An Evaluation of Modal Testing Results Based on the Force Appropriation Method

Abstract

Modal parameters of structural systems can be obtained in many different ways. The force appropriation method uses one sine signal to generate forces at different points of a structure and adjusts the relative values of those forces so as to isolate a single mode. Such tests provide very accurate information on the modeshapes which is then complemented by specific tests to determine the mode damping and scaling (modal mass). This approach has been traditionally used for ground vibration testing of aircraft where the use of sine inputs is compatible with the need for large forces at very low frequencies. After a presentation of the theory related to this testing methodology, this study applies, on the analytical model of actual aircraft, the tools generally used to extract experimental modal characteristics (frequency, damping, modeshape and modal mass) from test data. The knowledge of the true answer allows a real evaluation of the difficulties linked to different steps of the appropriation method. Issues addressed in particular are the definition of the actuator and sensor set-up, the determination of the appropriation forces rejecting unwanted modal contributions, and the accuracy of identified modal characteristics.