On-line modal monitoring of aircraft structures under unknown excitation

In this paper, we present the application of recently proposed subspace-based damage detection and isolation algorithms in on-line structural monitoring of an airplane structure under unknown excitation. The first step of the approach is a subspace-based modal analysis of the safe structure, using output-only measurements. The monitoring step consists in the computation of a chi(2)-variable, which reflects possible deviations of newly collected output-only data w.r.t. the reference (safe) modal behaviour. No re-identification 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 proposed monitoring algorithms are reported, in the case of the Paris MS760 airplane, on which mass as well as stiffness changes are introduced. Moreover, the results of the output-only subspace-based modal analysis are compared with the modal appropriation method. (C) 1999 Academic Press.