Residual stresses are those that exist in a part independent of external force or restraint. Neglect of these residual tensile stresses created during welding processes can lead to stress corrosion cracking, distortion, fatigue cracking, premature failures in components, and instances of over design. When applied and residual stresses are accumulated, the net tensile stress seen by the components are much greater than the actual load applied. This is why welded components usually fail at the weld.
The residual tensile stress from welding is created because the weld consumable is applied in its molten state. The weld is applied in its hottest, most expanded state. It then bonds to the base material, which is much cooler. The weld cools rapidly and attempts to shrink from the cooling. Since it has already bonded to the cooler, stronger base material it is unable to shrink. The net result is a weld that is essentially being “stretched” by the base material. The heat affected zone is usually most affected by the residual stress and hence where failure will usually occur. Inconsistency in the weld bonding material, chemistry, weld geometry, porosity, etc. act as a stress risers for residual and applied tensile stress to initiate fatigue failure.
Reference: Evaluation of Welding Residual Stress Levels through Shot Peening and Heat Treating
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