NASA-HDBK-7004B, NASA TECHNICAL HANDBOOK: FORCE LIMITED VIBRATION TESTING (31 JAN 2003)

NASA-HDBK-7004B, NASA TECHNICAL HANDBOOK: FORCE LIMITED VIBRATION TESTING (31 JAN 2003)., This handbook is approved for use by NASA Headquarters and all Centers and is intended to provide a common framework for consistent practices across NASA programs. This first revision of the handbook includes force data measured in two flight experiments and a few minor editorial changes. The flight data provide validation of the force limiting methodology. 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 only be met by implementing a realistic (flight-like) test with a specified positive margin. In most cases, the goal is not well served by traditional acceleration-controlled vibration tests that indeed screen out flight failures, but in addition may cause failures that would not occur in flight. The penalty of over testing is manifested in design and performance compromises, as well as in the high costs and schedule overruns associated with recovering from artificial test failures. It has been known for 30 years that the major cause of over testing in aerospace vibration tests is associated with the infinite mechanical impedance of the shaker and the standard practice of controlling the input acceleration to the frequency envelope of the flight data. This approach results in artificially high shaker forces and responses at the resonance frequencies of the test item. To alleviate this problem it has become common practice to limit the acceleration responses in the test to those predicted for flight, but this approach is very dependent on the analysis that the test is supposed to validate. Another difficulty with response limiting is that it requires placing accelerometers on the test item at many critical locations, some of which are often inaccessible. The advent of piezoelectric triaxial force gages has made possible an alternative, improved vibration-testing approach based on measuring and limiting the reaction force between the shaker and test item. Piezoelectric force gages are robust, relatively easy to install between the test item and shaker, and require the same signal conditioning as piezoelectric accelerometers commonly used in vibration testing. Also vibration test controllers now provide the capability to limit the measured forces and thereby notch the input acceleration in real time. To take advantage of this new capability to measure and control shaker force, a rationale for predicting the flight-limit forces has been developed, validated with flight measurements, and applied to many flight projects during the past five years. Force limited vibration tests are conducted routinely at the Jet Propulsion Laboratory (JPL) and also at several other NASA Centers, Government laboratories, and many aerospace contractors. This handbook describes an approach that may be used to facilitate and maximize the benefits of applying this relatively new technology throughout NASA in a consistent manner. A NASA monograph, NASA-RP-1403, which provides more detailed information on the same subject, is also available for reference.