On-line modal monitoring of aircraft structures under unknown excitation

In this paper, we present the application of recently proposed subspace-bas ed damage detection and isolation algorithms in on-line structural monitori ng of an airplane structure under unknown excitation. The first step of the approach is a subspace-based modal analysis of the safe structure, using o utput-only measurements. The monitoring step consists in the computation of a chi(2)-variable, which reflects possible deviations of newly collected o utput-only data w.r.t. the reference (safe) modal behaviour. No re-identifi cation of the possibly damaged structure is required. An on-line version of the detection algorithm is proposed. Monitoring certain critical modes can also be achieved through sensitivity tests. Experimental results for the p roposed monitoring algorithms are reported, in the case of the Paris MS760 airplane, on which mass as well as stiffness changes are introduced. Moreov er, the results of the output-only subspace-based modal analysis are compar ed with the modal appropriation method. (C) 1999 Academic Press.