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In the present paper, a frequency domain method for damping determination is presented. The described method is especially developed for low damped systems with well separated eigenfrequencies. Using the Short-Term Fourier transform and Resampling (STFR) of the signal, decay curves of several mode shapes can be identified and amplitude-dependent damping values can be calculated. Additionally, two common methods for damping determination are explained briefly. Finally, the quality of the introduced method is evaluated comparing the variances of the identified damping values by means of different methods. In this context, the damping for beams clamped in a suspended way is analyzed. Stainless steel is used as the specimen material.

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In turbomachinery blading the avoidance of High Cycle Fatigue (HCF) failures is of great importance. The prediction and reduction of vibration amplitudes is of primary objective in context of HCF risk reduction. For this reason the quantification of the mechanical damping is of essential relevance. During the last decades the research has been focused on the usage of nonlinear calculation tools to predict vibration amplitudes of blades. These calculations require the specification of contact parameters as well as of material damping values. Especially for weakly damped systems like turbine blades, it is necessary to specify adequate structural damping values which determine the accuracy of the calculated transfer function. However, many researchers use uncertain structural damping values to calculate transfer functions. In this paper, an experimental setup for specimen specific damping determination in a vacuum chamber is presented. Three different clamping mechanisms, as well as the mechanism for specimen excitation, are introduced. A suspended-like specimen clamping, where the specimens are clamped in their nodes of vibration, is described first. To analyze potential influence of the clamping procedure, a comparison is given to specimens clamped on one side (cantilever beams), where the magnitude of the clamping force is chosen in a way that the friction loss is minimized. To allow an application of static stresses the specimens are clamped on both sides in the third approach. The excitation of the analyzed specimens is performed with the help of a voice-coil actuator. Damping values are determined by analyzing the decay curve, which is measured with a laser doppler vibrometer. Further experimental results show the influence of ambient pressure, frequency, amplitude, geometry, mode shape, static stress and clamping mechanism on the specimen specific damping value.

Many low damped structures as turbine blades or drill strings are exposed to high dynamical loads causing high vibration amplitudes. These applications comprise sub-critical eigenfrequencies. Hereby, the lower eigenfrequencies have to be passed before reaching the operating point. Most investigations of vibration amplitudes caused by a resonance passage deal with the computation of single degree of freedom systems. Thereby, it has been shown that the stationary vibration response provides the highest possible amplitude. Further it can be stated that the maximum vibration response of the resonance passage decreases with an increasing sweep velocity [3]. Isolated modes of linear systems can be represented by single degree of freedom systems. Subsequently a mode shape can be described by the multiplication of the amplification function of the mode and the belonging eigenvector. There are only some recent works that deal with resonance passages of vicinal modes, e. g. [1].In this paper the resonance passage of a three dimensional system with nearby modes is studied. To calculate the transient vibration response an analytical approach is used. It is shown that the maximum amplitude of the stationary vibration response is not the upper limit for the maximum amplitude of the resonance passage. Thus, the maximum amplitude may rise while the sweep velocity increases. Hence, regarding a multi degree of freedom system the maximum amplitude of the resonance passage can exceed the maximum amplitude of the stationary vibration response.