NASA-HDBK-7004C, NASA TECHNICAL HANDBOOK: FORCE LIMITED VIBRATION TESTING (30-NOV-2012)

NASA-HDBK-7004C, NASA TECHNICAL HANDBOOK: FORCE LIMITED VIBRATION TESTING (30-NOV-2012)., This Handbook is published by the National Aeronautics and Space Administration (NASA) as a guidance document to provide engineering information; lessons learned; possible options to address technical issues; classification of similar items, materials, or processes; interpretative direction and techniques; and any other type of guidance information that may help the Government or its contractors in the design, construction, selection, management, support, or operation of systems, products, processes, or services. This Handbook is approved for use by NASA Headquarters and NASA Centers, including Component Facilities and Technical and Service Support Centers. This Handbook establishes a common framework for consistent practices across NASA programs. This third revision of the Handbook includes several advances in the calculation and application of vibration force limits, guidelines for the application of force limiting, and data from a third flight experiment that involved measuring the forces and accelerations at the interface between a spacecraft and launch vehicle. The primary goal of vibration tests of aerospace hardware is to identify problems that, if not remedied, would result in flight failures. This goal can best be met by implementing a realistic (flight-like) test with a specified positive margin. Usually, this goal is not well served by traditional acceleration-controlled vibration tests that historically do an adequate job of screening out flight failures but often result in failures that would not occur in flight. Vibration tests that are unrealistic and too severe are responsible not only for the cost and schedule overruns associated with hardware failures during unrealistic tests but also for the weight and performance penalties associated with designing for unrealistic tests. It has been known for 40 years that the major cause of overtesting in aerospace vibration tests is associated with the large mechanical impedance of the shaker and the standard practice of controlling the input acceleration to the frequency envelope of the flight data (Blake, 1954 [1]; Salter, 1964 [2]; Murfin, 1968 [3]; Ratz, 1966 [4]; Heinricks, 1967 [5]; Painter, 1967 [6]). This approach results in unrealistic, large base reaction forces and other large responses at the fixedbase resonance frequencies of the test item. The conventional method of alleviating this problem is to measure and limit the acceleration responses in the test to those predicted for flight, but this approach is highly dependent on the analysis that the test is supposed to validate and usually requires limiting the acceleration responses at many locations on large test items. This Handbook describes an improved vibration testing method that has been facilitated by two technological developments, circa 1980-1990: the advent of both three-axis piezoelectric force gages and shaker control systems with real-time response limiting. The subject method involves inserting the ring-shaped force gages under the test item to measure the reaction force between the test item and the shaker and limiting the measured force to a specification that is designed to replicate the flight environment. For most of the frequency range, the test is controlled, as in a conventional vibration test, by the input acceleration specification; but at the test item resonance frequencies, the force limit usually results in a reduction of the input acceleration (notching).