Null-Subspace Analysis for Structural Damage Monitoring in Fatigue Testing of Luminaires

A damage detection method of mechanical system based on output measurements only and on the subspace identification concept is presented. The method uses the definition of null subspace analysis of the Hankel matrix. It is demonstrated to be sensitive to small-size structural damages and suitable for continuous on-line monitoring. The measured response records of structures under environmental or artificial vibrations are assembled in the Hankel matrix that is further factorized into special subspaces. The active subspace defined by the first principal components is shown to be orthonormal to the null subspace defined by the remaining principal components. Therefore, any evident increase of the residues signifies possible structural damages. The method is illustrated by an application to fatigue tests of a street lighting device (luminaires). Experimental methods and numerical calculations on luminaires are simply described and discussed. The test results show that the developed methods may be easily used to effectively monitor the damage evolution during the fatigue tests.