Gokhan O. Ozgen

Further Developments in the Dynamic Stiffness Matrix (DSM) Based Direct Damping Identification Method

Theoretical development of a dynamic stiffness matrix (DSM) based direct damping matrix identification method is revisited in this paper. This method was proposed to identify both the mechanism and spatial distribution of damping in dynamic structures as a matrix of general function of frequency. The objective of this paper, in addition to the review of the theoretical development, is to investigate some major issues regarding the feasibility of this method. The first issue investigated is how the errors in measured frequency response functions (FRF) affect the accuracy of the DSM. It was already known that the DSM is highly sensitive to errors that are present in the FRF. A detailed analytical and computational study is conducted, which finally leads to a sound physical explanation of the high sensitivity of the DSM to measurement errors. A new and also important conclusion is that the leakage error drastically affects the accuracy of the computed DSM. The second major issue reported is the experimental implementation of the DSM based method to minimize the leakage error. Based on the findings presented in this study, a new and improved test setup is designed and developed, which enables the authors to obtain a good quality result that supports the theoretical and numerical analyses previously conducted.

Design and Development of a Complex Shear Modulus Measurement Setup for Viscoelastic Materials

Details of the design and development study of a complex shear modulus measurement setup for viscoelastic materials have been presented in this paper. The new setup is specifically designed for measuring the complex shear modulus of pressure sensitive adhesives. The setup consists of a rigid block that is connected to a rigid fixture through two identical shear specimens of the viscoelastic material to be tested. The rigid block and the shear specimens resemble a single degree of freedom system. Once the frequency response function of the rigid block is measured, the frequency dependent material properties of the viscoelastic material can be calculated in a frequency range of 20 to 200 Hz. The setup also has a forced liquid convection heating/cooling system that enables testing in a temperature range of 5° F to 200° F. Test results for a sample viscoelastic material have also been presented in this paper. Basic principals of this testing method can be found in various sources, yet very few of them provide detailed information for practical implementation. Also, unlike the widely-known vibrating beam technique, no detailed standards exist on the subject. The main objective of this paper is to be an aid for future implementations of the testing approach through summarizing this recent effort for the design, analysis and construction of a sample test setup.

Experimental Characterization of a Tuned Vibration Absorber

In this paper, experimental characterization studies conducted for a tuned vibration absorber is presented. The tuned vibration absorber has been particularly designed to reduce transverse resonant vibration response of a supported cylinder structure at its dominant two modes. Various testing configurations and techniques have been used such as transmissibility measurements, frequency response measurements, sweep sine testing, impact testing, and random testing. Different testing approaches were needed to explain the differences between actual TVA characteristics and the initial design objectives. Finally, structure and tuned vibration absorber are tested together to check the actual vibration reduction performance of the tuned vibration absorber.

A Modified Inverse Eigensensitivity Method for Large Finite Element Models

Finite element models should represent the dynamic behavior of real structures accurately to be subsequently used in design purposes. Therefore, finite element model updating methods have been developed in order to decrease the difference between analytical model and modal test results. In this paper, inverse eigensensitivity method as a sensitivity-based model updating method is summarized. Inverse eigensensitivity method with improved sensitivity computation which decreases the total calculation time of the updating procedure is introduced. A method based on the parameter sensitivities is integrated into the developed code in order to decrease the number of model updating parameters making it is possible to update large finite element models. Initially a simple plate structure is considered as an application. An aerospace structure is considered as an example where modal test is performed and modal parameters are extracted. The developed updating method is applied on the finite element model of the aerospace structure and the results of the updated model are compared with experimental ones.

Design of a Test Setup for Measuring Dynamic Stiffness of Vibration Isolators

In this paper, design efforts to develop a custom test setup for measuring dynamic stiffness of vibration isolators are presented. The setup is designed to conduct dynamic stiffness measurements for various static preload values and over a certain (target) frequency range. Direct Method has been selected among the methods defined by standards found in the literature. In order to investigate the effect of basic design parameters of the test setup on its overall performance, an equivalent eight degree of freedom lumped parameter model of the test setup is used which takes into account the basic dimensions and materials used for main structural components of the proposed setup design as well as the inertial characteristics of the isolators. Using the equivalent model, virtual tests are performed and the accuracy of the test setup is studied for various testing scenarios. A major work that is conducted as part of this work is to come up with a procedure that will enable tuning of the setup parameters such that the percent error on measured dynamic stiffness of various types of isolators are minimized for the case when various levels of error are present in measured displacement and force amplitudes.

Ground Vibration Test Planning of a Fighter Aircraft by Using a Rough Finite Element Model

Certification process is one of the crucial procedures for safety in the design of a new aerial platform. Flight flutter testing is the most critical component for the certification process. Usually a flutter analysis is performed beforehand for the planning of flight flutter testing of an aircraft which mostly requires the Finite Element Model (FEM) together with Ground Vibration Testing (GVT) to construct the structural dynamic model of the complete aircraft for the flutter analyses. GVT is not only required for new aircraft design but also when considerable changes are made to an existing aircraft or when new external load configurations are introduced. Experimental methods require high effort, high budget, long time, and much repetition. Therefore, the computational and theoretical studies seem more applicable in the early phase. However, GVT of an available fighter aircraft in defense projects becomes an issue for the designers if a detailed FEM of the aircraft is not available prior to test. Hence, planning of the GVT in early stage is vital for project leaders. In this study, a rough FEM of a fighter aircraft is developed and correlated to available GVT data for planning purpose. The representative mode shapes are evaluated by estimation of the several sections of the aircraft. It is also shown that a rough FEM of the aircraft can be utilized for determination of the measurement and excitation points on the aircraft in planning stage. The geometrical properties, physical limitations and basic requirements of GVT are also taken into account for an efficient planning.Copyright

Dynamic Stiffness-Based Test Systems for Viscoelastic Material Characterization: Design Considerations

In this paper, several important design issues for viscoelastic material characterization test systems which utilize dynamic stiffness measurements are discussed. These discussions are focused on structural dynamics aspects of the design of these test systems. These test systems are used to experimentally obtain the complex modulus of viscoelastic solids such as rubber, plastics, etc. Various standards exist on dynamic stiffness-based viscoelastic material characterization test methods, which give general guidelines on possible test procedures, associated theory, and limitations of recommended test procedures. In these standards however, there is not enough guidance on how to design the details of such a test system, specifically the mechanical structure of the test system. In this paper, authors’ past experiences on the structural design of such test systems are presented which may be utilized as design guidelines for design of similar test systems. Main design issues discussed in this paper include the effect of fixture compliance on the accuracy of calculated material data, frequency limitations, selection of actuators and sensors, and edge (Poisson’s) effect considerations for material specimens.