MHB Mass holding block CSA Damping shaker armature CRod Daming rod 1 Introduction High Cycle Fatigue (HCF) [1] is one of the commonest causes of component failure in gas turbine engines and there are many sources of HCF damage such as: (i) aerodynamic excitation, (ii) mechanical vibrations and (iii) airfoil flutter. Although new engine designs have reduced problems caused by vibratory stress, HCF is still one of the main concerns of turbo machineries. The standard practice for HCF risk assessment, for example of a blade, can be summarized briefly as follows. There are two important steps during the Finite Element (FE) model analysis: (i) stress analysis and (ii) structural dynamics analysis. One is to determine the mean stresses and the other is to determine resonant frequencies, mode shapes and dynamic stresses. Predicted resonant frequencies are compared with integral order engine excitation data on a Campbell diagram in which the crossing of natural frequencies of the blades and the engine excitation can be determined. This process is crucial to predict the strength of the excitation driver which is associated to dynamic stress. When this process is completed positively, a component can be manufactured and, subsequently, tested both in the laboratory and in engine verification testing. Technology for HCF testing of metallic components became more established during the past decades by using either, for example, electromagnetic (EM) shakers or an air jet or a pulsed air jet. These could produce levels of force sufficient for exciting up to large vibration test structures. Hence, a consistent database of fatigue test data could be produced and used for: (i) the design of metallic components and (ii) the stress analysis prediction. The introduction of new materials, such as carbon fibre based composites, in the design process required the production of a new database of fatigue or “endurance” data set, because of the mechanical failure of composite materials being very different from the metallic ones. The same technology used for running HCF on metallic component was so used for the composite components. This paper presents some experimental results, obtained during the course of several months, of HCF testing performed on composite components using an electromagnetic shaker. A mechanical test rig was designed and built to be tuned with one resonance of the component to be test for HCF. This was capable of exciting a specimen to high levels of vibration but driving the shaker to work out of specification. Impedance mismatched between the shaker and the test rig is the cause of the underperforming shaker and measurements are provided to support this study. 2 Problem background Carbon fibre materials, as already introduced, are increasingly used for manufacturing engine components such as fan blades, vanes or casings. Stress limits of composite are studied both statically and dynamically by performing several tests in the laboratory. The amount of data generated is then used to characterize the material properties which are used to produce more reliable FE models. Modal parameter estimation can be performed by modal analysis using different test approaches such as impact testing or phase resonance testing. The vibratory stress level of a component can be determined using either air jet or electromagnetic shaker excitation. Air jet excitation methods, such as constant or pulsed flow, present some limitations when compared with the EM 504
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