Effect of whipping stresses on the fatigue damage of ship structures

Abstract

Fatigue failures in ships are mainly caused by waves. Higher frequent stresses due to vibrations induced by wave impact, called whipping, are typically not considered in design. However, the bow and stern flare of many recently built container vessels has been increased causing higher and more frequent whipping stresses. Full-scale measurements have shown a significant increase of the fatigue damage caused by high-frequent loads if rainflow counting and Palmgren-Miner Rule are assumed. On the other hand, an increased damage rate has not yet been observed so that questions arose if this approach and the neglect of sequence effects of stress cycles are justified for these load processes. Therefore, fatigue tests have been performed with transverse stiffeners on a continuous plate. In a first investigation, low-frequent, constant amplitude loads were superimposed by higher frequent stress cycles. In successive investigations, stress histories extracted from the full-scale measurements on a Panamax containership, containing a typical whipping event, were applied. Fatigue lives obtained from the tests with and without whipping load cycles agree reasonably well with those calculated with the Palmgren-Miner Rule. Finally, a recent proposal of the authors for a simplified consideration of the whipping stresses in fatigue analyses for ships is discussed.