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Test Fixture Qualification

 

Upon the conclusion of the design of a vibration test fixture it is sometimes required that the fixture be qualified to determine its resonant characteristics. This requirement is imposed to demonstrate that the dynamic motion of the fixture will not adversely affect or skew the results of the vibration testing.

Test fixture qualifications can be performed numerically by hand analysis or finite element analysis (FEA), or in the laboratory by impact or bump testing. Another test technique is modal analysis. Except for simple fixtures, test methods are preferred, since hand analysis using basic static and dynamic methods usually results in imprecise data. FEA methods are complex and costly.

Finite Element Analysis

Finite element analysis is a computer-based analytic tool for solving field-flow problems. It allows for problem solving in the areas of structures, fluid flow, heat transfer, and electric fields due to the similarity in the partial differential equations which characterize these fields.

The FEA for structural design starts with modeling of the structure by dividing it into an equivalent system of simple elements, such as rectangles or triangles, with easily obtained stress and deflection characteristics. Upon specifying the material, material properties, boundary conditions, and loads, the analysis is completed by computer programs, utilizing arrays of matrix equations.

FEA allows for the determination of free-vibration natural frequencies and the associated mode shapes of a structure. It provides valuable information for use in the design stages of a program allowing optimization of the design by varying key parameters.

Impact or Bump Testing and Modal Analysis

Impact or bump testing is a powerful tool for testing the vibratory response of structures. It consists of a device for impacting energy, such as a hammer, into a structure. The frequency spectrum of the hammer force impact signal is nearly flat over a wide frequency range. Therefore, a single impact will excite all the natural frequencies within the frequency band. In other words, the energy content of an impulsive impact is broadband.

Utilizing a hammer with a force transducer and displacement transducers placed at numerous locations throughout the test item to measure the response motions, along with a Fast Fourier Transform (FFT) spectrum analyzer allows the determination of the structure's dynamic response. The response is defined by the natural frequencies, mode shapes and damping factors. This method is called modal analysis. Electric impact hammers are also available to yield controlled repeatable impact forces.

 

* EXCERPTED AND REPRINTED WITH PERMISSION OF THE PUBLISHER FROM TEST ENGINEERING AND MANAGEMENT MAGAZINE, JUNE/JULY 2004, PAGES 10-11. COPYRIGHT 2004 THE MATTINGLEY PUBLISHING COMPANY, INC., OAKLAND, CALIFORNIA. WWW.TESTMAGAZINE.BIZ

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