **decreases**, with the magnitude of this decrease depending on the mechanical properties of the soft layer itself. 3. Effect of residual stress: The impact of welding residual stress on the fatigue strength of structures is a matter of great concern. A great deal of experimental research has been conducted on this problem. Tests often involve comparing fatigue tests conducted on specimens with welding stresses and those on specimens whose internal stresses have been eliminated through heat treatment. Since the generation of welding residual stresses is often accompanied by changes in the material properties caused by the welding heat cycle, heat treatment restores or partially restores the material properties while eliminating internal stresses. Therefore, different interpretations of the test results arise ； Different evaluations have also been given to the impact on internal stress. The fatigue strength of the specimens after internal stress removal was higher than that of those without heat treatment, and the effect of internal stress was greater where stress concentration was higher. 4. Impact of defects: The effect of welding defects on fatigue strength depends on the type, size, direction, and location of the defects. The impact of sheet-like defects (such as cracks, lack of fusion, incomplete welding) is greater than that of defects with rounded edges (such as pores) ； The impact of surface defects > that of internal defects ； The effect of sheet-like defects perpendicular to the direction of the force > the effect in other directions ； The effect of defects in a residual tensile stress field is greater than that of defects in a residual compressive stress field ； The effect of defects located in stress concentration areas (such as weld toe cracks) > the effect of the same defects in a uniform stress field.