The influence of welding defects on the structural load-bearing strength

    When clusters or dense pore defects appear in the weld, the cross-section of the pores is larger, and at the same time, the mechanical properties of the weld may be reduced (such as oxidation, etc.), which will significantly reduce the bearing strength. Therefore, clusters of pores are more dangerous than a single pore. The influence of inclusions on strength is related to its shape and size. A single intermittent small spherical inclusion is not more harmful than a pore of the same size and shape. Continuous inclusions arranged in a straight line, thin strips and arranged perpendicular to the direction of force are more dangerous. Welding defects have varying degrees of influence on the static load failure and fatigue strength of the structure. In general, the failure form of the material is mostly plastic fracture. At this time, the strength reduction caused by the defect is roughly related to the reduction in the load-bearing cross-sectional area caused by it. Proportionally. Welding defects have a much greater impact on fatigue strength than static load strength. For example, due to the large stress concentration factor, the cracks inside the weld have a greater impact on the fatigue strength; when the load-bearing cross-sectional area caused by the pores is reduced by 10%, the fatigue strength can be reduced by up to 50%. The spherical inclusions inside the weld have little effect on the fatigue strength when the area is small and the number is small, but when the inclusions form sharp edges, the effect on the fatigue strength is very obvious.

    The effect of undercut on fatigue strength is much greater than that of pores and inclusions. The fatigue strength of undercut joints under 10° cycles is only about 40% of that of dense joints, and the degree of influence is also related to the direction of the load. In addition, poor weld formation, incomplete penetration, misalignment, and corner deformation in the weld toe area and the root will cause stress concentration, fatigue cracks and fatigue damage.

    Slag inclusions or inclusions reduce the tensile strength of the material in proportion to the size of its cross-sectional area, but have a small effect on the yield strength. Discontinuity defects caused by geometric shapes, such as undercut, poor weld formation, or weld penetration, not only reduce the effective cross-sectional area of the component, but also cause stress concentration. When these defects overlap with the residual stress in the structure or the embrittlement grain region of the heat-affected zone, it will cause brittle and unstable propagation cracks.

    Incomplete fusion and incomplete penetration are more harmful than pores and slag inclusions. Although when the welding seam has an increased amount or the welding joint is made with an electrode better than the base metal, the influence of incomplete fusion and incomplete penetration may not be very obvious. In fact, many welded structures have been working for many years, and the lack of fusion and lack of penetration inside the weld did not cause serious accidents. However, this type of defect may become the initiation point of brittle fracture under certain conditions.

    Cracks are considered to be dangerous welding defects that can easily cause structural fractures. Cracks generally occur in the larger tensile stress and coarse-grained structure in the heat-affected zone. Under static load non-brittle failure conditions, if plastic flow occurs before the crack instability and growth, the residual tensile stress in the structure will not have a great influence , And it will not produce brittle fracture; but once the crack instability expands, the impact on the welded structure is very serious.

(Huiputech)