Identification of weak non-linearities within complex aerospace structures

The modal identification of large aerospace structures like aircraft prototypes or satellites requires a great experimental effort and sophisticated test concepts in order to assure high quality test results. Nowadays, vibration characteristics like resonance frequency, mode shape, modal mass, and modal damping are extracted via experimental modal analysis during large scale testing. Although nearly all real aerospace structures exhibit some non-linear behavior, the identification methods used within modal survey testing assume linear dynamic behavior of the structures. First, this article presents the common test concept used by the German Aerospace Center (DLR) for the modal identification of large aerospace structures. Then, a strategy for non-linear modal identification of relatively weak non-linear effects is described. Typically 70% of the modes of most aerospace structures would tend to fall into the category in which reasonably accurate identification using classical linear methods is possible. Most of the non-linear modes can be considered as uncoupled, i.e. other modes are not influenced by the non-linearity of the respective mode. However, some of the non-linear modes may be coupled with other modes. The article describes the theoretical basis for the non-linear modal identification of one single uncoupled mode and for the case of non-linear coupled modes. Both methods are applied to an analytical model and to measured data of a large aircraft.