Vibration-based damage localisation: Impulse response identification and model updating methods

Structural health monitoring has gained more and more interest over the recent decades. As the technology has matured and monitoring systems are employed commercially, the development of more powerful and precise methods is the logical next step in this field. Especially vibration sensor networks with few measurement points combined with utilisation of ambient vibration sources are attractive for practical applications, as this approach promises to be cost-effective while requiring minimal modification to the monitored structures. Since efficient methods for damage detection have already been developed for such sensor networks, the research focus shifts towards extracting more information from the measurement data, in particular to the localisation and quantification of damage. Two main concepts have produced promising results for damage localisation. The first approach involves a mechanical model of the structure, which is used in a model updating scheme to find the damaged areas of the structure. Second, there is a purely data-driven approach, which relies on residuals of vibration estimations to find regions where damage is probable. While much research has been conducted following these two concepts, different approaches are rarely directly compared using the same data sets. Therefore, this thesis presents advanced methods for vibration-based damage localisation using model updating as well as a data-driven method and provides a direct comparison using the same vibration measurement data. The model updating approach presented in this thesis relies on multiobjective optimisation. Hence, the applied numerical optimisation algorithms are presented first. On this basis, the model updating parameterisation and objective function formulation is developed. The data-driven approach employs residuals from vibration estimations obtained using multiple-input finite impulse response filters. Both approaches are then verified using a simulated cantilever beam considering multiple damage scenarios. Finally, experimentally obtained data from an outdoor girder mast structure is used to validate the approaches. In summary, this thesis provides an assessment of model updating and residual-based damage localisation by means of verification and validation cases. It is found that the residual-based method exhibits numerical performance sufficient for real-time applications while providing a high sensitivity towards damage. However, the localisation accuracy is found to be superior using the model updating method

Über die numerische Berechnung von Faserbeulen durch Mesoskalenansätze

The present treatise is concerned with the application of numerical models to the prediction of compressive strength and associated phenomena in fiber reinforced polymer matrix composites. This topic has received much attention by the scientific community, and the basic mechanisms at microscopic scale are well understood. Even so, microscale models and theories offer no predictive capability at scales relevant for practical application, and the problem of devising suitable approaches for this purpose is still wide open. The main obstacle in this endeavor is that relevant mechanisms are spread over several length scales, hindering their integration. To address this challenge, the topic is thoroughly reviewed and mesoscale approaches are identified as an essential stepping stone towards an eventual transfer of fundamental scientific research to engineering application. Subsequently, the mesoscopic approach based on a homogenized representation of the fiber/matrix composite is developed further and its application for the prediction of the aforementioned mechanisms is demonstrated: Random flaws in local fiber alignment are the main source of uncertainty with regard to compressive strength and introduce a dependence of compressive strength on domain size. Methods for the proper representation of these flaws and their effect on compressive strength are considered and extended. Compressive failure in the materials under consideration is caused by shear strain localization and features characteristic width and orientation. To make these phenomena amenable to mesoscale modelling as a homogenized solid, the application of an extended solid theory with additional rotational degrees of freedom is considered. The versatility of the approach is demonstrated by predicting phenomena ranging from very small sizes, i.e. the bandwidth, to large sizes via the predicted scale law for compressive strength. Hence, it is argued that the mesoscale approach provides an excellent platform for further work concerned with component scale applications