Uncertainty in Stress Concentration Factor Computation for Ship Fatigue Design

Abstract

In stress-based fatigue design and analysis of marine structures, a stress concentration factor (SCF) is used to compute the local stress in structural details when the hot-spot approach is used. For a typical steel material, the fatigue damage is computed as proportional to the power 3–5 of the stress values, defined by the chosen S-N curve. This means that a small change in the SCF value can lead to a large difference in the fatigue damage result and corresponding fatigue life prediction. Thus, the methodology used to compute the SCF should be clearly defined in classification society guidelines. This study presents a review of different direct calculation procedures for how to obtain the SCF based on fatigue assessment guidelines. The effect of different element types and local stress extrapolation methods to the fatigue damage estimation is studied for both longitudinal load and bending load conditions in a container ship. A simple structural detail with cracks observed after only a few years in service is used for the case study. For this structural detail, two alternative methods to compute the local stress for fatigue assessments are compared. The difference in fatigue life prediction using the proposed approaches is compared; with at least 50% difference expected even within the guidelines from the same classification society. It is further discussed how to reduce the SCF with the objective to increase the fatigue life.