Wind turbines are structures predominantly subjected to dynamic loads throughout their period of life. In that sense, fatigue design plays a central role. Particularly, support structure design might be conservative with respect to fatigue, which may lead to a real fatigue life of considerably more than 20 years. For these reasons, the implementation of a fatigue monitoring system can be an important advantage for the management of wind farms, providing the following outputs: (i) estimation of the evolution of real fatigue condition; (ii) since the real condition of fatigue damage is known, these results could be an essential element for a decision about extending the lifespan of the structure and the possibility of repowering or overpowering; and (iii) the results of the instrumented wind turbines can be extrapolated to other wind turbines of the same wind farm. This paper reviews the procedures for calculating the fatigue damage of wind turbine towers using strain measurements. The applicability of the described procedures is demonstrated with experimental data acquired in an extensive experimental campaign developed at Tocha Wind Farm, an onshore wind farm located in Portugal, exploring the impact of several user-defined parameters on the fatigue results. The paper also includes the description of the data processing needed to convert raw measurements into bending moments and several validation and calibration steps.

Press brake air bending, a process of obtaining products by sheet metal forming, can be considered at first sight a simple geometric problem. However the accuracy of the obtained geometries involves the combination of multiple parameters directly associated with the tools and the processing parameters, as well as with the sheet metal materials and dimensions. The main topic herein presented deals with the capability of predicting the punch displacement process parameter that enables the product to be accurately shaped to a desired bending angle, in press brake air bending. In our approach, it is considered separately the forming process and the elastic recovery (i.e. the springback effect). Current solutions in press brake numerical control (computer numerical control) are normally configured by analytical models developed from geometrical analysis and including correcting factors. In our approach, it is proposed to combine the use of a learning tool, artificial neural networks, with a simulation and data generation tool (finite element analysis). This combination enables modeling the complex nonlinear behavior of the forming process and springback effect, including the validation of results obtained. A developed model taking into account different process parameters and tool geometries allow extending the range of applications with practical interest in industry. The final solution is compatible with its incorporation in a computer numerical control press brake controller. It was concluded that, using this methodology, it is possible to predict efficient and accurate final geometries after bending, being also a step forward to a ''first time right'' solution. In addition, the developed models, methodologies and obtained results were validated by comparison with experimental tests.

The present paper has two main objectives: (1) to create a digital twin of an onshore wind turbine to provide to the owner of the wind farm a tool for continuous tracking of accumulated fatigue damage and evaluation of alternative operation strategies; (2) to perform the first tasks for the creation a reliable numerical of a floating wind turbine to simulate experimental data for the testing of monitoring tools on this type of wind turbines. A numerical model of an onshore wind turbine was created with FAST software developed by the US National Renewable Energy Laboratory (NREL). The structural and mechanical properties of the structure were calculated from the geometric properties of the tower and blades. The aerodynamic properties of different sections of the blades were computed with 2D models created with ANSYS Fluent. Having the wind turbine fully characterised, the control mechanisms were calibrated using data from the manufacturer catalogues. Numerous time series of wind excitation for different operation conditions were generated, and the structural response computed. Additionally, the Supervisory Control and Data Acquisition (SCADA) operating in parallel with a series of extensometers and accelerometers located in strategic points along a real wind turbine tower and blades provided a full description of the internal forces in both locations and the estimation of the modal properties for different operating conditions. Both measured and simulated response allowed the identification and validation of structural dynamic properties and static and dynamic internal loads. In spite of several approximations and simplifications, the results obtained with the numerical model were in good agreement with the ones measured in the field.

The sheet metal bending is one of the metal forming processes with the simplest geometric interpretation and usually a 2D analysis is considered. The bend over a sheet metal blank consists of a V shape forming by using a punch, with a certain nose radius, forcing the plate against an open die, with a V section. The forming result is a part with an angle obtained between the V legs, flanges, which is known as bending angle. The operation to get the required V angle is called air bending, or free bending. The most common used machines for this forming process are press brakes, special long presses, where the tools, punch and die, are attached to. With the spread use of CNC machines, and their computer control capabilities, most of them using graphical user interface (GUI), became important to get the required shape at first trial. Beyond the required bending angle obtained with just one hit, it is also important to position the gauge system in order to get the successive flange lengths to complete the programmed shape. The main variables controlled by the CNC are the punch penetration inside the die and the position of the back gauge, which is determined by the bend allowance. However this penetration is not the only responsible for the resulting bending angle and the gauging position is not the only responsible for the flange length. Additionally, the radius inside the V shape edge, known as bending radius, influences the geometry and correspondingly the bend allowance. Some authors believe that the punch nose radius has direct influence, both in the bending angle and bend allowance. In this paper, results are presented describing the use of finite element analysis as an aid in the prediction of the inside bending radius, that influences both punch penetration for the final bending angle and the bend allowance for the final flange length. From the air bending analysis, a natural inside bending radius is presented as an important variable in these kind of processes, as well as its minor dependence on the punch nose radius.

Abstract. The main goal of the recently started WindFarmSHM research project is the development, validation and optimization of monitoring strategies to be applied at the level of the wind farm. These strategies should be able to evaluate the structural condition of a set of wind turbines and their consumed fatigue life using the response to operation loads. In this context, a quite extensive experimental campaign is being performed at the Tocha wind farm, an onshore wind farm located in Portugal, which includes the simultaneous instrumentation of several wind turbines, adopting strain gauges, clinometers and accelerometers distributed in the tower and blades. This paper introduces the Tocha wind farm, presents the different layouts adopted in the instrumentation of the wind turbines and shows some initial results from the already fully instrumented wind turbine. At this preliminary stage, the capabilities of the very extensive monitoring layout are demonstrated. The results presented in this paper demonstrate the ability of the different monitoring components to track the modal parameters of the system composed of a tower and rotor and to characterize the internal loads at the tower base and blade roots.