Application of Nullspace-Based Fault Detection to an Aircraft Structural Part Under Changing Excitation

A suitable combination of different methods and measurement techniques is often required for Structural Health Monitoring (SHM) of objects in situ. In the field of aerospace applications, the current research project "Combined acoustic and modal structure monitoring" is dealing with the development of a robust SHM system for damage identification in carbon-fiber-reinforced polymer (CFRP) structures under realistic and varying loads. The methods combined within the project are based on guided waves, acoustic emission, and different vibration monitoring techniques. The present paper deals with the application of the nullspace-based fault detection algorithm (NSFD) as a part of a holistic concept for the monitoring of an aircraft door surround structure. Due to the great sensitivity to changes in the statistical properties of the measured data, the algorithm reacts very sensitively to structural changes, but also to changes in the environmental conditions, such as changes in the external loads. In this paper, the impact of changing excitation conditions on the NSFD damage indicator is analyzed. Therefore, two different formulations for the NSFD algorithm are studied and compared concerning their sensitivity and robustness against changes of the external loads. Since the sensitivity and the results depend also on the algorithm set-up parameters, these are analyzed and taken into account. The evaluation of the results considers both, the robustness to the influence of the varying excitation and the sensitivity to damages in the analyzed structure. The theory and the algorithms are successfully tested with measured data sets from a CFRP-airplane structure.