Dynamic qualification of large non-linear space structures by means of an advanced boosted modal survey test concept

Abstract

The dynamic qualification of today's space structures is a challenging task which normally is performed by means of driven base tests in order to simulate the dynamic loads during launch. This method is limited to structures which fit onto the shaker facility. Thus, the increase of dimensions and recent advances in the lightweight design of space systems has led to the requirement of new structural dynamic qualification techniques. For these large and unwieldy structures an alternative approach can be utilised which applies modal forces for the qualification. This testing concept is called Modal Force Combination (MFC) technique and replaces the base acceleration by a set of forces acting directly in a selected number of structural points. Modal forces are patterns of single forces that are able to excite single resonances and which can be applied by movable electrodynamic shakers. An advanced test strategy is proposed in this paper which is based on the MFC technique. In contrast to common MFC techniques, the presented strategy considers structural non-linearities and applies narrow-banded swept sine excitation. In extension to MFC technique introduced up to now, a non-linear model which is identified from the narrow-banded swept sine runs on a low or intermediate level is used for the redefinition of the transients which are needed to achieve the final qualification of the structure. Since within the strategy single modes are used for the qualification the method is called boosted modal survey test. The proposed method is utilised on measured data from the boosted modal survey test on the flight model of the European Robotic Arm (ERA) in order to demonstrate the suitability of the concept. The pros and cons of the approach are discussed and recommendations concerning the application are given. Furthermore, the limitations are point out.