BLAST MITIGATION SEAT ANALYSIS – DROP TOWER DATA

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BLAST MITIGATION SEAT ANALYSIS –
DROP TOWER DATA REVIEW
Kelly Bosch, PE; Katrina Harris; David Clark, PE; Risa Scherer; Joseph Melotik
TARDEC Ground Systems Survivability
Disclaimer: Reference herein to any specific commercial company, product, process, or service by trade name, trademark,
manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United
States Government or the Department of the Army (DoA). The opinions of the authors expressed herein do not necessarily state or
reflect those of the United States Government or the DoA, and shall not be used for advertising or product endorsement purposes.
14 August 2014
UNCLASSIFIED: Distribution Statement
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A. Approved for public release.
Testing Background
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Baseline drop tower data collected from Anthropomorphic Test Devices (ATDs) seated in 12
models of Commercial Off-The-Shelf (COTS) and prototype blast energy-attenuating (EA)
seats in various phases of engineering design development
ATD data quality-checked and preliminary comparisons conducted (~7000 channels
reviewed)
Testing completed with:
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5th percentile Female, 50th percentile Male, 95th percentile Male HIII ATDs
With or without personal protective equipment (PPE)
200 g or 350 g pulse
ATD injury assessment values compared to Occupant Centric
Protection (OCP) and Army Research Lab / Survivability / Lethality /
Analysis Directorate (ARL/SLAD) Injury Assessment Reference Values (IARVs)
Current ATD analysis includes:
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Accelerations
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Head (Resultant, HIC15, HIC36)
Chest (Resultant)
Pelvis (DRI)
Forces/Moments
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Upper Neck
Lumbar
Femur
Upper Tibia
Lower Tibia
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Testing Background
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Drop tower located at TARDEC Occupant Protection (OP) Laboratory
Testing simulated the initial vertical loading event during an
underbody blast
Pulse profile variables include:
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Pulse profile tuning is achieved by changing:
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Maximum acceleration
Time to peak
Delta velocity
Drop height
Platform payload
Energy absorbing medium
Test matrix designed to maximize information gained
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Preliminary Conclusions
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The test methodology prescribed for the evaluation of blast mitigation seats
using a drop tower with the 5th Female, 50th Male, and 95th Male at 200 g and
350 g while complying with the OCP and ARL IARV values has been
demonstrated
Compliance with the IARV criteria is achievable by some of the current
production and development seats
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Most of these seats require a specific EA device for occupant weight or impulse
loading, multiple EA mechanism changes (e.g. after two drops), or the seat is a
specific system for a specific impulse.
Two of these seats were in development to mitigate multiple impulses and provide
protection for the 5th, 50th, and 95th Hybrid III ATDs without seat EA modifications
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Data Caveats
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• Caution should be used in directly comparing test results to each other based on
differences in:
• Test setup (ATD positioning, reusing seats)
• Energy absorption devices
• Suitability of each seat based on occupant size and impulse
• Seats were reused in multiple tests, so some seats experienced issues that may
have affected results
• Energy absorption malfunctions
• Deformation to seat frames
Did not
stroke
• Limited data sets pose
challenges in drawing concrete
conclusions such as the effects of PPE
• Lab HVAC temperature control difficulties
may introduce variability in data
• Impact velocity not recorded
• Rebound of platform resulted in higher
delta velocity than impact velocity
• All caveats have not yet been identified
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Lumbar FZ Compression
Normalized
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•113 drop tower tests
performed from July 2012 to
July 2013
•Evaluated against OCP and
ARL/SLAD IARVs to
determine if methodology is
correct
•OCP IARV limits are
attainable with respect to
lumbar compression (see
brown bar chart below)
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50th Male - Lumbar FZ
Compression Normalized
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•56% pass rate for all
tests at OCP limit
•72% pass rate at ARL
limit
•Highest test result over
OCP threshold is 175%
the limit
Data Takeaways:
•ATD lumbar
compression response
seems to be the go/no-go
injury criteria for seat
performance assessment
•Most seats are designed
for the 50th percentile
male
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95th Male - Lumbar FZ
Compression Normalized
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•91% pass rate for
lumbar compression
across all tests
•Worst test result for
lumbar -Fz is 110% of
IARV limit
Data Takeaways:
•95th male at 200 g is
easiest configuration
to pass for lumbar
compression
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5th Female - Lumbar Fz
Compression Normalized
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•17% pass rate for
combined testing
•8% pass rate for at 350 g
•Tests above IARV limit
produce lumbar
compression values up to
400% the limit
Data Takeaways:
•ATD lumbar compression
response seems to be the
go/no-go injury criteria for
seat performance
assessment
•Most difficult to pass: 5th
female – 350 g
• Lighter ATD may not
allow seat to fully
stroke
• Female has lower
IARV limits than males
• 80% pass rate for
lumbar -Fz against the
95th male limit
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5th Female - Lumbar Fz
Compression Normalized
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•Some testing conducted
with and without PPE on
same seat models with
same configurations
•Purpose was to
determine if extra weight
(for an encumbered 95th)
would cause the seat to
bottom out or less weight
(unencumbered 5th) would
prevent the seat from
stroking
•Data displayed is from 5th
Female testing at 200 g
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Data Takeaways:
•Encumbrance level does
not seem to have a major
effect on injury levels
•Due to limited matched
pair testing, it was difficult
to complete comparative
analyses between ATDs
with and without PPE
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5th Female – Upper Neck Fz
Compression Normalized
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•Some IARVs included
associated dwell curves
•Time history data can flag
issues in dwell curves that
does not register on
maximum/minimum peak
analyses
•Upper neck compression
in 5th female may have
been exceeded due to:
•High lumbar
loads/force
transmission
•Weight of helmet
•Artificially low IARV
limits
Passed peak limit but
exceeded lower threshold over
the allowable dwell time
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IARV Pass Rate – Upper Body
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•Pass/fail chart provides
quick reference for all
IARV performance across
a seat or ability to pass an
IARV across all seats
•Green indicates all tests
passed an IARV in that
configuration
•Red (pink) indicates all
tests failed an IARV in that
configuration
•Yellow means at least
one test passed
•Results for PPE and no
PPE were combined
Data Takeaways:
•Lumbar compression (and DRIz [Dynamic
Response Index] calculated from pelvis Az) is most
sensitive parameter for upper body
•Upper neck and head are least likely to exceed
IARVs – exception is 5th female neck compression
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IARV Pass Rate – Lower Body
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•Seats were tested with and without flooring
solutions
•Seats B, C, and L featured footrests
•Seat K had a blast mat under the ATD’s feet
Data Takeaways:
•Lower extremity injuries are less likely with a flooring
solution
•Flooring solutions directly affect several channels
(lower/upper tibia compression, femur tension, lumbar
tension)
•Lower leg issues arise at both 200 g and 350 g
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Performance Assessment Plot
5th Female – 200 G
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Upper Body/Seat
Lower Body/Floor
•Top 8 difficult channels to
pass were normalized to
their respective IARV and
plotted against each other
•Allows quick reference to
evaluate seat/flooring
solution trends
•All IARVs normalized
•Seat performance affects
upper neck and lumbar
•Flooring solutions affect
lower extremities
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Performance Assessment Plot
5th Female – 350 G
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Upper Body/Seat
Lower Body/Floor
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General Data Observations
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General Data Observations for “2012 Seat Market Study” Drop Tower Testing conducted
at TARDEC’s OP LAB from 2012-2013:
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Lumbar Compression (-Fz)
[DRIz]
Channels to monitor (dependent on seat
design):
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Go/no-go channels:
Upper Neck –Fz (5th only)
Lumbar Shear ±Fx
Chest resultant acceleration
Channels to monitor (dependent on
flooring solution):
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Lumbar Tension +Fz
Femur Shear +Fx
Femur Tension +Fz
Femur +My
Upper Tibia Compression –Fz
Upper Tibia +Mx (5th only), -My
Lower Tibia Compression –Fz
Lower Tibia ±Mx, ±My
Channels least likely to exceed IARVs:
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Head acceleration
HIC15 and HIC36
Upper Neck ±Fx, ±Fy, +Fz, ±Mx, ±My
Lumbar ±Fy
Lumbar ±Mx, ±My
DRIx, DRIy
Femur Shear –Fx, ±Fy
Femur Compression –Fz
Femur ±Mx, -My
Upper Tibia Tension +Fz
Upper Tibia –Mx, +My
Lower Tibia Tension +Fz
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Stroke vs No Stroke (Seat F –
Lumbar Compression)
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• First 4 tests on
Seat F did not
stroke
• Test 11 was a
repeat of Test 1
with only 1 of 2
spring clips
attached
• 39% reduction in
lumbar load and
51% reduction
for pelvis
acceleration (not
shown) when
stroking seat
works correctly
39%
reduction
Data Takeaways:
•Properly stroking seats reduce lumbar load peak value
•EA mechanisms extend the “ride down”, cushioning the pelvis
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Stroke VS No Stroke (Seat F –
Lower Tibia Compression)
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• Lack of seat stroke
has small effect on
tibia compression
values (reduction
of 7% for tests 1
and 11 on 5th
Female)
• Tibia compression
occurs before seat
has an opportunity
to stroke
Minimal
Reduction
(7%)
Data Takeaway:
•Lower extremity injuries are independent of seat
performance
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Conclusions
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• The drop tower testing and data analysis provided an objective assessment of the
seats’ performance with respect to the injury criteria
• The test methodology and OCP IARV assessment criteria were evaluated and
deemed acceptable for future use
• Complications with test setups led to a list of caveats for this data analysis
• Caution should be used in directly comparing test results to each other based on
differences in test setup, energy absorption devices, and suitability of each seat
based on occupant size and impulse
• General data trends and observations were compiled
• Lumbar compression seems to be the go/no-go injury criteria
• Flooring solutions result in less lower extremity issues
• Head/upper neck IARVs are least likely to be exceeded except for the 5th female
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Future Work/Next Steps
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• Drop tower data should be compared to live fire data to identify similarities and
differences in ATD and seat response
• Further analysis of this data with respect to seat construction may allow an
evaluation of seat characteristics to create an optimum seat design
• This information may be used to aid in the development and selection of flooring
solutions
• Use lessons learned from data analysis to improve lab procedures and best practices
• A further evaluation of the OCP criteria may be needed to verify that the 5th percentile
female and 95th percentile male IARVs were appropriate
• Drop tower is currently being moved – lessons learned will be incorporated
• Future test plans can be developed to evaluate seats efficiently
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