Im Rahmen eines kooperativen Forschungsprojekts der Universität Siegen zwischen der Arbeitsgruppe Technische Mechanik (ATM) des Departments Maschinenbau und des Lehrstuhls Stahlbau und Stahlverbundbau des Departments Bauingenieurwesen wurde eine Methode zur zerstörungsfreien Schadensdetektion an Schrauben und Bolzen entwickelt und auf ihre grundsätzliche Anwendbarkeit hin überprüft. Die Methode basiert auf der Messung von elektromechanischen Impedanzspektren, welche sich bei variierenden Strukturzuständen ändern und so eine Zustandsbewertung ermöglichen. Die Untersuchungsmethode könnte bspw. im Monitoring von Brückenbauwerken etabliert werden. Im Zuge des Projekts wurden Untersuchungen an Stahlplatten mit orthogonal aufgeschweißten Gewindebolzen durchgeführt. Um bei den Probekörpern das generelle Verhalten von elektromechanischen Impedanzspektren bei unterschiedlichen Zuständen festzustellen, wurden Voruntersuchungen gemacht. Hierbei ist eine gegenläufige Entwicklung der untersuchten Frequenzspektren unter den Einwirkungen „Vorspannung“ und „Sägeschnitt am Bolzenfuß“ festgestellt worden. Nachfolgend durchgeführte zyklische Versuche haben schließlich die Anwendbarkeit des Verfahrens gezeigt. Im Beitrag werden erste Versuchsergebnisse vorgestellt und darüber hinaus wird die Anwendbarkeit der Messmethodik für das Bauwerksmonitoring diskutiert.

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Fatigue crack growth is one of the most common damage mechanisms of structural components and can result in a sudden and unexpected abrupt fracture which can cause catastrophic failure. Thus, monitoring of crack propagation is an important topic of structural health monitoring. For that reason, this work aims at the detection of crack propagation by the application of a high-frequency vibration-based method, using the electromechanical admittance (EMA). It enables the local monitoring of the coupled dynamic behavior of the structure and the piezoelectric transducer which changes, for example, due to damage. It is the purpose of this study to develop a method from the recorded data reflecting fatigue crack growth in weld zones. As it is known that the data are affected by temperature, a temperature compensation strategy is considered. In this study, a presentation of the coupled EMA data has been found, which allows the isolation of the crack propagation effects. Within the scope of fatigue experiments, fatigue cracks were introduced into threaded bolts of rail support points, welded onto base plates. In practice, they are embedded in a layer of mortar and, therefore, have poor accessibility. Electromechanical impedance measurements were carried out at different states of fatigue crack growth. The fatigue crack growth was validated by strain gauge measurements executed during the fatigue experiment. The proposed feature was calculated from the recorded EMA data. The technique produces promising results detecting fatigue cracks and can also be transferred to similar types of welded joints or rods where fatigue crack growth occurs.

Within a research project carried out at the Department of Civil Engineering and the Department of Mechanical Engineering of the University of Siegen, a new monitoring method for non‐destructive damage detection on bolts was developed and tested according to its applicability on real life structures. The so‐called EMI‐method is based on the measurement of electromechanical impedance spectra, which change with varying structural states and thus allow for a condition assessment. Structures vibrate in a combination of their natural modes, which can provide information about their structural condition. Their modal characteristics are system‐inherent quantities and are governed by geometry, system stiffnesses and mass allocation. If a system parameter changes, e.g. the bending stiffness of a bridge, this is reflected in the vibration analysis and the modal characteristics. To identify the modal characteristics of civil engineering structures, these are excited and the system response is measured. This principle is also used for damage detection by means of electro‐mechanical impedance spectra (EMI). The observed system‐inherent quantity is the frequency‐dependent mechanical impedance Zs(ω) of the structure. The method is based on the idea that the mechanical impedance of a structure or component to be measured changes as a result of damage (or loss of prestress in the case of bolts) and thus these changes can be detected. In the project, investigations were carried out on steel plates with orthogonally welded threaded bolts, which are used, for example, as fasteners for rail supporting points on bridges. Research objectives were on one hand the detection of the prestress stage and on the other hand the detection of fatigue damage in the weld seam. Within the scope of the project, it has been investigated to what extent a change of the prestressing load can be reliably detected by means of the EMI method. Furthermore, the influence of damage to the bolt on the impedance spectra was investigated.