Residual strength estimation and imperfection modelling for plastically deformed stiffeners

Accurate modelling of the deflection curve of a member is essential to assess strength calculations, furthermore a well predicted residual stress distribution presents a better result for residual strength assessment. In a real-time simulation to obtain these results in a desired accuracy demands computer utilization with finite element analysis and modelling. Additionally, measurement of residual stresses is not affordable for every case considering the need of equipment and crew to conduct measurements so they need to be predicted. In this study a new method is proposed for curve fitting of the deflection curve discretization by means of fixed curvature segments employing G1 tangential continuity. The method uses nodal coordinates and inclination at the 1st node to fit segments through nodes with a fixed curvature within using only either measured or predicted imperfection curve. Additionally, the method presents the methodology to use curvatures for residual strength assessment employing moment-curvature relationship explained within Euler-Bernoulli beam theory. Since its known that any curvature of a beam leads to strains in fibers, and strains lead to stresses; area integration of the stresses through section leads to the subjected or resultant moment. Using the mentioned relationship for any metarial model or composition of materials for any type of section the moment-curvature relationship can be obtained directly. Thus, it will be possible to predict the reduction in moment carrying capacity due to the damages of the structure considering the residual curvatures and possibly loaded moment could be predicted using moment-curvature diagram. In the thesis, mesh independency study for the proposed imperfection modelling method is conducted by evaluation of the modelling for 3 different cases and these results are compared to analytical curvature value. For validation of model in prediction of residual curvatures the results are compared with a commercial FEA program, ANSYS using a loading-unloading simulation of a beam for the given moments. It has been seen that proposed method can be used for modelling of any kind of imperfection and assessment of residual strength of beams.