Craigmyle, Benjamin B.

Davis, Robert Ben

Fulcher, Clay W.

Jones, Douglas C.

LaVerde, Bruce T.

Smith, Andrew M.

Waldon, James M.

NASA Technical Reports Server

Correlating Attenuation of Vibroacoustic Response 
to a S stem Damping Sched le sing Gro nd Test  y   u  u  u   
Measurements, the Finite Element Method and the 
DampID Optimization Tool
Andrew M. Smith 
Vibroacoustics Specialist
Vehicle Loads and Strength Branch (EV31)
Dr. Robert Ben Davis
Acoustics and Structural Dynamics Specialist
Propulsion Structural & Dynamics Analysis Branch (ER41)
NASA Marshall Space Flight Center
      
NASA Marshall Space Flight Center
Bruce T. LaVerde,  ERC Inc
Vibroacoustics Lead Engineer
ESTS Contract Support to
Clay W. Fulcher (ER41), Douglas C. Jones (EV31) ,
James M. Waldon (EV31), & Benjamin  B. Craigmyle (ES20)
Jacobs Engineering Structural Dynamics    
Vehicle Loads and Strength Branch (EV31)
NASA Marshall Space Flight Center
 ,   
Specialists providing support  through ESTS Contract
NASA Marshall Space Flight Center
Spacecraft and Launch Vehicle Dynamic Environments Workshop
19–21 June 2012
© The Aerospace Corporation 2010
https://ntrs.nasa.gov/search.jsp?R=20120015314 2019-08-30T22:18:19+00:00Z
Validation of Measured Damping Trends 
Agenda
•Addressing the need:
• Validate/Refine the approach used to estimate the vibration environments 
associated with Equipment-Mass-Loaded-Exterior-Panels of launch vehicles.  
This is of major importance to New Vehicle Programs.
• This Validation has been identified by the NASA Engineering and Safety Center 
(NESC) as an area of uncertainty that is worthy of on-going study.
• System damping can increase with greater levels of integration.
• Important to test validate damping under flight like conditions. 
•Test Program & Preliminary Observations
•Use of Finite Elements with Modal Updating for Damping
•Updating for Damping  with use of DampID Optimizer Tool:
1. Software tool that can help identify the damping associated with the cables 
without explicitly modeling the complex energy loss mechanisms they introduce 
to the system
2. Results represent a best fit solution over multiple response channels.  These 
should be used with Engineering Judgment  to Guide Analyst to construct 
Reasonable System Damping Schedule.
3. Can reveal areas of the analysis that require further study.
•Conclusions (Assessment of test results is a work in Progress)
2
Test Program & Preliminary Observations
Test setup
a. Acoustic or Fluctuating 
Pressures affect the exterior 
surface of Vehicle panels.
b. Ground test setup.
(a.)
c. Flight like excitation of 
exterior surface in Baffled 
panel test setup. 
(Reverberant)
d. View of Flight like test article 
from Anechoic receiver room.
(b.)
• Note that the Vehicle Panel Test      
Article can be configured  as a 
bare panel as in (d.) or further 
integrated with 
equipment/cables.
3
(c.) (d.)3
In figure (a ) at the left bottom the test article is
Test Program & Preliminary Observations
(b )
Test setup
•   .          
configured with Equipment only
• Very incidental wires are present for measurement 
transducers
56 test cases covering 14 configurations and 4
.
•         
different excitation levels were addressed in the 
test series.
(a ) (b.).
• In figure (b.) at the right top the test article 
is configured with both Equipment and a 
significant set of Flight like cables.
• The results from these two configurations
4
      
are presented on subsequent slides in this 
paper.
Test Program & Preliminary Observations
Cable Installation
(b.)(a.)
Potential damping 
sources: 
a Cables routed over.    
brackets and secured 
with plastic tie wraps.
b. Cables attached to (d.)(c.)
flight-like equipment 
box with pin 
connectors. 
c Cables routed and.    
secured using p-
clamps. 
d. Close-up views of a 
5
bracket (left) and p-
clamps (right).
Preliminary Observations From Test
• The plots labeled a and b below present response at locations 1 and 2              
for configurations with and without cables.
• Both provide evidence of attenuation in the range from 100 to 400 Hz.  
Note these two responses have nearly identical spectral shapes below 
600 Hz In this frequency range the structural bending wavelengths .          
remain large relative to the orthogrid cell size.  Global panel behavior is 
exhibited.
• Above 600 Hz, the responses shown diverge from each other:
• the bending wavelengths are small enough for the response at the           
center of an orthogrid cell to be different from the response on the 
perimeter. 
(a.) (b.)
Location1 Location 2
6
Preliminary Observations From Test
• The plots labeled a and b below present response at locations 4 and 16              
for configurations with and without cables.
• Attenuation effects are perhaps best observed in figure (a.) below, which 
seems to have been sensitive to the presence of the cables over a wide 
frequency range .
• The observed attenuation is not necessarily purely the result of additional 
damping introduced by the cable harnesses :
• Attenuation of response, accompanied by shifts in the frequency 
of certain response peaks indicates that inertial mass effects of the           
added cables are a likely contributor to the response reduction.
• Without an appreciable shift,  damping is thought to be chief 
contributor.
( ) (b )a. .
Location 4 Location 16
7
Preliminary Observations From Test
(a ) 4z Acceleration.    
Response Spectral 
Density (5 Hz 
frequency resolution),
(b.) Frequency 
uncertainty 
considered -
measured spectrum 
shifted by 15% in 
either direction .  
(c.) Smooth envelope 
over frequency 
dispersed 
measurement, 
(d ) E l t i d f.  nve ope re a ne  or 
comparison
8
Preliminary Observations From Test
• The measurement from the case with no cables was processed by repeating the             
same steps.  The figure below provides a comparison of the smoothed envelope 
for this pair of measurements.  
• Not all measurement locations will be as sensitive to proximity to cable 
harnesses, but this reduction may be appropriate for light weight equipment that 
may stretch across panels that are not heavily mass attenuated. 
• The damping attenuation provided by cable harnesses may prove to be most 
effective in the zones which are large responders       .
9
Use of Finite Elements with Modal Updating 
for Damping
• Analysts seek agreement between analysis and test to:
• Validate models
• Improve predictive capability in untested configurations
• Validate new analysis methods or tools
• Typically, agreement is sought through model updating
• Without knowledge of test damping, damping becomes a “knob” in the model 
updating process 
Wh d i l i ll• en up at ng, ana ysts typ ca y:
1. Assume flat damping across spectrum 
• Easiest, but can yield poor agreement     
2. Estimate modal damping using half power points
• Better agreement, but can be time consuming and tedious
• Intractable for systems with high modal density
D t l h tit ti f t
10
• o no  a ways ave quan a ve measures o  agreemen
3. The Subjective Eyeball Method
Updating for Damping  with use Of DampID
Optimizer Tool
• New MATLAB-based software tool developed at MSFC 
– Employs MATLAB optimization toolbox
• Systematically arrives at a damping schedule such that the difference between 
FEM response and test data is minimized
• Req ired ser inp tsu  u  u :
• Frequency response data at each test channel
• FEM nodes that correspond to each test channel
• FEM natural frequencies and modes    
• Excitation Definition
• Drive point and forcing function for Base Shake
• Spatially Correlated Pressure PSD for Panels excited by Fluctuating Pressure 
such as Acoustic Field
DAMP ID eliminates need to assume flat damping or create 
complex damping schedules via trial and error
11
      
DAMP ID Features
• Key Capabilities
• Identifies damping based on multiple
User Defined Options
• Frequency range of interest
• Number of “bins” in frequency range     
channels of response data simultaneously
• Allows multiple channels of input spectra
     
• Lower and upper bounds on damping fraction
• Weight agreement at peaks more heavily
• Apply penalty to non-conservative solutions
• Filter out contributions from other bins     
• Either correlated or uncorrelated
• Accepts forcing input in form of:
• Apply linear or log scaling
• Save plots at each iteration for purposes of 
making movies
      
• Force spectral density
• Acceleration spectral density (via large 
mass method)
Statistics in Objective Function
1. Bin Peak
2. Bin Average
3. Bin Standard Deviation
• Acoustic pressure spectral density 4. Bin Variance
5. Bin RSS
6. Bin Correlation Coefficient
7. Bin Minimum
12
Methodology
• DAMP ID uses MATLAB Optimization Toolkit function FMINCON
• Based on conjugate gradient-based nonlinear constrained 
optimization technology* 
• Seeks a local minimum of a user-defined objective function
• At each iteration: 
1. Trial damping schedule used to find frequency response via 
MATLAB functions developed at MSFC
2. Objective function and its gradients calculated 
3. Calculates new trial damping schedule
• Identification process ends when objective function converges within 
prescribed tolerance
13
*  Good reference: Matlab Optimization Toolkit User Manual, Chapter 6, "Constrained 
Nonlinear Optimization Algorithms," Mathworks, 2011
Case 1
Simple Structure with Known Damping
Solution Steps
1 Model cantilevered beam with square cross-section.      
2. Develop arbitrary, but known, damping schedule and 
generate response to flat acceleration input      
spectrum (SOL 111)
• Consider this the “test” response
3. Initialize analytical response with a flat 0.01 damping
4. Observe Damp ID attempt to match analytical and 
“ ” b dj i d i h d l
14
test  response y a ust ng amp ng sc e u e
Case 1 (cont’d)
Simple Structure with Known Damping
Iteration 0
15
Case 1 (cont’d)
Simple Structure with Known Damping
Iteration 0
Iteration 55
• Excellent agreement between “test” and analysis responses
• Damp ID finds known damping schedule quite well
16
• Areas of damping discrepancy indicate response is insensitive to damping at those 
frequencies 
Case 2
Vehicle Panel Responding to Fluctuating Acoustic Pressure Excitation
Solution Steps 
1. Use model matching 3 different  Test 
configurations:
• Bare Panel
• Loaded without cables
• Loaded with cables   
1.
2. Use “test” measured response and 
optimize for 6 channels of response at 
2 locations
Observe Damp ID attempt to match 
analytical and “test” response by 
adjusting damping schedule
17
  .
Case 2a, 2b,& 2c (cont’d)
Vehicle Panel Responding to Fluctuating Acoustic Pressure Excitation
(a.) Bare Panel 
Configuration -FEM 
Result with optimized 
damping compared to 
(a.)
test measured 
response
(b.) Equipment Loaded 
(No Cables)
Configuration -FEM 
Result with optimized 
damping compared to 
test measured 
(b.)
response
(c.) Equipment Loaded 
(With Cables) 
Configuration -FEM 
Result with optimized 
damping compared to 
test measured 
response
(c.)
18
Case 2a, 2b,& 2c (cont’d)
Vehicle Panel Responding to Fluctuating Acoustic Pressure Excitation
(a ) The FEM Predicted response for three.        
configurations are compared. Bare Panel, 
Equipment Loaded (No Cables), Equipment 
Loaded (With Cables) Configurations - Using 
System Damping Schedules from Optimizer
(a.)
    
(b.) Comparison of the System Damping 
Schedules from Optimizer for the same three 
configurations.
Note that system Damping increases with 
increasing levels of integration:
• Loaded Panel greater than Bare Panel 
(correlates with presence of more bolted      
joints and friction mechanisms).
• Loaded Panel (With Cables) greater than 
Loaded Panel (No Cables) (correlates 
with presence of energy sinks and friction
(b.)
       
mechanisms).
Optimized damping from 1500-1800 Hz is 
Curious and warrants further study.
19
Conclusions and Future Work
• Three example configurations were assessed and compared. 
• Assessment of Cable Harness test results is a work in Progress.
• Attenuation due to damping was significant at certain measurement locations.
• DAMP ID  Optimization routine was demonstrated addressing a long-standing need to 
systematically update FEM Response assessments by adjusting damping.
• Computationally efficient and flexible with applications to both acoustic and base drive 
excitation.   
Cautions:
• Damping schedules are not unique.
• Damping schedules may not be consistent across all spatial locations and/or forcing 
functions.  Nonlinear with excitations amplitude.
• Care should be taken when assigning physical significance to damping schedules.
Future work:
• Use Damp ID to assist in completing the assessment of Cable Harness test cases in 
recent acoustic panel testing conducted at MSFC
• Present results in NASA report and or Technical paper.
• Report sensitivity to amount of cable harnesses
• Report Linearity/Nonlinearity of observations for different levels of excitation.
• Investigate curious results from 1500-1800 Hz.
Reference:
“
20
1Smith, A., Davis, R.B., LaVerde, R., Fulcher, C., Jones, D., Waldon, J., Craigmyle, B., Validation of Measured Damping 
Trends for Flight-Like Vehicle Panel/Equipment Including a Range of Cable Harness Assemblies”, AIAA SDM April 2012.
Correlating Attenuation of Vibroacoustic Response 
to a S stem Damping Sched le sing Gro nd Test  y   u  u  u   
Measurements, the Finite Element Method and the 
DampID Optimization Tool
Andrew M. Smith 
Vibroacoustics Specialist
Vehicle Loads and Strength Branch (EV31)
Dr. Robert Ben Davis
Acoustics and Structural Dynamics Specialist
Propulsion Structural & Dynamics Analysis Branch (ER41)
NASA Marshall Space Flight Center
      
NASA Marshall Space Flight Center
Bruce T. LaVerde,  ERC Inc
Vibroacoustics Lead Engineer
ESTS Contract Support to
Clay W. Fulcher (ER41), Douglas C. Jones (EV31) ,
James M. Waldon (EV31), & Benjamin  B. Craigmyle (ES20)
Jacobs Engineering Structural Dynamics    
Vehicle Loads and Strength Branch (EV31)
NASA Marshall Space Flight Center
 ,   
Specialists providing support  through ESTS Contract
NASA Marshall Space Flight Center
Spacecraft and Launch Vehicle Dynamic Environments Workshop
19–21 June 2012
© The Aerospace Corporation 2010
© The Aerospace Corporation 2012
Thank you 
© The Aerospace Corporation 2010
© The Aerospace Corporation 2012


Correlating Attenuation of Vibroacoustic Response to a System Damping Schedule using Ground Test Measurements, the Finite Element Method and the DampID Optimization Tool

Correlating Attenuation of Vibroacoustic Response to a System Damping Schedule using Ground Test Measurements, the Finite Element Method and the DampID Optimization Tool

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