99 Diseases of Pressure Equipment: Stress Raisers

Speaking of stress raisers, they are another insidious type of flaw that can and do lead to equipment failures. Stress raisers (aka stress intensification sites) can be mechanical or metallurgical notches. Undercutting, physical weld flaws, mismatched thicknesses, and sharp geometric intersections can all become stress raisers. But so too can so-called "metallurgical notches" like one finds at the edge of a weld where the cast structure of the weld pool meets the wrought structure of the heat-affected zone. Equipment in many static services without significant cyclic stresses can tolerate the presence of some stress raisers because of hefty code design margins. But equipment in services with significant fatigue stresses (thermal and mechanical) are especially vulnerable to such notches and deserve special QA/QC attention during design, specification, fabrication, and repairs.