FATIGUE STRENGTH OF THIN-PLATED BLOCK JOINTS WITH TYPICAL SHIPBUILDING IMPERFECTIONS

During design of block joints in ship structures it is assumed that the permissible tolerances for axial and angular misalignment given in shipbuilding standards are not exceeded. This is particularly important for fatigue assessments based on the nominal stress approach which implicitly incorporates stress magnification due to imperfections. Preliminary investigations of thin-plated ship structures (t = 4 mm) showed that (a) it is very difficult to keep the thickness-dependent tolerances for thin plates, and (b) even if the tolerances are kept, the fatigue strength might not be sufficient as the stress magnification incorporated in the nominal stress approach is far exceeded. The present paper highlights the problem in the beginning. The main part describes fatigue tests of components performed in a joint industry project. Stiffened panels were fabricated by different shipyards under realistic shipbuilding constraints. The partly large imperfections (buckles, axial and angular misalignment) were extensively recorded using photogrammetric technique in order to obtain the characteristics of typical pre-deformations. The component tests establishing the link to the fatigue strength were carried out with a stress ratio R = -1 which is typical for naval vessels. Concurrent nonlinear finite element calculations containing the whole test set-up were carried out to obtain relations between pre-deformation and stress increase. After description of the test results, a procedure for the assessment of imperfections is discussed which are measured in the usual way in shipbuilding.