2 NEW HEXAPOD TEST RIG FOR TESTS UNDER DYNAMIC LOADING

In this paper new research approaches based upon the opportunities of a new unique testing rig are presented. The Hexapod test rig at the TUHH enables researchers to excite large test specimens of, for example aircraft cabin interiors, under various transient and stationary periodic dynamic loads in all six degrees of freedom. The research projects treat the lack of dynamic parameters of material and structure behaviour for better simulation based predictions in dimensioning of these lightweight products. Also the aspect of different product variants in dimensioning is targeted. As of great importance for such experimental parameter determination, the experimental design with the adoptions necessary in practice and their influence on the outcome are a research topic described. 1 LIGHTWEIGHT DESIGN UNDER DYNAMIC LOADING FOR VARIANT CABIN MONUMENTS Various dynamic load cases are to be taken into account when dimensioning aircraft cabin interior resulting from different causes. Beside the ordinary loads from flight manoeuvres and turbulences there are also conditions to be considered like the emergency landing. In this case the cabin monument has to stay in place under a transient shock pulse. This paper, however, will focus on the periodic-stationary conditions of low frequency mechanical vibrations. Stationary-periodic excitations can occur when a turbine fan blade is lost and the rotor shaft continues turning due to the air flow even though the engine is switched off. This condition is called “Sustained Engine Imbalance” (SEI) or Turbine Blade Loss Windmilling. It is a safety issue which has to be substantiated for most new aircraft. Another excitation condition is excitation of vibrations during take-off and landing due to runway unevenness transmitted into the cabin through the landing gear. Monitors should not vibrate too much in order to increase the comfort of the passenger. This is clearly a comfort driven issue but it adds a selling point for cabin monument suppliers. B. Plaumann, O. Rasmussen, R. Seemann, D. Krause 2 1.1 Lightweight design through detailed analysis with better simulation models Correct and detailed models are extremely helpful or sometimes crucial when a high performance design is to be dimensioned. Weight optimisation in aircraft design is such a case. Good simulation models can save many iterative trial-and-error tests, although they cannot completely replace some smaller tests on the general behaviour of single components or sometimes a full scale substantiation test. A general lightweight design dimensioning with detailed mechanical analysis is described under [1]. A common workaround for resonance problems is to stiffen the structure so that resonance frequencies are pushed out of the excitation range. This, however, adds a lot of mass which contradicts lightweight design. An alternative approach is using a precise knowledge of the amplification in resonance. If the critical damping rate parameter is known the model can predict the amplification under the real excitation. This Frequency Response Analysis (FRA) can be used to predict interface forces and local stresses once damping parameters have been established. Following this, an analysis of the calculated results will show whether the configuration is critical at all. Maybe the damping is high enough so that the general behaviour is not critical even in resonance. And if damping is not sufficient, a detailed model will help to dimension the product properly. 1.2 Impact of variant product structures on the dimensioning Due to the high number of variants of cabin monuments like galleys, stowages and partitions fatigue investigations are often only conducted on small subcomponents which have been identified as being critical. However, when looking at dynamic loading of larger components like cabin monuments the vibrational behaviour has to be taken into account. Only after looking at this full scale dynamic behaviour, local fatigue analysis can be performed because resonances can multiply the loads and stresses locally and globally. If the worst-case-combinations of the requirements are not known or easily derived, additional design loops will have to be taken. The most unwanted situation would be the full dimensioning and substantiation of every possible combination. This is impractical for the design of cabin interiors monuments as described here. Cabin monuments of the research project partner aircraft supplier Diehl Service Modules are highly individualized and often only one piece will be built of every variant. In reality the combinational variety will be narrowed down by similarity approaches and vast simplifications in order to come up with only a few candidates for worstcase-identification. Conservative estimations and high safety factors are used in order to guarantee a high level of safety without precise knowledge of the real behaviour, which contradicts lightweight design principles. This is particularly applicable for the dimensioning under dynamic loads where critical worst-case-combinations with resonances are not easily estimated. Many variants and the corresponding extra effort in dimensioning limits the development time that could be used on lightweight design optimizations. Often a design will be chosen that will have the least chance of causing problems in later substantiation. AST 2013, April 23 24, Hamburg, Germany 3 Multiplying the numbers of the influencing factors load cases, geometrical variants and materials to be considered gives the theoretical number of simulations to build up, calculate and evaluate. Generally this number can easily get very large. As described before, this number should be minimized to a few critical worst-casecombinations for further investigation and corresponding design optimization.