Original Date: 04/26/1999
Revision Date: 01/18/2007
Best Practice : Modal Test Facility
The Modal Test Facility at Marshall Space Flight Center (MSFC) uses three primary modal test beds: (1) the Shuttle Payload Universal Modal Test Bed; (2) the International Space Station (ISS) Rack Modal Test Bed; and (3) the Shuttle Spacelab Pallet Modal Test Bed. The purpose of each is to obtain dynamic characteristics of flight structures by using experimental modal testing methods. Modal test data enable engineers to determine the resonant frequency, damping, and mode-shape information about the tested structure. This information is used to verify finite element analysis (FEA) models of the flight vehicle with the test article mounted in place. This verification is necessary to assure that no unacceptable dynamic loads will be imposed on the flight vehicle by the structural resonance of articles being carried into space.
The Shuttle Payload Universal Modal Test Bed is capable of mounting the test article in a fixed-free configuration that represents the Shuttle trunnion mounting system. Typically, for a five-trunnion system with 30 degrees-of- freedom, seven degrees-of-freedom are fixed and the remainder are free relative to the fixed condition. The ISS Rack Modal Test Bed is capable of mounting the test article in a fixed configuration representing typical rack mounting. The Shuttle Spacelab Pallet Modal Test Bed is capable of mounting a Shuttle Payload attached to a Spacelab Pallet in a free-free test configuration. Up to 500 triaxial accelerometers (1,500 linear accelerometers) can be attached to the test article before applying up to six independent, benign, low-level, vibrational inputs over a frequency range of 0 Hertz (Hz) to 100 Hz. Engineers then measure the modal response of the test article, and can record up to 224 channels of data simultaneously. The recorded directional accelerometer information is used to compute the frequency response functions which, when curve-fitted, yield the desired modal parameters (e.g., resonant frequencies, damping coefficients, mode shapes) for verifying the FEA models.
MSFC’s Modal Test Facility is considered a unique capability within the United States, and possibly the world. No other facility is known to be capable of performing modal testing on specimens that span up to 45 feet in length and weigh up to 40,000 pounds. The Facility’s modal test data collecting, recording, and processing capabilities are considered unparallel.
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