Modal testing

About: Modal testing is a research topic. Over the lifetime, 4047 publications have been published within this topic receiving 64772 citations.

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.

Investigation of water length effects on the modal behavior of a prototype arch dam using operational and analytical modal analyses

Abstract: This study determines the water length effects on the modal behavior of a prototype arch dam using Operational and Analytical Modal Analyses. Achievement of this purpose involves construction of a prototype arch dam-reservoir-foundation model under laboratory conditions. In the model, reservoir length was taken to be as much as three times the dam height. To determine the experimental dynamic characteristics of the arch dam using Operational Modal Analysis, ambient vibration tests were implemented for empty reservoir and three different reservoir water lengths. In the ambient vibration tests, the dam was vibrated by natural excitations provided from small impact effects and the response signals were measured using sensitive accelerometers. Operational Modal Analysis software process signals collected from the ambient vibration tests, and Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification techniques estimated modal parameters of the dams. To validate the experimental results, 3D finite element model of the prototype arch dam was modeled by ANSYS software for empty reservoir and three different reservoir water lengths, and dynamic characteristics of each model were determined analytically. At the end of the study, experimentally and analytically identified dynamic characteristics compared to each other. Also, changes on the natural frequencies along to water length are plotted as graphs. Results suggest that reservoir water complicates the modal behavior of the arch dam significantly.

Thermoelastic vibration test techniques

Abstract: The structural integrity of proposed high speed aircraft can be seriously affected by the extremely high surface temperatures and large temperature gradients throughout the vehicle's structure. Variations in the structure's elastic characteristics as a result of thermal effects can be observed by changes in vibration frequency, damping, and mode shape. Analysis codes that predict these changes must be correlated and verified with experimental data. The experimental modal test techniques and procedures used to conduct uniform, nonuniform, and transient thermoelastic vibration tests are presented. Experimental setup and elevated temperature instrumentation considerations are also discussed. Modal data for a 12 by 50 inch aluminum plate heated to a temperature of 475 F are presented. These data show the effect of heat on the plate's modal characteristics. The results indicated that frequency decreased, damping increased, and mode shape remained unchanged as the temperature of the plate was increased.