The creation of a database for the validation of finite element (FE) models is one of the most common reasons for carrying out structural dynamic experiments. The Institute has been involved in the development and application of the so-called model updating methods for many years. These methods adjust the systematic stiffness and mass parameters of an FE model in order to minimise the deviations of natural frequencies and vibration patterns between simulations and experiments. In so doing, FE models are being used as a standard in the planning and preparation of ground vibration tests (GVT). During these structural dynamics experiments, the number of measurement degrees of freedom does not come close to that of models. However, it is necessary to identify an equivalent structural dynamic model of a real construction in order to carry out a GVT. Relevant test preparation is therefore necessary, even against the backdrop of the extremely tight and limited time frame for such tests. In order to meet these requirements, various methods are implemented for test planning, which, with the help of an FE model, are able to determine the optimal position of sensors and vibration exciters. The aim of this procedure is to increase the efficiency of the tests, for example to minimise the effort needed during the process of installing a GVT by using as few sensors as possible, whilst at the same time maximising the information content of the measurement data.
The above image is an example of the optimisation of the sensor positioning, as determined by one of the methods used. The purpose for using these methods is to significantly reduce the amount of time needed for the GVT development phase. These methods enable the measurement degrees of freedom to be reduced from the initial more than 400 (in the case of a GVT on the A340-300) to no more than 100. In addition, it is also important that these 100 sensors be placed in the optimal position in order that all the aircraft natural vibration patterns can be identified up to 15Hz. Validating the method for identifying the optimal position of the sensors is carried out as part of a research ground vibration test on an A340-600 in the joint FTEG project in the aeronautical research programme IV (LuFo IV).