Evaluation of the frequency range of operation of a vibration test fixture using the finite element method

Equipment embedded in land, water and aerospace vehicles must support intense dynamic loading during its operational life. Vibration tests are essential for the acceptance and qualification of this equipment. The purpose of such tests is check the mechanical strength, and eventually the equipment operational performance when subjected to a dynamic environment similar to which it will be exposed during operation. In a vibration test the device under test is fixed to the moving part of an electromechanical vibrator ("shaker") by a structure, the vibration test fixture. This structure must transmit the motion to the device under test without interfering with its dynamic behavior, and should therefore be as rigid as possible. However, limitations on the weight to be tested by the shaker prevent devices with very high mass. Therefore, every fixture will have some degree of flexibility and it is necessary to assess whether its dynamic characteristics will interfere with the test, increasing or reducing the motion transmitted to the device under test. This involves assessing the fixture's modes frequencies and the transmissibility relating the accelerations imposed on fixture base and on the regions where the device under test can be fixed. In this work, a numerical model of a fixture is built by using the finite element method and is used to evaluate the dynamic response in several regions of the fixture to a harmonic excitation applied to its base, obtaining the transmissibility over a wide range of frequencies. Then it is possible to evaluate a usable frequency range, as well as the points on the fixture where the component can be fixed to be tested.