Factors Affecting Fatigue Strength of Welded Structures

1.Fatigue fracture of welded structure

Fatigue fracture refers to the failure phenomenon of parts after long-term operation under variable load. The characteristic of fatigue fracture is low stress brittle fracture.

The fracture occurs under low stress, the maximum cyclic stress is lower than the tensile strength, there is no macro plastic deformation at the fracture part, and the fatigue fracture occurs only after hundreds or even millions of times under the action of alternating stress.

Fatigue fracture is a process of damage accumulation

The internal structure of metal first changes locally and is damaged. Fatigue fracture will occur when the damage reaches a certain degree, which can be divided into three stages: the formation, propagation and fracture of fatigue crack.

The main failure mode of welded steel structure is fatigue fracture, which accounts for about 90% of the fracture accidents of welded structure.

2.Stress concentration caused by welding defects

The influence degree of joint fatigue strength depends on the type, location and direction of defects.

Defect type: planar defect and volumetric defect.

(1) Crack: crack is a serious source of stress concentration, which reduces the fatigue strength of structure and joint.

(2) Incomplete penetration: incomplete penetration is not always a defect. Some structures require local penetration of joints. Incomplete penetration defects include surface defects and internal defects, which have no serious impact on fatigue strength.

3.Influence of welding defects on joint fatigue strength

The influence of surface defects is greater than that of internal defects, the influence of defects in the residual tensile stress area is greater than that in the residual compressive stress area, the influence of plane defects perpendicular to the force direction is greater than that in other directions, and the influence of defects in the stress concentration area is greater than that of the same defects in the uniform stress field.

4.Effect of material strength on joint fatigue strength

With the increase of the tensile strength of the material itself, the fatigue strength increases by about 50%. The fatigue strength of the joint has nothing to do with the tensile strength of the material itself. The fatigue life of the joint is short, and the fatigue strength of high-strength steel joint is higher than that of low-strength steel joint.

5.Fatigue damage and influencing factors

The fatigue crack first forms on the matrix with the highest stress and the weakest strength. When the initial crack is under the action of alternating load and the crack tip is in the tensile stress field, the crack opens and the tip extends forward because of the large stress concentration. When the crack extends to the material strength limit, it reaches the critical crack size and leads to fracture.