Characterization of in-phase and out-of-phase vibration modes of secondary structures attached to a primary structure by experimentation and finite element analysis

Abstract

Ground vibration testing (GVT), is one of the most critical testing procedures for airplane certification. With proper GVT, analysts can determine the stiffness distribution, natural frequencies, mode shapes, and structural damping of each component, all of which are needed to perform flutter and dynamic load analyses. The problem identified in this dissertation is one that might lead to defects in the structural design of aircraft and catastrophic situations if proper attention is not given to GVT results. During this type of testing, technicians and engineers provide instrumentation for most of the primary components of the airplane, entirely avoiding secondary structures such as bungees, gears, control surfaces, etc. However, secondary surfaces might also be important for identifying in-phase and out-of-phase modes; otherwise, tuning of an airplane’s stiffness might not be accurate. The primary goals of this dissertation are to re-create the above-mentioned problem of in-phase and out-of-phase modes for a small simple replicate wing gear-like structure using validated modeling procedures and to quantify the effects of secondary structures in GVT. To achieve these objectives the detailed methodologies of this work include the following: - Simulating the modal experiments and finite element analysis (FEA) on a cantilever plate and free-free beam with and without flexible links attached to them.

- Determining the stiffness distribution of a primary structural component using GVT. - Identifying the modal characteristics of primary and secondary structures. - Characterizing in-phase and out-of-phase modes of a secondary structure attached to a primary structure by using signal processing. Results from this study are incorporated in GVT on a component before performing the same on the fully assembled structure and instrumenting the secondary structures.