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The formation process of welding defects is highly complex, influenced both by metallurgical factors and by stress and deformation. These defects have a significant impact on the load-bearing capacity of welded structures; more importantly, stress and deformation coexist alongside the defects. Welding defects tend to occur in the weld area and its surrounding regions, which are precisely the areas with the highest tensile residual stresses in the structure. The reason why welding defects reduce the strength of welded structures is that they decrease the effective area of the structure’s load-bearing cross-section and cause stress concentration around the defects. In general welded structures, stress concentration and changes in the load-bearing cross-section can also occur due to improper design or construction. Welding defects generally include lack of penetration, lack of fusion, cracks, slag inclusions, pores, undercutting, burn-through, and poor weld shape. Welding defects can be planar or three-dimensional; planar defects have a much greater impact on stress increases than three-dimensional defects, and are therefore far more dangerous. Those belonging to the former include cracks, lack of penetration, and lack of fusion ； Examples of the latter include pores and slag inclusions. 01 Mechanism of stress concentration caused by welding defects: Materials experience an increase in local stress due to sudden changes in the cross-section through which loads are transmitted; this phenomenon is known as stress concentration. Different shapes of defects result in varying degrees of cross-sectional change, as well as different angles relative to the direction of the load, all of which lead to significantly different levels of stress concentration around the defect. Taking an elliptical void defect as an example, the void is surrounded by an isotropic infinite elastic body under stress; as the elliptical void gradually transforms into a sheet-like crack, stress concentration becomes extremely severe. In addition to void-type pores, cracks, and lack of weld penetration, slag inclusions are also common welding defects. When the distance between multiple defects is small (such as numerous pores and slag inclusions), high stress concentrations occur in those defect areas, leading to cracks that form between the defects and connect the pores together. In this case, the greatest stress concentration occurs at the edges of the two outer holes. In welded joints, geometric discontinuities such as weld bead elevation, misalignment, and angular deformation exist; although some of these are permitted by current codes, they all cause stress concentration. Furthermore, different stress concentrations occur due to variations in joint types. Among the joint types commonly used in welded structures, butt joints have the lowest degree of stress concentration, while corner joints, T-joints, and end lap joints have similar levels of stress concentration. For T-joints in critical structures, such as H-shaped plate girders subjected to dynamic loads, the stress concentration at the joint can be significantly reduced by using grooves cut in the edges of the plates. However, this is not possible for lap joints; the stress distribution along the entire length of a side lap weld is highly uneven, and the greater the length of the weld, the more severe this unevenness becomes. Therefore, standard codes for steel structure design specify that the calculated length of side lap welds should not exceed 60 times the size of the weld leg. Because beyond this limit, even by increasing the length of the side lap welds, it is not possible to reduce the stress peaks at both ends of the weld. 02 The influence of welding defects on the non-fragile failure of structures under static loads Welding defects have varying degrees of impact on the static load failure of structures. Under normal circumstances, the failure mode of the material is plastic fracture; in such cases, the reduction in strength caused by the defects is roughly proportional to the decrease in the load-bearing cross-sectional area resulting from those defects. Under general standards, individual, isolated, or sparsely distributed pores in the weld are permitted; provided that the total cross-sectional area of these pores accounts for only 5% of the working cross-section, their impact on the yield strength and tensile strength limits is minimal. However, when pores appear in clusters and their total cross-sectional area exceeds 2% of the weld cross-section, the strength limits of the joint decrease significantly. The main reason for this phenomenon is the interruption of the protective atmosphere during welding, which leads to the formation of clusters of pores as well as a decline in the mechanical properties of the weld metal itself. Therefore, limiting the amount of pores also helps to prevent a deterioration in the mechanical properties of the weld. Porosities on the surface of a weld or in areas adjacent to it are more dangerous than those that are deep within the weld; clusters or dense groups of porosities are far more hazardous than individual porosities. Inclusions or slag reduce the tensile strength of the material in proportion to their cross-sectional area, but have a lesser impact on the yield strength. The size and shape of such defects have a significant impact on strength; a single isolated spherical inclusions or impurities are not more harmful than pores of the same size and shape. Continuously arranged, fine slag inclusions that are aligned perpendicular to the direction of stress are quite dangerous. Discontinuity defects caused by geometric shapes, such as undercutting, 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 regions of high residual tensile stress in the structure or with areas of large, brittle grains in the heat-affected zone, it often leads to the brittle, unstable propagation of cracks. Lack of fusion and incomplete penetration are more harmful than pores and slag inclusions. When the weld has an increase in thickness or when the weld joint is made using welding electrodes that are superior to the base material, the effects of lack of fusion and incomplete penetration may not be very apparent. In fact, many welding structures in use have been operating for many years, and the lack of fusion and incomplete welding hidden within the welds has not caused any serious accidents. However, such defects can become the starting point for brittle fracture under certain conditions. Cracks are considered the most dangerous welding defects, and their presence is not permitted in standard specifications. Due to phenomena such as the tip notch effect that occurs in sharp cracks, the development of a triaxial stress state, and temperature drops, the cracks may become unstable and propagate, leading to the failure of the structure. Cracks generally form in tensile stress fields and in regions of poor microstructure within the heat-affected zone. Under conditions of static loading and non-fragile failure, if plastic flow occurs before the unstable propagation of cracks, the residual tensile stresses in the structure have no harmful effect, nor does it lead to brittle fracture. Unless there are adverse conditions such as a sharp deterioration in the material properties at the crack tip, poor microstructure in the surrounding area, high residual tensile stress, and an operating temperature below the critical temperature, cracks in the material generally tend to be halted once they move away from the tensile stress field or the areas with deteriorated microstructure. 03 The impact of welding defects on structural brittle failure: Welded structures often experience brittle fracture at defective areas or points of structural discontinuity, leading to catastrophic damage. It is generally believed that the more severe the stress concentration caused by defects in a structure, the greater the risk of brittle fracture. Cracks pose the greatest hazard because the sharpness of their tips is much greater than that of defects such as lack of penetration, lack of fusion, undercutting, and pores. When the presence of volume defects such as pores and inclusions is below 5%, and if the operating temperature of the structure is at or above the material’s plasticity-fragility transition temperature, they pose no threat to the safety of the structure. The critical temperature of cracked components is much higher than that of components with inclusions. In addition to using the transition temperature to measure the impact of various defects on brittle fracture, fracture mechanics is employed as a basis for evaluation in many important welded structures, as it allows for determining the relationship between fracture stress, crack size, and fracture toughness. Many brittle fractures in welded structures are initiated by tiny cracks; under normal circumstances, since these small cracks do not reach the critical size, the structure does not fracture immediately after operation. However, small welding defects and discontinuities are likely to grow steadily over time during use, eventually reaching a critical value and resulting in brittle fracture. Therefore, conducting regular inspections during the service life of the structure to promptly identify and monitor defects approaching critical conditions is the most effective measure to prevent brittle fracture in welded structures. When welded structures are subjected to impacts, high local strains, or harsh environmental conditions, welding defects can easily lead to brittle fracture. For example, fatigue loads and corrosive environments can make defects such as cracks sharper, cause their size to increase over time, and accelerate their progression to critical levels. 04 Conclusion: Understanding and grasping the impact of various welding defects on the strength of structures is essential for properly assessing the safety of welded structures. It also helps us identify which welding defects can have catastrophic consequences for such structures, as well as those that do not significantly affect their strength under normal use. This provides valuable guidance for establishing standards for welding quality inspection.