Validation of Force Limited Vibration Testing at NASA ... Validation of Force Limited Vibration...

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  • NASA /TM-2003-212404

    Validation of Force Limited Vibration Testing at NASA Langley Research Center

    Chad E. Rice and Ralph D. Buehrle Langley Research Center, Hampton, Virginia

    May 2003

    https://ntrs.nasa.gov/search.jsp?R=20030056584 2020-04-02T23:02:57+00:00Z

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  • NASA/TM-2003-212404

    Validation of Force Limited Vibration Testing at NASA Langley Research Center

    Chad E. Riceand Ralph D. Buehrle Langley Research Center, Hampton, Virginia

    National Aeronautics and Space Administration

    Langley Research Center Hampton, Virginia 23681-2199

    May 2003

  • Validation of Force Limited Vibration Testing at NASA Langley Research Center

    Chad E. Rice Systems Integration and Test Branch

    NASA Langley Research Center Hampton, Va. 23681

    Abstract

    Ralph D. Buehrle Structural Acoustics Branch

    NASA Langley Research Center Hampton, Va. 23681

    Vibration tests were performed to develop and validate the forced limited vibration testing capability at the NASA Langley Research Center. The force limited vibration test technique has been utilized at the Jet Propulsion Laboratory and other NASA centers to provide more realistic vibration test environments for aerospace flight hardware. In standard random vibration tests, the payload is mounted to a rigid fixture and the interface acceleration is controlled to a specified level based on a conservative estimate of the expected flight environment. In force limited vibration tests, both the acceleration and force are controlled at the mounting interface to compensate for differences between the flexible flight mounting and rigid test fixture. This minimizes the over test at the payload natural frequencies and results in more realistic forces being transmitted at the mounting interface. Force and acceleration response data was provided by NASA Goddard Space Flight Center for a test article that was flown in 1998 on a Black Brant sounding rocket. The measured flight interface acceleration data was used as the reference acceleration spectrum. Using this acceleration spectrum, three analytical methods were used to estimate the force limits. Standard random and force limited vibration tests were performed and the results are compared with the flight data. Implementing the force limiting technique resulted in a reduction in the interface force and response acceleration at the payload natural frequency by over two-orders of magnitude as compared to the standard vibration test. The response acceleration and interface force data from the force limited vibration test still provided a conservative envelope of the flight data over the range force limiting was in effect.

    Introduction

    In standard random vibration tests, the payload is mounted to a rigid fixture and driven to a specified acceleration spectral density (ASD). This test approach results in an over test at the payload’s fixed-base natural frequencies. Two major factors contribute to this over test problem. The first factor is that the ASD specification is based on a conservative envelope of maximum expected acceleration values based on flight tests with similar payloads/launch vehicles, ground tests, analytical predictions, or a

    1

  • combination of analytical and empirical methods. Unfortunately, enveloping eliminates notches in the interface acceleration associated with resonant response of the payload. The second factor contributing to the over test problem is the difference in impedance between the rigid test fixture and the flight mounting. Mounting the test article to a three-inch thick aluminum test fixture during a standard vibration test is normal practice at NASA Langley Research Center. When the test article is mounted to the flight vehicle, the payload acts as a vibration absorber at frequencies near its fixed-base natural frequencies.’ This reduces the interface force and acceleration at these particular frequencies resulting in notches in the flight data. After enveloping, these notches are removed from the test specification. Applying the enveloped ASD to a rigidly mounted payload results in excessive interface forces and response accelerations at the fixed-base natural frequencies of the payload.

    The force limited vibration testing technique’-4 was developed to minimize the over testing associated with enveloping the acceleration spectral density and differences in flight versus test mounting impedance. Force limiting is implemented by dual control testing where both the interface acceleration and force are controlled during the vibration tests. Force limited vibration testing has been established as an accepted practice at NASA.’” NASA Langley Research Center (LaRC) has not used this technique on a flight payload. This paper documents the demonstration of the force limited vibration test technique at NASA LaRC. Validation of the technique was based on flight data5 provided by NASA Goddard Space Flight Center (GSFC) for a test article that was flown in 1998 on a Black Brant sounding rocket. This paper will describe the force limit prediction methods, test article, test setup, and results for the force limited vibration tests. For this study, the ASD used for all tests and force limit predictions was the actual flight interface acceleration data (not enveloped) provided by GSFC. This reduced the amount of conservatism usually seen in the acceleration specification and allowed for a better assessment of the force limit prediction methods. Comparisons will be made between the force limited vibration test results, standard vibration test results, and the flight data.

    Force Limit Prediction Methods

    The force limit prediction methods are used to determine the force spectral density needed when conducting force limited vibration tests. Several prediction methods are described in references 1 and 2. For this study, three of the prediction methods were used to define the force limits. The methods used were:

    1. Simple Two-Degree-of-Freedom System (Simple TDFS) 2. Complex Two-Degree-of-Freedom System (Complex TDFS) 3. Semi-Empirical (S-E)

    A detailed description of the force limit prediction methods is provided in references 1 and 2. In this section, a brief summary of the methods will be provided.

    2

  • Simple TDFS Method

    The Simple TDFS method’I2 was derived from the TDFS shown in figure 1. The equation used to develop the force limits for this method was:

    The frequency ratio can be written in terms of the masses as:

    ( m / u 0 ) , =l+(M,/M,)/2~[(M,/M,)+(M,/M,)2/4]0.s (2)

    where, SFF =Force spectral density S , =Acceleration spectral density MI = Frequency dependent residual mass for the source M , = Frequency dependent residual mas