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Residual stresses are stresses that persist in a structure even after the original load is removed. They arise from uneven plastic deformations across different parts of a structure. When a rod is stretched beyond its yield point, it permanently deforms and doesn't regain its original length. These remaining stresses exist because plastic deformations are permanent and distributed unevenly throughout the material.

During welding, the plug's temperature increases significantly, reducing both the modulus of elasticity and stress. As the plug cools post-welding, its elasticity increases, causing yielding at a constant stress level. This creates residual stress in the plug and weld close to the steel's yield strength. The cooling process generates stresses that remain locked in the material after the weld solidifies.

During cooling of cast or hot-rolled metals, outer layers cool faster than the inner core, regaining stiffness more quickly. On returning to room temperature, the inner core contracts more than the outer layers, resulting in residual longitudinal tensile stresses at the inner core. The outer layers develop residual compressive stresses. This differential cooling creates opposing stress distributions throughout the material.

Residual stresses form because plastic deformations are permanent and uneven across a structure. When material undergoes plastic behavior beyond its yield point, it cannot fully recover its original shape or stress state. The permanent deformation from plastic behavior locks stresses into the material, creating residual stresses that persist even after external loads are removed.

Large residual stresses are removed through reheating and slow cooling. This process allows the material to relax and redistribute stresses more uniformly. By raising the temperature and cooling gradually, the material regains ductility and can relieve the locked-in stresses. This thermal treatment is essential for ensuring the reliability and durability of structures and components in engineering applications.

Understanding and managing residual stresses is crucial because they affect structural reliability and durability. Residual stresses can influence how materials respond to future loads and environmental conditions. They may cause unexpected failures or reduce component lifespan if not properly managed. Engineers must account for residual stresses during design and manufacturing to ensure structures perform safely and predictably throughout their service life.

During welding, elevated temperatures reduce the modulus of elasticity in the heated plug region. As the plug cools, the modulus of elasticity increases, causing the material to become stiffer. This stiffness increase combined with thermal contraction forces the material to yield at a constant stress level, generating residual stresses. The variation in elastic properties across the weld zone drives residual stress formation.