FE modelling of friction connections in tubular tower for wind turbines

Abstract: During the last few decades the demand for renewable energy has led to a myriad of innovative solutions. The present assembling solution between two segments of a tubular tower is accomplished by conventional ring flange connection. This connection suffers from economical and technological barriers. One possible improvement could be achieved by implementing friction connections. Performance of this connection exposed to static loading is considered in this thesis. FEM is used as the main tool to improve understanding of the friction connection behaviour for application in towers for wind turbines. This new solution consists of the upper tower segment, where normal clearance holes are made and where bolts can be pre-installed, and the lower tower segment where opened slotted holes are made. It is easier to produce and 80% less expensive than traditional flange connection which leads to a total reduction of 10% to 15% of the tower costs. The influence of the level of assembling tolerances and the loss of pretension force in the bolts has been thoroughly investigated by a large number of FE models in the search for a model that is the most successful match with regards to its comparison with test data. Experiments are performed in an international project financed by RFCS, HISTWIN. The starting point is a small scale friction connection, best described as a single shear lap joint. This specimen represents a strip of the friction connection of a tubular tower. The ability of the FE models to capture three-dimensional contact problems, with realistic modelling of the assembling process and effects of the secondary bending during the loading are investigated. Resistance according to European codes, EN 1993-1-8, for the preloaded bolts is compared with the FE solutions. The second type of specimen is a down-scaled bending test of the beam. The test is used to check the performance of the connection and to calibrate the FE model. A very realistic FE model is created and compared with results from experiments. Failure modes and the respective ultimate resistances are predicted by FEA. The same model is used to investigate the influence of the gap between tubular segments and the influence of residual stresses generated in the segment which contains slotted holes on the tower resistance in bending, as well as in compression and tension.