An Investigation on the Influence of Root Defects on the Fatigue Life of the Welded Structure of a Large Two-Stroke Diesel Engine

The crankshaft housings of large two-stroke diesel engines are welded structures subjected to constant amplitude loading and designed for infinite life at full design load. A new design of the so-called frame box has been introduced in the engine using butt welded joints of thick plates, welded from one side only, with no access to the root side. Linear elastic fracture mechanics applied to three-dimensional finite element models has been used to assess this new design from the fatigue viewpoint. The methodologies used, from coarse models of the complete engine structure to refined sub-models of the welded joints, are described and the results presented. In addition, large-scale test specimens with controlled lack-of-fusion weld root geometry were manufactured and fatigue tested to develop S-N curves and determine threshold stress intensity factor range values. These were established for opening mode loading both under the influence of residual stresses from production and in stress relieved specimens.