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This post was last edited by Wang Wei2 on 2026-5-11 10:06. Cracks are the most dangerous defects in composite steel welds; their harmfulness far exceeds that of other defects, and they are the main cause of sudden structural failure. Cracks are typical sharp-notch defects that generate extremely high stress concentrations when subjected to forces, allowing them to expand rapidly and lead to brittle fracture. Especially in composite steels, due to the large difference in thermal expansion coefficients between the base layer and the clad material, high welding residual stresses occur, which makes cracks more likely to form; once such cracks appear, the material usually has to be repaired or discarded. Comparison with other major defects: Defect type, Hazard characteristics, Whether cracks are allowed. Stress concentration is the most severe; crack propagation occurs rapidly, leading to sudden catastrophic failures. Cracks are absolutely not allowed; any occurrence requires immediate repair. Undercutting is a surface-type defect that reduces the load-bearing area and increases the risk of cracks; its hazard level is second only to that of cracks. It is not allowed in welds of grades I, II, and III. Lack of penetration reduces the effective cross-sectional area, causes stress concentration, and may develop into a source of cracks. Ingrade I joints, slag inclusions are not allowed as they disrupt metal continuity and reduce fatigue strength, though the risk of crack propagation is lower. Porosity can be tolerated to a certain extent, depending on its size and location. It is a volume-type defect with relatively minor effects on strength. A small amount of porosity is allowed. In practical engineering, cracks, lack of penetration, and undercutting are considered the most serious welding defects; however, cracks are regarded as the most hazardous due to their unpredictable tendency to propagate and their fatal consequences.