A study of mechanical impedance in mechanical test rigs performing endurance testing using electromagnetic shakers

Aircraft engine components, such as bladed discs, are subjected to high levels of vibration due to their service conditions and vibratory stresses can reduce the expected operational life. High Cycle Fatigue (HCF) is the most common cause of component failure in gas turbine engines. Laboratory testing is important for understanding the fatigue properties of materials and for producing a database eventually used during the design of new components. HCF test can be performed in the laboratory using a test rig on which is installed a test structure, connected to an electromagnetic shaker supplying the excitation. A simple test rig can be made comprising a holding block connected by push rod to the armature of a shaker so as to produce a base excitation. Such a test rig can be specifically designed to increase the test piece vibration levels by tuning it to one resonance of the structure. However, in doing so, a test rig of this type can present an impedance which is mismatched with the shaker, thereby causing dissipation of the excitation force. Any power loss can be a problem, exacerbated by components presenting high levels of structural damping, because of the higher force levels required to achieve high levels of vibration. Hence, any HCF test can be ineffective because of the diversion of shaker power from the test component to other parts of the test setup. The aim of this paper is to study the impedance mismatch between test rig and shaker by modelling a simple test rig, using a lumped-parameter model, for designing and measuring vibrations of the test rig to identify its weakness for HCF.