What are the factors that affect the fatigue strength of welded structures?

Fatigue fracture of welded structure

Fatigue fracture refers to the failure phenomenon of mechanical parts after long-time operation under variable load. Fatigue fracture is characterized by low stress brittle fracture.

1.The fracture occurs at a lower stress, and the maximum cyclic stress is lower than the tensile strength;

2.There is no macroscopic plastic deformation at the fracture site;

3.The fracture is sudden and has no forewarning;

4.Fatigue fracture occurs after hundreds of cycles under alternating stress.

Fatigue fracture is a process of damage accumulation

The internal structure of the metal material changes, the local area is damaged at first, and the fatigue fracture occurs after the damage accumulates to a certain extent.

Stress concentration caused by welding defects

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

Defect types: plane defect and volume defect.

1.Cracks

If the crack area is 10% of the cross section of the specimen, the fatigue strength decreases by 55% ~ 65% under alternating load.

2.Incomplete penetration

Some structures require partial penetration;

Incomplete penetration defect: surface defect; Internal defects;

The effect of crack on fatigue strength is more serious than that of incomplete penetration.

Influence of welding defects on fatigue strength of welded joints

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

Effect of material strength on fatigue strength of joint

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

Fatigue failure and its influencing factors

The fatigue crack is formed on the matrix with the highest stress and the weakest strength. Under the action of alternating load, the crack tip is in the tensile stress field. The stress at the crack tip is large, the crack opens, the grain slips, and the tip extends forward. When the fatigue crack extends to the strength limit, the crack reaches the critical point and the instantaneous fracture occurs.