1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-11a71 Center Point of Contact: JPLa71 Submitted by: Wil HarkinsSubject: Random Vibration Testing Practice: Define an appropriate random vibration test, and subject all assemblies and selected subsystems to the test for des
2、ign qualification and workmanship flight acceptance.Abstract: Preferred Practice for Design whichever is greater. The flight acceptance (FA) test level should be equal to or Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-greater than the maximum pre
3、dicted flight environment, but not less than a level which has been found to provide an adequate workmanship screen for the type of hardware being tested. Qualification and protoflight test levels should have margin above the FA level.Random vibration testing has two principal objectives:1. To verif
4、y the test item designs capability, with some margin, to withstand the launch vibroacoustic environment, and2. To screen the workmanship integrity of the flight equipment.Random vibration criteria should be developed by the process described in the following four steps:1. Determine the Power Spectra
5、l Density (PSD) of the random vibration directly transmitted into the flight article through its mounts from the launch vehicle ources such as engine firing, turbopumps, etc (see Figure 1). These vibration conditions at the launch vehicle-to-payload interface are typically available from the launch
6、vehicle builder.2. Perform an analysis to predict the payload/flight articles vibration response to the launch vibroacoustic environment. Statistical energy analysis (SEA) methods such as the VAPEPS (VibroAcoustic Payload Environment Prediction System) program are effective predictors in the higher
7、frequencies (see Figure 2). The VAPEPS program can also effectively extrapolate from a database using SEA techniques to provide predictions for a similar configuration. If random vibration predictions are needed for the lower frequencies, finite element analysis methods, such as NASTRAN, are commonl
8、y used. The vibration is induced into the test article both directly and indirectly (through its mounting).3. refer to D descriptionD Figure 1: Vibration levels transmitted to flight article through mounts refer to D descriptionD Figure 2: Payload/flight article response to vibroacoustic environment
9、 4. Establish a minimum level of vibration which is necessary to ferret out workmanship defects-both existing and potential failures (see Figure 3). This is particularly applicable to electronic assemblies for which minimum effective workmanship levels have been established based on extensive test e
10、xperience.5. Envelope the curves from 1-3 to produce a composite random vibration specification for the test article as follows: This resultant random vibration specification (Figure 4), which is employed as the flight acceptance test level, covers the two primary sources of this vibration while als
11、o providing an effective process for uncovering workmanship defects, particularly for electronics. Qualification and Protoflight test levels are increased Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-typically 3 to 6dB above flight acceptance to v
12、erify that the design is not marginal.refer to D descriptionD Figure 3: Minimum vibration levels for workmanship defect detection refer to D descriptionD Figure 4: Composite Conventional rigid fixture vibration tests can severely overtest the hardware at resonances. It is accepted practice to respon
13、se limit, or notch the input, at resonances of fragile hardware where it can be technically justified with flight or system test data, or analysis. Recently developed techniques to alleviate the overtest at resonances by specifying force limiting criteria potentially provides a much more accurate si
14、mulation of the flight vibration environment, but have not yet been implemented NASA-wide.The launch vehicle acoustically excites the spacecraft. This excitation is impractical to simulate for electronic assemblies at the assembly level because of fixture complexity, etc. Therefore, random vibration
15、 is substituted to excite the hardware.Random vibration is currently the most widely adopted type of dynamics environmental testing for spaceflight hardware. It is generally perceived by users to be the most realistic environment to reproduce in the vibration test laboratory as well as an effective
16、tool for uncovering workmanship defects- especially in electronics assemblies.Impact of Non-Practice: Increased probability of in-flight failure due to design deficiencies or defective workmanship.Related Practices: N/AAdditional Info: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Approval Info: a71 Approval Date: 2000-04-11a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
