O’Donnell Consulting Engineers (OCE) performed an analysis and redesign on a heater that had a limited structural life. The heater was constructed of .025 inch thick stainless steel. The configuration includes an edge weld joint type, where two .025 sheets are placed side-by-side and the top surfaces are welded. When this is connected to the header, a three point T weld joint configuration is created by the intersection of the radial and tangential faces of the header and tube sheet. Many fatigue cracks initiated in this area.
Imperfections in fabrication processes such as welding, forming and assembling parts for welding and cutting can contribute to the concentration of strains in localized areas to a degree that significantly shortens the fatigue life of a fabricated unit. Sharp corners, large weld profiles, and undercutting also reduce fatigue life. An examination of the fabricators processes was undertaken. We then made recommendations in the manufacturing process to reduce the likelihood of early fatigue failures.