DTIC AD1033663: High Frequency Shock During Random Vibration Testing

High Frequency Shock During
Random Vibration Testing

Michael Mastovich

LINCOLN LABORATORY

Massachusetts Institute of Technology

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This material is based upon work supported under Air Force Contract No. FA8721-05-C-0002 and/or FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect
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Overview

* Experienced high frequency shock during random vibration
testing

* Geometry is a bali and gothic arch mount

• incorrect stiffness of fiexures in finite eiement model led to
much higher reaction forces at interface

• What if this was not an audible event? Occurred above 2kHz,
therefore outside of normal monitored range

LINCOLN LABORATORY

Massachusetts Institute of Technology

Geometry

X'-> Y

Gothic

Arc

Ball

LINCOLN LABORATORY

Massachusetts Institute of Technology

Y-axis Testing Resuits

Testing aborted at -6dB: “peening
sound heard acoustically

Analysis of data shows a max
446.32g shock event at 13kHz
originating from flexure (off-axis)

- Corresponding natural frequency
shift during -6 dB test

Testing Level

Natural Frequency (Hz)
Ch. 17

Q Factor

Test 1: White Noise

192.5

47.97

Test2:-18dB

192.5

51.11

Test 3: -15 dB

192.5

50.14

Test4:-12dB

192.5

52.55

Test 5: -9 dB

192.5

44.99

Test 6: -6 dB

190

47.22

Test 7: White Noise

192.5

48.78

LINCOLN LABORATORY

Massachusetts Institute of Technology

Shock Event

• -6dB Random Input

• Shock origination: Bottom Flexure

LINCOLN LABORATORY

Massachusetts Institute of Technology

Shock Event Response at CG and Front

Face

• Ydir Input -6dB Random

• Response at CG accelerometer and
Front Face accelerometer

CG Accelerometer

Front Face Accelerometer

173.89 s 173.94 173.89 s 173.95

Time (seconds) Time (seconds)

LINCOLN LABORATORY

Massachusetts Institute of Technology

Shock Event Microphone Response

* Input -6dB Random

* Response on microphone normal to
bottom flexure (channel 21)

High frequency event signature matches

173 88 s . , 17395

Time (seconds)

LINCOLN LABORATORY

Massachusetts Institute of Technology

Pre and Post White Noise Comparison

Resonant frequencies and damping are unchanged after the shock event during vibration at -6dB

Test Level: 0.000 dB Reference RMS: 0.500 Test Range: 20.000,2000.000 Hz

Test Time: 000:01:00 Clipping: Off Resolution: 2.500 Hz

[Log] Frequency (Hz)

LINCOLN LABORATORY

Massachusetts Institute of Technology

Post Vibration Test Inspection

* No decisive macro-level scratching/chipping at interface of
tooling balls or flexure

• No noticeable fractures on flexure staking

LINCOLN LABORATORY

Massachusetts Institute of Technology

Solution

• System already in final configuration

• Not realistic to repiace flexures, but needed to fix rotational
stiffness

• Reduced shock down to acceptabie levels by stiffening flexures
using Constrained Damping Layer

LINCOLN LABORATORY

Massachusetts Institute of Technology

Shock Event

Shock on Flexure

LINCOLN LABORATORY

Massachusetts Institute of Technology

Summary

• Need to monitor boundary conditions

• Be aware of modeied stiffness vs actual stiffness

• Understand possible responses beyond 2kHz

LINCOLN LABORATORY

Massachusetts Institute of Technology