Uncertainty in stress concentration factor computation for ship fatigue design

Paper in proceeding, 2014

The stress concentration factor is often used to compute the local stress of ship structures when the hot-spot approach is chosen for fatigue assessment, where the computed damage is proportional to the power 3 or 5 of the stress concentration factor (SCF). It means a little change of the SCF value could lead to a huge difference in fatigue life predictions. Hence the methodology of computing the SCF should be defined clearly in classification society guidelines. In this study, different methods to compute SCF based on class guidelines are firstly surveyed. Then some fuzzy explanations in the computation methods are identified. The consequences of the fuzzy definitions may lead to large variation of SCF computations. A rather simple structural detail with cracks observed is used for this investigation. To this structural detail, two alternatives to compute the local stress for fatigue assessments are compared. Some recommendations to reduce the SCF to increase its fatigue life are also taken into account to guide the local design of such structural details. The difference of fatigue life predictions using these proposed approaches is compared, while at least 50% difference is expected even though the guidelines from the same classification are employed. Further, it is supposed to have more impacts to the final results if a more complex detail is chosen.