Knight, J. Brent

NASA Technical Reports Server

Undue Conservatism in Random Vibration Design 
Loads
J. Brent Knight
Dynamic Loads Analyst
© The Aerospace Corporation 2010
Spacecraft and Launch Vehicle Dynamic Environments Workshop
19–21 June 2012
Space Systems Thermal and Mechanical Analysis Branch (ES22)
NASA Marshall Space Flight Center
https://ntrs.nasa.gov/search.jsp?R=20120015317 2019-08-30T22:18:21+00:00Z
Random Vibration Design Loads
– Background and Objectives
– “Higher Frequencies” - How High is High?
– Underlying Premise
– Case Study – Simple Cantilevered Beam
– Conclusion
– Future Work
– Acknowledgements
Agenda
2
Background and Objectives
– Structural design loads due to random vibrations are often 
suspected of significant undue conservatism
• Some reasons for that perception include
– Environments are sometimes (maybe all the time) labeled 
as unrealistic
• “Heritage” prediction methods (mass scaling) often are based 
on data relative to less than ideally similar structures
• Modern methods (SEA, hybrid methods,...) are new and some 
Random Vibration Design Loads
3
(c.) (d.)
are skeptical of their applicability
– Dynamic loads specified as static loads are unduly 
conservative as applied (statically)
• A recently retired MSFC vibro-acoustic engineer has 
been heard to say (numerous times)  “it’s a dynamic 
load, if you want to apply it statically, be my guest”
– At “higher frequencies” the effect of vibrations on 
structures is minimal – this of course begs the question, 
how high is high?
Background and Objectives
– This effort focuses on the notion that above some frequency the 
strain field associated with a random vibration environment will 
be insignificant for typical aerospace hardware
• This effort is geared towards things such as an avionics box 
and its interface to primary structure as opposed to circuitry 
inside the box
Random Vibration Design Loads
4
(c.) (d.)
– The objectives of this presentation are: (1) to depict the order of 
magnitude of potential undue conservatism in specified design 
loads associated with “higher frequency” modes and (2) to 
discuss future work in support of this effort
Higher Frequencies - How High is High?
– While a member of the SPACEHAB Science Double Module 
(SDM) Design Team (1997 time frame), since pertinent hardware 
was not NASA hardware, significant flexibility in analysis 
methods existed 
• If a component or hardware item was inside the SDM it posed 
no threat to NASA hardware so scrutiny was minimal
• Methodologies used were reviewed internally on a case by 
Random Vibration Design Loads
5
(c.) (d.)
case basis
• “Engineering judgment” led to the threshold of 300Hz for 
hardware with typical robustness
– For the hardware in question and the applicable 
environments modes above 300 Hz were not considered in 
the loads development process
– Recently ISS vibro-acoustic test  data was observed that 
indicated that at around 300 Hz measured strain stopped 
accumulating
– The real answer is that the frequency threshold of concern 
is dependent on the structure in question and the 
environment
Higher Frequencies - How High is High?
Random Vibration Design Loads
6
(c.) (d.)
• The specific answer for a given system has to be 
determined via analysis and/or test of that hardware
– Nonetheless, for a given structural design and a given 
environment there is a frequency above which the effects 
of the random vibration environment is benign
Underlying Premise
Random Vibration Design Loads
• Strain is the phenomena of concern
• For harmonic motion
7
(c.) (d.)
Disregarding the sense we can say,
– Plotting                  for an arbitrarily selected 100 g
Underlying Premise
Random Vibration Design Loads
8
(c.) (d.)
– As one goes up in frequency the corresponding displacement goes 
down dramatically
• The displacement associated with strain, ∆l, is but a fraction of that
– Obviously, the above trend would be notably different for a higher or a 
lower acceleration
• Clearly, for a given situation (component and environment), as one goes 
up in frequency the significance of acceleration goes down with respect 
to the corresponding strain field
– At times, load factors or static equivalent loads are specified for 
structural design that correspond to high frequency modes and this 
principle is not employed
Underlying Premise
Random Vibration Design Loads
9
– In one case, 700 - 800 g’s was specified as a structural design load 
for a fairing and those loads corresponded to a mode at about 1100 
Hz
– Another example is that of a camera mounted on Ares IX that had in 
one direction 577 g specified and that was associated with a 934 Hz 
mode
• This is perceived as extremly2 (“extremely squared”) conservative
– The goal of this study was to as closely as possible 
compare “apples to apples” and investigate the strain 
due to an excited dominant/significant mode with its 
frequency at about 40 Hz to that of a one with its 
frequency at about 300 Hz
• In both cases the same cross section was reacting the load
Case Study – Simple Cantilevered Beam
Random Vibration Design Loads
10
(c.) (d.)
• In both cases the mode in question was a significant mode
• An approximately equivalent oscillating force was used
– A simple cantilevered beam was designed as the Test Article (TA)
• It accommodated addition of mass to produce the desired  low 
frequency mode
– Test Configurations
Random Vibration Design Loads
Case Study – Simple Cantilevered Beam
11
(c.) (d.)
Configuration 1 – Beam without the mass
f≈300 Hz, M= 6.5 Lb., MCL≈ 4 Lb. Configuration 2 - Beam with added mass
f≈40 Hz, M= 48.5 Lb., MCL≈ 46 Lb.
– The beam had a strain gage rosette installed
Random Vibration Design Loads
Case Study – Simple Cantilevered Beam
12
(c.) (d.)
– The fundamental mode of each configuration was 
measured
• Case 1, the beam without the added mass, had a 304 
Hz mode
• Case 2, the beam with the added mass, had a 40 Hz 
mode
– The high frequency case was then excited via sine dwell at 
Random Vibration Design Loads
Case Study – Simple Cantilevered Beam
13
(c.) (d.)
304 Hz with response amplitude of about 24.5 g
• It had 4.0 Lbs. oscillating (the weight of the cantilevered 
part of the beam)
• That yields an oscillating force of approximately 98 
pounds
– The low frequency case was then excited via sine dwell at 
40 Hz with response amplitude of 2 g
• It had 46 Lbs. oscillating which yields an oscillating force 
of about 92 Lb.
Random Vibration Design Loads
Case Study – Simple Cantilevered Beam
14
(c.) (d.)
– Therefore the oscillating effective forces were close to 
equal in both cases (which was the goal)
– The measured strain traces both correspond to (1) the 
same cross section reacting the load, (2) a very similar 
oscillating force, and (3) a dominant mode
• Results – measured 40 Hz and 304 Hz strain
Random Vibration Design Loads
Case Study – Simple Cantilevered Beam
0
0.00002
0.00004
0.00006
0.00008
0.0001
S
t
r
a
i
n
40 Hz Strain Vs. 304 Hz strain
40 Hz Strain
15
(c.) (d.)
• A factor of approximately 3 was observed between the 
two cases
-0.0001
-0.00008
-0.00006
-0.00004
-0.00002
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Time, Sec.
304 Hz strain
• Efforts in this arena are planned to continue until a method to leverage 
this principle is evolved and accepted by the dynamics community
– This notion will only aid a subset of components, those with dominant 
modes above their frequency threshold
• This would have most likely helped the fairing with 800 g’s 
specified as structural design criteria
Random Vibration Design Loads
Conclusion
16
– A method of identifying the frequency threshold for a given structure 
and environment is needed. So, again, “how high is high?”
– While, all the nuts and bolts of the pursued methodology are not 
established, the underlying premise is valid and the can contribute to 
mitigating undue conservatism 
– There are other areas of future focus relative to mitigation of undue 
conservatism, this one was seen as the lowest hanging fruit
– Acquiring additional data from different structures is planned 
• Additional data points will be acquired with the TA described in 
this presentation
– Data has been acquired but not yet post-processed
• The recent MSFC/AD01/AE01 test series included two flight 
Random Vibration Design Loads
Future Work
17
(c.) (d.)
like avionics boxes
– Strain and acceleration were measured in those tests
– The predicted modal frequencies of those boxes are high 
enough to include them in these studies
• Other hardware with significant modes above 300 will be 
acquired and small scale testing is planned
• Ultimately, all pertinent results will be utilized in efforts to 
develop a methodology that will facilitate using this 
principle to mitigate undue conservatism
•
Random Vibration Design Loads
Future Work
18
(c.) (d.)
The longer term pursuit is that of dynamics analysts 
writing out stress/strain from dynamic analyses and 
those being combined with stress/strain from static 
loadings
– The MSFC Technical Excellence program provided resources with 
which raw materials as well as tools and measurement equipment were 
acquired in support of these efforts. It is planned for these efforts to 
continue utilizing those resources
– Multiple individuals have supported the efforts “under the radar”
• MSFC/ES21 supported these efforts thru their Mechanical 
Development Facility and their personnel. ES21’s Bob Beard 
fabricated the TA
Random Vibration Design Loads
Acknowledgements
19
(c.) (d.)
• MSFC/ET40 Steve Rodgers provided the shaker table/excitation to 
the TA
• MSFC/ET40 Dan Lazor post processed measured strain data
– The efforts and resources above are much 
appreciated!
Undue Conservatism in Random Vibration Design 
Loads
J. Brent Knight
Dynamic Loads Analyst
© The Aerospace Corporation 2010© The Aerospace Corporation 2012
Spacecraft and Launch Vehicle Dynamic Environments Workshop
19–21 June 2012
Space Systems Thermal and Mechanical Analysis Branch (ES22)
NASA Marshall Space Flight Center
Thank you
© The Aerospace Corporation 2010© The Aerospace Corporation 2012


Undue Conservatism in Random Vibration Design Loads

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