1

Development of an Instrumentation Suite to Measure Helmet Windblast

and Impact Loading23 June 2004

John PlagaHuman Effectiveness Directorate

Air Force Research LaboratoryDoug CoppessGlenn Thomas

Greg ThompsonGeneral Dynamics

2

Background

• Windblast forces during ejection impart potentially injurious head and neck loads

• Integrated Chin/Nape Straps (ICNS) have been recently installed on USAF helmets to increase stability

• Ejections have shown potential for head/headrest impacts

• Expanded crewmember range may result in lower tolerances to injuries

2

3

Problem

• Traditional ejection test manikins have limited head sensors– Upper neck forces/moments– Triaxial accelerometer– Possibly angular (pitch) accelerometer

• Difficult to determine what exactly is contributing to the neck loading

4

Application

• Newton’s Second LawF = maΣF = maF1 + F2 + F3 + Fn = ma

• Forces include– Inertial– Aerodynamic– Reaction e.g. headrest force/impact– Neck reaction force

∑ = amF vv∑ =Γ αv

vI

3

5

Objectives

• Determine loads acting from helmet into head

– Helmet lift loads

– Chin strap tension

– Aft headrest impact

• Correlate a measurable force with neck tension as helmet is being lifted from head

• Determine if headrest impacts are injurious

6

Approach

• Measure neck loads

– Upper and lower neck forces and moments

• Measure head accelerations

– CG, Earplugs, angular pitch

• Measure other loading

– Aerodynamic

– Helmet static pressure

– Reaction (impact, e.g. headrest)

• Determine aerodynamic loading

4

7

Chin Strap Load Cell

• Small and light weight– 15 grams– Minimize inducing loads– Titanium alloy

• Shaped for minimized friction– Filleted or cylindrical edges– Free rolling center shaft

• Two pairs of strain gages– Full bridge– Greater output

• Small, durable wiring– Channels/pockets

8

Chin Strap Load Cell

5

9

Chin Strap Load Cell Loading

10

Chin Strap Calibration

Chin Strap SN#6 (5-06-2004)

0

5

10

15

0 50 100 150 200 250

Pounds

Mv'

s

6

11

Skull Cap Load Cell

12

Skull Cap Load Cell

• CRABI Load Cell, Denton 2254 (FTSS-IF-954)

• Rotated so that Fx load is lift load and Fz is horizontal compression load

• Capacity

– Fx & Fy – 200 lbs

– Fz – 500 lbs

– Mx & My – 500 in-lb

– Mz – 300 in-lb

7

13

Skull Cap Load Cell

14

Modified Head

• Existing cap – 1.56 lbs

• Modified head

– New Cap – 0.63 lbs

– Load cell – 0.31 lbs

– Adapter Plate – 0.50 lbs

• Head ballasted to match 95th percentile head weight and CG

– 10.49 lbs (10.55” target)

– CGx = 0.25” (met target)

– CGx = 1.79” (1.77” target)

8

15

Testing Using Instrumentation

• Actuator is an MTS hydraulic system that pulls the head downward out of helmet • Load cells in manikin neck (internal) and top of bracket (not shown)

• Loading rate of approximately 33,400 N/s (7,500 lb/s)

• Test ends when MTS load reaches 4500 N (1,000 lbs) or strap failure

16

ICNS Dynamic Test

• Test terminated @ 4500 N

• 3 Test Configurations:

• No Mask

• MBU-12/P Mask

• MBU-20/P Mask

9

17

MTS Test Data

0 100 200 300 400 500Time (msec)

0100200300400500600700800900

1000Fo

rce

(lbs)

INT NECK Z FORCE (LB)INT SKULLCAP Z FORCE (LB)RIGHT CHIN STRAP FORCE (LB)

18

MTS Test Data

0 100 200 300 400 500Time (msec)

-100

0

100

200

300

400

500

Forc

e (lb

s) o

r Tor

que

(in-lb

s) INT SKULLCAP X FORCE (LB)INT SKULLCAP Y FORCE (LB)INT SKULLCAP Z FORCE (LB)INT SKULLCAP Mx TORQUE (IN-LB)INT SKULLCAP My TORQUE (IN-LB)INT SKULLCAP Mz TORQUE (IN-LB)

10

19

Results

Helmet Pull Tests

0.10 0.09

0.17

0.09

0.00

0.05

0.10

0.15

0.20

SeparateChin/Nape, no

mask

ICNS, no mask ICNS, MBU-12/P ICNS, MBU-20/P

Configuration

Chi

n St

rap/

Nec

k Z

Average peak neck tensile load

Linear relationship between neck loadand chin strap load

Chin FzLbs Lbs

SCNS, no mask 99 602ICNS, no mask 76 817ICNS, MBU-12/P 72 743ICNS, MBU-20/P 74 832

20

Results

Average peak neck tensile load

Linear relationship between neck loadand chin strap load

Helmet Pull Tests

0.320.300.35

0.00

0.10

0.20

0.30

0.40

ICNS, no mask ICNS, MBU-12/P ICNS, MBU-20/PConfiguration

Skul

l Cap

FZ/

Nec

k Z

Neck Fz Skull FzLbs Lbs

ICNS, no mask 817 285ICNS, MBU-12/P 743 226ICNS, MBU-20/P 832 267

11

21

Comparison with Rocket Sled Ejection Test Data (ICNS)

Rocket Sled Ejection Tests

0.000.050.100.150.200.250.300.35

430

445

470

515

595

599

600

Averag

e

Velocity (KEAS)

Chi

n S

trap/

Nec

k Z

0.24

100

130

390200

350470 360 286

Peak neck tensile forces (lbs)

22

Ejection Test Differences

• Chin strap/neck tensile force

– MTS: 0.10

– Ejection: 0.24

• Not pure axial loading

• Aerodynamics

– Stagnation pressure

– Force on strap/load cell

12

23

Head/Neck Sensors for Follow-up Rocket Sled Testing

HEAD ANGULARACCELEROMETER

0/0 Ejection Seat Test

13

25

0/0 Ejection Seat Test

8.3 8.4 8.5 8.6 8.7 8.8 8.9 9 9.1 9.2 9.3 9.4-200-150-100

-500

50100150200250300350

Skull Cap FXSkull Cap FYSkull Cap FZ

8.3 8.4 8.5 8.6 8.7 8.8 8.9 9 9.1 9.2 9.3 9.4-2500

-2000

-1500

-1000

-500

0

500

1000

1500

2000

Neck Force XNeck Force YNeck Force Z

26

Other Uses – Skull Cap

• Measure impacts to the back of the head• Commercial and industrial helmet systems

– Motorsports– Motorcycle– Bicycle– Hardhats

• Evaluation of the crashworthiness of vehicles– Rearward impacts– Rollover

• Falls from objects such as ladders• The data can be used to assess the probability of

injury

14

27

Results - Skull Cap Load Cell

• Testing "altered" the sensor

– Five of the six channels were out of zero spec.

– The metal wasn't located in the same geometry it was located when the gauges were initially applied

– Recommend at least a three-fold increase in range

28

Results – Chin Strap Load Cell

• Effective in helmet pull and ejection seat tests

• Intermittent dropouts during windblast tests

– Enamelized wires likely contacting conductive surface

• Newest sensors use Teflon® insulation

• Use of “pockets” around stain gages

– Better protection of strain gages

– Possibly better concentrate strain

15

29

Conclusions

• Both the skull cap load cell and chin strap load cell have been used in several test programs

• A few problems occurred that can be corrected in the future

– Higher range skull cap load cell

– Teflon® insulated wires on chin strap load cell

– “Pockets” cut in chin strap load cell

• Use of this instrumentation suite can yield valuable data in the area of reducing neck injuries during ejection

– Can also be useful for other commercial applications