Component Cushion Test Development

1
Component Cushion Test Development

• Presented at the Fourth International Fire and
  Cabin Safety Research Conference
• November 18, 2004
• Lisbon, Portugal
• Steve Hooper, PhD
• and
• Marilyn Henderson
• J.B. Dwerlkotte Assoc., Inc.
• Wichita, KS
• USA
• 316-269-6970, ext 14
• Email shooper474_at_earthlink.net

2
Outline

• Motivation to develop component test
• Review dynamic seat certification requirements
• Background
• Review foam material properties
• Describe a dynamic component test method
• Material test results
• Correlate with dynamic sled (seat) test results
• Present proposed component test method
• Summary and conclusions

3
Motivation to Develop Component Test

• Seat Cushions are a significant maintenance item
• Typical life 3-5 years
• Cushions identical to the original cushions not
  always available
• Full-scale seat tests are destructive
• Airlines are not willing to obtain / destroy
  seats for full-scale tests
• Bottom cushion design strongly influences lumbar
  load
• During 14-G download tests
• Component tests are useful in validating dynamic
  simulations of seats / restraints / and occupants

4
Download Test Description

• Test Conditions
• 14g triangular pulse, tr 0.08s (peak g), DV
  35 fps
• Applied 90 deg to flight path vector
• Pass / Fail Criteria
• Lumbar Load 1500 lb.

Amdt. 25-64, 53 FR 17646, May 17, 1988
5
Background Foam Material Properties

• Gibson and Ashby describe foam compression as
• Three significant regions
• Two significant material properties eD and spl
• Some matls exhibit a work hardening response
• As shown on the right

6
Background Foam Material Properties

• Upper R.H. portion of curve is an artifact of the
  test
• Not a Material Property

7
Background IFD Test

• ASTM D3574 Indentation Force Deflection (IFD)
  Test
• Measure Resistive Force at 25 and 65 deflection
  of cushion thickness
• 7 ½-in (190.5-mm) Diameter Specimen
• Static loading
• No unloading measurements

8
Background Other Efforts

• Lim method developed during the FAA / NASA AGATE1
  Program
• Addressed rate effects
• Hooper and Henderson developed dynamic component
  test
• Included diaphragm in the test article definition
• Lims method not documented in the public
  literature
• Not validated as a robust method
• Hooper and Hendersons method
• Too expensive (diaphragm issue)
• These investigators identified the significance
  of the densification strain on lumbar load during
  dynamic seat tests

9
Description of Dynamic IFD Test

• Dynamic test based on ASTM D 3574-03 (IFD Test)
• Utilize high-rate servo hydraulic test stand
• 220 kip load frame
• 110 kip actuator
• 10 kip piezoresistive load cell
• MTS Testar-IIm Controller
• MTS Multi-Purpose Testware (MPT) software
• Sampling rate 12,288 samples/sec

10
Fixture Design
Dimensions in inches
11
Test Stand Performance
Position
Velocity
Unfiltered Data
12
Test Description

• Test articles fabricated and supplied by three
  aircraft seat cushion suppliers
• Monolithic (nonflotation) cushions
• 4 polymers
• 3 densities (3.1 4.4 lb/ft3)
• 3 thicknesses
• Laminated (flotation) cushions
• 3 laminates
• 2 polymers comfort foam
• 3 polymers flotation foam
• 3 densities (comfort foam)
• Same matl as used in monolithic specimens
• Flotation foam thickness established to satisfy
  TSO-C72c
• 3 densities (1.4 2.6 lb/ft3)
• 3 thicknesses
• The entire test matrix was not tested

13
Test Results Monolithic Materials
14
Test Results Laminated Materials
15
Stress- Strain Curve - Monolithic Foam
16
Stress-Strain (cont.)

• Shifting the unloading curves to a common stress
  value produces a common stress-strain curve

17
14-g Sled Tests

• 14-g sled tests were conducted of a limited
  number of cushions

18
Correlation of Sled Results w/ Matl Properties
2-in. monolithic
3.25-in. monolithic

• Matl plotted in black produced highest lumbar
  load in every test
• Blue and red curves (matls) always in same
  relative position on s-e curve and always
  produced lower lumbar loads.

19
Comparison of Static and Dynamic Test Results

• Measurable rate effects may include
• Increased plateau strength
• Reduced apparent densification strain

20
Correlation of Sled Results w/ Matl Properties
(cont.)
21
Effect of thickness on lumbar load

• Caution this trend is probably material
  specific
• Points to importance of Densification Strain

22
Correlation of Sled Results w/ Matl Properties
2-in. laminated
4-in. laminated

• Matl plotted in black produced highest lumbar
  load in every test
• Blue and red curves (matls) always in same
  relative position on s-e curve and always
  produced lower lumbar loads.

23
Physical Explanation of Results

• 14-g tests of an ATD installed in a Rigid iron
  seats with no cushion produce lumbar loads lt1000
  lb.
• The F d curve of this steel cushion lies above
  all of the foam cushion curves
• Consider the performance of a very soft cushion
  that is installed on the iron seat
• This cushion is so soft that it is completely
  consolidated under the ATDs 1-g preload
• But, the F d curve to the right of the
  consolidation strain is nearly as stiff as the F
  d curve for steel
• Therefore, the lumbar load for this cushion will
  approach 1000 lb. as well

24
Regression Analysis

• Outlying data point in second plot due to
  difference in 1-g cushion deflection under ATD
  load
• Quadratic curve selected as the Criterion Curve

25
Criterion Curve

• Definition Criterion Curve
• The load-deflection curve for a specified cushion
  thickness that produces the largest lumbar load

26
Criterion Stress Log Strain Curve
27
Proposed Component Test Method

• Perform dynamic component test of Certified
  Cushion Specimen
• Compute criterion curve for specimen thickness
• From stress log strain curve
• Compare F - d data for certified cushion with
  criterion curve
• If above, then show replacement cushion in Usable
  Region

Monolithic or Laminated Cushions
28
Proposed Component Test Method (cont.)

• If below, then show replacement cushion in Usable
  Region

Monolithic Cushions Only
29
Evaluation Region

• The Usable Region is determined by analyzing the
  position of the F - d curves in the Evaluation
  Region

30
Ineligible Material
31
Static Requirement

• Replacement cushion specimen displacement under
  130-lb. Load must be equal to, or greater than,
  the corresponding displacement of the original
  certified cushion

32
Acknowledgements

• This research was funded by the FAA William J.
  Hughes Technical Center by a subcontract through
  the National Institute for Aviation Research at
  Wichita State University.
• Mr. Timothy G. Smith, FAA COTR
• Dr. John Tomblin, WSU P.I.
• The authors would like to acknowledge the
  contributions of
• Ms. Lamia Salah and Wadii Benjilany of Wichita
  State
• Mr. Rick DeWeese and David Moorcroft of FAA CAMI
• Mr. Mike Thompson of the FAA Transport Aircraft
  Directorate
• Mr. Matt Riggins and Mr. Habtom Gebremeskel of
  JBDA
• The late Van Gowdy, who contributed to the design
  of this research.

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Summary Conclusions

• High-rate material tests were performed of
  typical flotation and nonflotation seat cushions
• Seat cushions exhibit a measurable rate
  sensitivity
• The material properties from these tests were
  qualitatively correlated with lumbar loads
  measured during dynamic seat tests
• These results point to the existence of a F - d
  curve that maximizes the lumbar load for a
  specified cushion thickness
• These results show that a replacement cushion can
  be designed that will produce a lumbar load that
  is equal to, or less than, the lumbar load
  produced by an original certified seat cushion
• The replacement cushion does not have to exhibit
  properties that are identical to the original
  certified cushion