Design of high impedance test rig for composite structures vibration measurements

Experimental vibration measurement of mechanical components are very important for studying the dynamic properties. Electromagnetic (EM) shakers are the most widely used exciters in mechanical testing because of both the broadband dynamic range of excitation and the excitation power. However, there are applications where these exciters can work inefficiently, and so underperform. This can be caused by an impedance mismatch between the shaker armature and test rig, which causes dissipation of the generated power into heating the armature rather than moving the test structure. Clearly, mechanical components presenting a high level of structural damping will require higher level of power to obtain high levels of vibration. Hence, it is important to minimize as much as possible any unwanted power dissipation due to both the test rig design and the connection between the shaker and/or the test rig. This paper demonstrates that a bladed disc type of structure can be used as a high impedance connector for a test rig in order to increase the excitation force level. This is possible thanks to otherwise an undesirably dynamic characteristic of bladed discs, which is represented by mistuning of the blades. When this mistuning characteristic is enhanced, it is possible to produce several resonances each with a high impedance match between the shaker and the test rig and this can increase the force applied to the specimen and thus its displacement amplitude. Also, the test rig proposed here can be used of several resonance frequencies depending on the number of blades. Hence the proposed test rig can improve both the performance of a shaker and increase the amplitude of vibration of the test structure. Further to this the application of the amplification process can be used for fatigue trials of composite material component. This has been an application which has caused some considerable difficulty: few cases have been successful and the results in this paper show evidence of how to proceed for future trials.