115 Residualstresses

FIGURE 1.13 Typical residual-stress distributions (+ indicates tension and — compression).

thickness. Tensile stresses are present on one surface and compressive stresses on the opposite surface.

The residual-stress distributions mentioned above are usually relatively constant along the length of the member. However, residua! stresses also may occur at particular locations in a member, because of localized plastic flow from fabrication operations, such as cold straightening or heat straightening.

When loads are applied to structural members, the presence of residual stresses usually causes some premature inelastic action; that is, yielding occurs in localized portions before the nominal stress reaches the yield point. Because of the ductility of steel, the effect on strength of tension members is not usually significant, but excessive tensile residual stresses, in combination with other conditions, can cause fracture. In compression members, residual stresses decrease the buckling load from that of an ideal or perfect member. However, current design criteria in general use for compression members account for the influence of residual stress.

In bending members that have residual stresses, a small inelastic deflection of insignificant magnitude may occur with the first application of load. However, under subsequent loads of the same magnitude, the behavior is elastic. Furthermore, in "compact" bending members, the presence of residual stresses has no effect on the ultimate moment (plastic moment). Consequently, in the design of statically loaded members, it is not usually necessary to consider residual stresses.

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