1997 spine injury criteria for military seats

8/3/2019 1997 Spine Injury Criteria for Military Seats

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ESTABLISmNG ASPINAL INJURY CRITERION FOR MILITARY SEATS

Martin Rapaport, Estrella Forster and Ann SchoenbeckNaval Air Warfare Center

Aircraft DivisionPatuxent River, MD

LeonDomzalskiARCCA, Inc.

ABSTRACTPeons Park, PA

INTRODUCTIONCurrently, military specifications require use of anacceleration-time criterion to assess the physiologicalacceptabilityof crash resistant seats and restraint systems.With the evolution of crash test dummies which nowexhibit greaterbio-fidelic performance,military researchersare considering "direct force" measurements taken withinthe Hybrid III anthropomorphic test device (ATD) as analternate evaluation criterion. The Federal AviationAdministration(FAA) recently established such a criterionas part of a new dynamic test requirement for thecertificationof airline passenger seats.I Prior to militarytri-service implementation of a similar requirement,validation of the methodolgy with the "aerospace" modelof the Hybrid III manikin was necessary. This paperdescribesthe test program cOl1ductedwith Hybrid III crashtest dummies to establish a lumbar spinal load injurycriterion for military crash resistant seat compliancetesting. The effort was sponsored under the Naval AirSystems Command's (PMA-202) Advanced CrashworthyAircrewSurvivalSystems(ACASS)Program.Dynamic testing was. conducted on the HorizontalAccelerator Facility at the Naval Air Warfare Center,Aircraft Division, Warminster, PA. The test programexamined the lumbar spinal response of Hybrid III testdummies when ex.posed to predominantly verticalhelicopter crash pulses under controJled laboratoryconditions. Sufficient data was acquired to conduct astatistical analysis of the Hybrid Ill's lumbar spineresponse to compressive loading (Le., + Gz ~eadwarddirecLion). The analysis supports the recommendationtoemploy lumbar force as a primary physiological criterionfor military crash resistant seat compliance. Also, themerits of using this injury indicator during escape systemtestingarediscussed. .

Military specifications do not state perfonnaneerequirements in tenns of specific injury mechanisms orquantifiableinjurythreshold lcvc:ls.Typically, perfonnancerequirements for crash resistant seats and restraints are"indirectly" validated against potential injury dUriri~qualification testing. For example, MIL-STD-1290stipulates an impact velocity change parameter for theaircraft's crash survivability requirement which is basedonmishap data from operational aircraft accidents and crashtesting of specific aircraft models. Crash resistant systemsproposed by industry must meet structural strengthrequirements, and an occupant survivability criterion basedupon the Eiband Curv~3 when tested to the standard'sgeneric crash pulse. The Eiband Curve criterion essentiallydefines a maximum acceleration-time profile for Lheheadward (+ Gz) direction to which the scat/occupantresponse must comply (Figure t). However, since injurymechanisms are intrinsically related Lothe applied forcesustained by the body member, military crash safetyresearchers are now investigating the feasibility ofemploying a "direct" parameter. In the case of vertebralfractures, a "spinal force" parameter is proposed. Althoughthis investigation focused on lumbar forces to establish analtemative criterion to the Eiband Curve, the process canbe applied to other injury mechanisms during thedevelopment/qualificationphase of crash resistant systems.As part of the ACASS program, test methodolgy changesare under consideration for various injury mechanisms forthe head, brain, cervical neck, face, upper torso/thorax,lower limbs, and lumbar spine. Reference 4 reviewsinjury criteria which provide the basis for advancedmanikin use in the development of automotive restrainttechnology.

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Establishing a Spinal Injury Criterion for Military Seats

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Naval Air Warfare Center,Aircraft Division,Patuxent River,MD,20670

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..thespineof anAID couldbemeasuredithoutadverselyaffecting the flexural characteristics of the lumbar spine.The FAA's newly adopted seat dynamic perfonnancestandard which incorporates a lumbar spinal load injurycriterion is described in Reference I I. A maximwlIcompressive load of 1,500 pounds between the pelvis andthe lumbar spine of the Part 5728, 50th percentile maleATO has been established for the crash environment inwhich the predominant impact load component is directedalong the spinal column of the occupant. Figure 2 depictsthe installation of the pelvic lumbar spine load cell in a Part572B anthropomorphic dummy associated with the FAA'sstandard.

I,!I!,III

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Figure2: Part 572 Pelvic Segment with Lumbar SpineandLoad Cell AssemblyA key element in the application of injury threshold levelsto the evaluationof a system's cra.


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Test ObiectivesNote that a direct correlation between the Hybrid Ill'simpact performance and human injury tolerance was notconsidered within the scope of this program. Test programobjectives were intended to support the establishment of anew Navy injury cri terion based on direct measurements oflumbar spinal forces/moments taken during dynamic crashtests of candidate crash resistant systems. Specifical ly, thefollowing test objectives were established:(a) To quantify the effects of various load paths within

the Hybrid Ill 's lumbar spine construction.(b) To determine the effects of the following variables:- Hybrid III size/weight;

- Upper torso stiffness;- Crash pulse parameters such as:Peak accelemtion;Pulse velocity-change.

The influence of these variables on the response of theHybrid III's lumbar spine to typical crash impulses wasstudied by an analysis of variance (ANOV A) statisticalmethod.

EXPERIMENTAL PROCEDURETest FacilityThe dynamic tests were conducted at the Naval AirWarfare Center Aircraft Division Horizontal AcceleratorFacility previously located in Warrninster, PA. TheHorizontalAccelerator simulates typical decelerativecrashforces associated with vehicle mishaps by reversing theorientation of the test article and acceleratingthe systemfrom an initial velocity of zero. It consists of three mainassemblies:(1) acceleratingmechanism, (2) test sled, and(3) a set of guide rails, 100 feet long. The acceleratingmechanismis a 12-inchHYGE actuator which generatesamaximumforce of225,OOOpounds of grossthrust.

Test ArticlesTest Seat I Restraint SystemThe test article consisted of a rigid seat structureconfigured with a standard Mil-S-58095 five-point restraintsystem. The Mil-S-58095 restraint's inertia reel wasreplaced with an adjustable fixed anchor fitting to precludethe possibility of inertia reel failures during this study.Since the restraint system would experience minimal crashloads under the predominant vertical impact vector duringthis test series, replacement of test restraints was madesparingly.The generic seat system (non-energy absorbing) Wasmountedto the sled platfonn in an orientation to simulate

the vertical impact mode. The test seat was rotated 90degrees to the vertical axis to align the manikin's spineparallel to the input crash pulse. Standard operational seatcushions were not used to preclude any adverseanlplification factor in the controlled testing.Figure 3 shows a post-test side view of the 95th percentileATD seated in the test seat as located within the vertica]orientation test fixture.

Figure 3: PosHest SideView of 95th PercentileATDShowing Vertical-OrientationTest Fixture:Run No. 95030

Anthropomorphic Test Devices (ATO)Hybrid 1lI male ATDs (5th, 50th and 95th percentiles)instrumentedwith lumbar load cellswere seatedwithin thetest seat. The Hybrid III lumbar spine is a polyacrylatcelastomer member with molded-in end plates forattachment between the thoracic spine and pelvis. Acurved lumbar spine is incorporated in the automotiveconfiguration to replicate the typical automotive seateddriving position. However, the "erect" (i.e.. straight)lumbar spine and the articulated hip assembly of the"aerospace model" were selected for this test series topermit axial loading of the spinal column. In addition tothese designmodifications, structural enhancementsto theshoulder-clavicular assembly have been incorporated intothe aerospaceconfigurations.Prior to the test phase of this program, each of the ATDswere disassembled to identify alternate load paths presentwithin the upper torso. The abdominal insert assembliespresented a secondary load path for reacting a portion ofupper torso mass during compressive loading of the spinalcolumn. Each oCthe ATD's upper torso components werescale-weighed. In addition, the test dummies wereelectronically weighed "with and without" the abdominalinserts positioned to determine the 1g effect of the alternateload path. Table II lists the component weights, totalweights, and delta measurements attributed to the insertsfor each of the test dummies.

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TEST CONDITIONSThecrashpulse:parametersshuwn below (Table I) definethe impact profi les which were employed during the testprogram. The pulse shape selected was essentiallya "half-sine" wavefonn, with peak acceleration and velocity-change the critical parameters defining the severity of thecrash pulse:. Pulse sevt:ri ty was based on the premise thatthe analysis would be conducted on a "fixed" scat structureand, therefore, would not provide any energyabsorption/load- limiting effect to the seated occupant .TABLE I: SUMMARY OF IMPACT PARAMETERS

Figure 4 shows sample overlay plots of the test facility'scrash pulse waveforms selected foi this test program.Representative. pulse signatures for the three peakaccelerations at the 25 fps velocity change level are given.

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A preliminary assessment of lumbar load test datameasureduringearlierNavytestprogramshadindicatedthat the Hybrid Ill's lumbar spinal load cell did notmeasurethe total fQrcetheoretically generatedby the uppertorso mass during crash impact testing. The influence ofthe abdominal insert was investigated relative to thisapparent anomaly.Table II shows the delta readings between the normal scalemeasurementsand a static 1 g measurement of the uppertorso as measuredby the lumbar load cell with andwithoutthe abdominal inserts positioned. The influence of theabdominal insert is evident from the I g rotationweighings resulting in weight deltas ranging from aminimumof 6.8 pounds for the 5th percentilemanikin to amaximimof 26.9 pounds for the 95th percentile manikin.The higher mass readings recorded with the abdominalinserts removed support the assertion that the insertsproduce an alternate load path to compressive/axial spinalloading.Also of significancewas the finding that the upper torsostructure of the 95th percentile ATO did not exhibit aproportional increase of mass as compared to thedifferential between the 5th versus 50th percentiles.Subsequentload cell measurements of the 95th-percentileATD with the insert installedreduced the "effective" uppertorsomass to 69.3 pounds.This result coupled with what appears to be nonlinearscaling of the 95th percentile's upper torso masspresenteda set of test dummies which skewed the test results. Itshould be noted that at this time, essentially noconfiguration control of the aerospace model Hybrid 1IIATD exists. Caution shouldbe exercised when comparingtest data from various facilities, or even within the sametest organization,using this particular configuration of thebasicHybrid IIIdesign.

TABLE II: Hybrid III Aerospace ATDWeights

Upper Torso Mass consisted offollowinl'. components:- Head/neck assembly-Upper torso assembly minus foreanns. Instrumentat ion (t ri-ax accelerometers in head & thorax; loadcells in lower neck: & lumbar spine)

. Skin & abdominal insertsObservatiQos

Figure 6 displays the characteristic response of the HybridIll' s lumbar spinal column when subjected to a typicalhalf-sine crashpulse as produced by this test facility. Thephase lags associated between the thorax and pelvisaccelerations typically recorded during this test series arealso evident.

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TABLE III: Test Results; Means (Phases I & II )DeltaPki 5th 5th 5th5Qth:::~Qd,,~Oth 95th 95th 95th&;~)~:dP~;. ~~r. L~~.;~~;. .~~'~f' P~~.T~~r.L~:.

.(O's) eG's) (G's) (Jbs)q~~) t(:t~) (II's): (g's) (G's) (lbs)16.9 1056 :


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Program14 currently investigating the possiblity ofproducing a load-limiter which optimizes perfonnancerelat ive to lumbar load response as opposed to the existingcriterion of seat/pelvic acceleration (Le., Eiband Curve).

Lumbar .Tal...nc. CU.veun2800 "'''''''''''''''''''''''''' . ............ . , ..' ... ... .

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Figure 10: Proposed Lumbar Load Tolerance CurveThe parametersof the sled's impact pulse (Table I) wereselected for comparabilityto known load-limiting (energyabsorbing) seat design responses.IS This approach wasfollowed because it was not practical to employ a load-limiting device for each test condition. As a consequenceof this limitation, lumbar forces measured within theHybrid Ills actually reached levels considered beyondvertebral fracture levels at the more severe impact levelsstudied. The related pelvic and thoracic accelerationsmeasuredat the more severe pulses are consistent with the"high" lumbarforce levels.Tolerance levels for the three manikins employed duringthis test series were 1940 pounds (5th AID = 160 pounds),2170 pounds (50th ATD = 180 pounds), and 2500 pounds(95th ATD ""210 pounds), respectively. Clearly, even thethe 5thpercentilemaleAID would not have compliedwiththe tolerance criterion under the higher impulse severitylevels (i.e., 20 and 30 Gpk). Similarly, the 50th percentileATO would not have met the criterion at the upper impactseverity levels without some means of load limiting. Theabove results are explained by the selection of test impactparameters and are essentially in agreement with theperformance of standard non-energy absorbing seatdesigns.However,the response of the 95th percentileATDpresentedmisleading results, indicating compliance at theless severe impactseverities. As previously indicated,thisis directly related to the nonproportional mass of the 95thaerospaceATO's uppertorso.The rationaleto support use of a lumbar force criterion forejection seats is based on the similarity of lumbar spinalinjury mechanisms associated with the two acceleration-time profiles. The inertial loading vector to the aviator'sspine is essentially similar whether ejected under

accelerativeforcesor decelerated to zero'velocity in avertical crash scenario. Historically, crash resistant seatshave beendesigned to essentiallythe same injury tolerancecriteria applied to ejection seats (i.e.. Eiband Curve andDRI). Since lumbar injuries are directly related to theultimate strength of spinal vertebrae, a lumbar forcecriterionappears valid for both seat applications.

CONCLUSIONS

The analysis performed on the test data supports thefollowing conclusions:1. The Hybrid III anthropometric test device(ATD) inthe 5th and 50th percentilesizes can be usedas atest surrogate during "vertically-oriented" (i.e., alongspinal axis of occupant) impact tests. This conclusion isrelated to the "straight" spinal segment of the Hybrid IIIATDaerospaceconfiguration.2. The 95th-percentile ATD selected for thisinvestigation exhibited a nonproportional upper torsoweight distribution which resulted in lumbar load responselevelsbelow those of the 50th-percentilemanikin. The 5thpercenlilemale ATD's response appeared commensuratelybelow that of the 50th percentilemaleATD.3. Lumbar force provides a suitable spinal injurymeasurement within the Hybrid III ATD for comparativeanalysis of manikin response during evaluation of militarycrash resistant seat systems and ejection seat systems.Lumbarforce can serve as an additional injury parameterinthe assessment of spinal injury prediction, sUPPQrtingthecurrent indicators, such as the Eiband Curve and theDynamic Response Index (ORI). Assessment of lumbarforce data in this test series would have indicated thepotential for spinal injury had the test seat employed beenunder evaluation. However, as noted this result is related'to the expedient use of a nonstroking seat mechanism in asevere crash impact environment. Current crash resistantseat designs employ load-limiting mechanisms to managethe seat occupant 's response to vertical impacts at theseverity levelsemployed.4. The abdominal inserts provide an alternate loadpath to the spinal column of the Hybrid III ATD and canattenuate the level of lumbar force measured by the lumbarload cell. The etTect of the abdominal insert wa.~shown tobe statistically significant, but no attempt was made toquanti ty the unloading effect relat ive to manikin size. Theanalysis identified the existence of this alternate load pathin each of the three manikins tested. Allhough these datashow that the abdominal inserts atTected lumbar loadresponse, calibration test methods could be employed toquantify the effect, thus permit ting the usc of lumbar forceas an effective comparative criterion for assessing thepot enti al for spi nal in jury.

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5. All threeindependentvariableshada significanteffecton lumbarload. Lumbarloadsas measured withinthe Hybrid III ATD were responsive to the peakacceleration level and velocity change parameters of thecrash pulse, and manikin size. The analysis of variancedetennined that a three-way interaction existed betweenmanikin size, velocity change, and sled peak acceleration(F= 6.27, P= .000).6. The etTect of upper torso restraint was studiedand found to have a statistically significant effect onlumbar load. However, the effect was minimal for the 5thand 50th percentile ATDs, i .e., approximately, a 100 pounddifference between the mean levels.

RECOMMENDATIONS

I. Lumbar forcemeasurements are recommendedasa spinalinjury assessmentcriterion,both for crash resistantseatsandejectionseats.2. A configuration study should be conducted todetermine the variance of the 95th percentile aerospaceATD's upper torso mass within the Navy's inventory oftest dummies.3. To support use of a lumbar force criterion,additional calibration testing of both the "aerospace" and"automotive" versions of Hybrid III ATDs should beconductedto assess the following factors:

a. Responseofthe 5th percentile femaleaerospaceHybrid IIIATD.b. Effect of the "curved" automotive lumbarspinal element on lumbar force.c. Repeatabil ity (i.e. , similari ty of results of asingle ATD under identical test conditions) andreproducibility (i.e., variability between ATDs)coefficients should be established for the + Gzheadward loading direction.

REFERENCESI. Code of Federal Regulations; 14 FAR 25.562,JanuaryI, 1989.2. MIL-STD-1290A(AV), Military Standard, LightFixed And Rotary-Wing Aircraft, Crash Resistance, 26September1988.3. Eiband, A. M., "Human Tolerance to RapidlyApplied Accelerations: A Summary of the Literature,"NASA Memorandum5-I9-59E, National Aeronautics andSpaceAdministration,Washington,DC, USA,June 1959.

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4. "Human Tolerance to Impact Conditions asRelated to Motor Vehicle Design," SAE J885, JUL 86,HandbookSupplement HSJ885 JUL 1990Edition, SocietyofAutomotiveEngineers,Warrendale, PA.5. Stapp, J. P., "Human Exposure to LinearAcceleration," The forward Facing Position andDevelopment of a Crash Harness, WADC TR-5915,Wright-PattersonAir ForceBase,Ohio, 1951.6. Ewing, C. L., and Thomas, D.1., "Human HeadAnd Neck Response to Impact Acceleration," NavalAerospaceMedical ResearchLaboratory, 1972.7. Pike, Jeffrey, A., "AUTOMOTIVE SAFETY,Anatomy, Injury, Testing, Regulation," Copyright 1990SAE,Warrendale,PA.8. SAE PT-43, "BIOtv1ECHANICS of IMPACTINJURY and INJURY TOLERANCES of the I-IEAD-NECK COMPLEX," Edited by Stanley H. Backaitis,1993,society of Automotive Engineers, Warrendale, PA.9. SAE PT-186, "BIOMECHANICS andMEDICALASPECTSof LOWERUMB INJURIES,"Symposium on Biomechanics, Oct. 29-30, 1986, SAE,Warrendale , PA.10. Laananen, D. H., and Coltman, 1. W.,"Measurement of Spinal Loads in Two ModifiedAnthropomophicDummies," Simula Inc., TR-82405, FinalReport, US Anny AeromedicalResearch Laboratory, FortRucker, AL, May 1982.II. Chandler,Richard,F., General,Aviation SafetyPanel, CrashworthinessWorking Group, Federal AviationAdmistratlon, 1984.12. "Hybrid III: The First Human-Like Crash TestDummy," S~ PT-44, 1984, Society of AutomotiveEngineers,Warrendale,PA.13. Frisch, Georg, D., et aI., "Structural IntegrityTestsof a ModifiedHybridIII Manikinand SupportingInstrumentationSystem,"14. "A Third Generation Energy Absorber For CrashAttenuating Helicopter Seating," American HelicopterSociety 50thAnnual Forum, dated May 11, 1994, LanceC. Labun, Simula, Inc., Phoenix, AZ., Martin Rapaport,NavalAirWarfare Center,Warminster,PA.15. Aircraft Crash Survival Design Guide, Volume II- Aircraft Design Crash Impact Conditions and HumanTolerance, USAAVSCOM TR-89-D-22B, Dee 1989.

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BIOGRAPHIESANN SCHOENBECK Ms. Schoenbeck has beenemployed by the Naval Air Warfare Center AircraftDivision in Patuxent River, MD, since 1996. She isinvolved in the Advanced Crashworthiness AircrewSurvival Systems (ACASS) team and the Cockpit AirbagSystem program. Ms. Schoenbeck received a B.S. inMechanical Engineering from the University of Missouri -Columbia in 1992 and a M.S. in Mechanical andAerospaceEngineering from the University of Virginia in1996 with research in automotive crashworthiness fordisabled persons.

ESTRELLA FORSTER Ms. Forster is the principalinvestigatorfor the Navy Smart Aircrew Mission SupportSystem(SAMSS) and the Aircrew Integrated Life SupportSystem (AILSS) programs. Her thirteen years ofexperience include acceleration physiology, +Gz-inducedloss of consciousness (G-LOC), aircrew perfonnance,statistical applications in research, and programmanagement. This experiencewas gained in BrooksAFB,Texas (1984-1989) and the Naval Air Warfare Center.AircraftDivision(1989-present). She is currentlypursuinga Ph.D. degree at Drexel University and is a member ofvarious associations addressing aviation medicine andtechnology. Ms. Forster has a M.S. in Statistics ITomDrexel University. Philadelphia, PA, and a B.S. inPhysiology fromtheUniversity of Houston, Houston,TX.

MARTINRAPAPORT Mr. Rapaport was employed bytheNaval AirWarfareCenter Aircraft Division from 1990until 1997. He served as a crash safety engineer involvedin supplementaryrotary wingrestraint systems, such lISth~Cockpit Airbag System, and crew seating. He currentlyholds a position with Volvo Cars of North America, Inc.Mr. Rapaport received a B.S. in Physics ITom CarnegieMellon University in 1985 and a M.S. in AppliedMechanicsfromPolytechnicUniversity in 1990.

LEONDOMZALSKI Mr. Domzalski has thirty-six yearsexperiencein the field

1997 spine injury criteria for military seats

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