On-line Damage Assessment Using Operating Deflection Shapes

Abstract

Structural health monitoring using measured vibration data may (or not) be based on a numerical model. If a structural model is not available, the measurements of the nominally healthy structure have to be used as the baseline for comparison. In this way, alterations of the behavior may be tracked. This approach can be considered as a reactive way to handle the problem since the engineer will detect that the characteristics of the structure changed, but he will not know if the modification(s) render(s) the structure unreliable. The exploitation of a numerical model allows the application of a more rich, proactive strategy. In some conditions, the analyst may even be able to diagnose the remaining lifetime of the structure. A common approach to health monitoring is to use identified modal data. In this paper, the direct use of operating deflection shapes (ODS) is considered The main advantage of this approach is that ODS are more sensitive to structure modifications than mode shapes which have to be identified; moreover, the modal analysis efforts (and errors) are avoided. In this paper, a two level approach for damage assessment is presented. In the reactive level, current experimental ODS are compared to the healthy measured ODS. The Frequency Domain Assurance Criterion -FDAC- is used to track a global evolution and the shifted residual ODS technique is used to obtain a first damage localization. If changes in the ODS are significant, the proactive level of damage assessment is activated. It uses a FE model and is based on the theory of Minimization of Errors on Constitutive Equations -MECE- to locate and quantify damage in terms of physical parameters. The proposed methods are applied on the example of an actual civil engineering structure on which the time evolution of damage is known.