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Int. J. Mech. Eng. & Rob. Res. 2014 Praful R Randive et al., 2014

OPTIMIZATION SEAT OF BACK REST OF A CAR

Praful R Randive1*, Mohan D Karambe2 and S M Kamble1

*Corresponding Author: Praful R Randive, raj.dhabekar@yahoo.com

With the goal of reduced weight, free-size finite element based optimization with constraints onstresses and deflection of a car seat backrest manufactured from low strength steel plate isperformed using ANSYS software. In the free-size optimization, sheet metal thickness in a finiteelement plate of the backrest is design variables with stress and deflection limits as theconstraints, with an objective to minimize mass. Using the results from the free-size optimizationand a minimum draw-able sheet metal thickness, a final design is derived which obtains a totalmass reduction. To verify the functional performance of the final design, the final optimized seatbackrest is performed using the ANSYS software package. Results from the analysis providean accurate prediction of the material yielding and load distribution on the backrest plate providefactor of safety estimates on yield and ultimate strength.

Keywords: Finite element, Optimization, Stresses, ANSYS

INTRODUCTIONCar seat research is related to comfort qualityand mainly to human safety. Passengers arein the permanent contact with seat. Materialcharacteristics of applied material areimportant for level of comfort and safety.Pressure distribution in contact betweenhuman body and seat determine the seatingquality. Safety of sitting human in car duringcrash (front side and rear impact) is veryimportant in car. Automotive seats aregenerally constructed from metallic framescovered by foam. Cushions, backrests,

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Int. J. Mech. Eng. & Rob. Res. 2014

1 Department of Mechanical Engg, DMIETR, Wardha.2 Department of Mechanical Engg, S.S.P.P, Bhandhara.

headrests, armrests, and other parts that makeup a vehicle seat are designed according tothree principal criteria integration within thevehicle like safety, aesthetics, and comfort.The design of seat backrest, however, isprimarily based on safety requirements whichgive support to spinal cord of body.

Optimization is an important and necessarydesign tool that generally is used to design ofcomponent in the conceptual design stage.The importance of optimization in theautomotive industry is to reduce the mass andimprove safety.

Research Paper

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Int. J. Mech. Eng. & Rob. Res. 2014 Praful R Randive et al., 2014

In the present work, optimization will bedone to show the potential mass reduction inavailable sheet metal of seat backrest of car.As part of this work, a reference seat backrestwill be considered for optimization under theside and rear load on the basis of result byANSYS software. The feasibility of theoptimization and the subsequent weightreduction can be attributed to the large safetymargin, obtained from the results of finiteelement analysis of the reference seatbackrest.

Finite element analysis is accepted acrossa wide range of industries as a important toolfor product design and optimization. Whendesigning car seats, most of the variables tobe considered relate to either geometry ormaterials. A valuable tool for facilitating andshortening this complex design process isnumerical simulation using Finite ElementAnalysis (FEA). FEA can predict the responseof a particular design under specif iccircumstances and supply data that can beused to optimize geometry and materials.

OBJECTIVEThe objectives of the present work are:

• To optimize automotive seat backrestframe manufactured from low carbon ductilesteel sheets for light weight design, whileconforming to the load and strengthrequirements of ECE R-17 backrestmoment under specified constraints onstresses and displacements.

• A new range of sheet metal thickness isobtained from the results of free-sizeoptimization using CAD software.

• Using the results from the free-sizeoptimization and also by conforming to a

minimum draw-able sheet metal thickness,a final design is derived which obtain totalmass reduction.

• To verify the functional performance of thefinal design and to determine Factor ofSafety for strength, a non-linear finiteelement analysis including an rear and sideimpact of the reference seat and the finaloptimized seat backrest frame is performedusing the ANSYS package.

DESCRIPTION OF THEREFERENCE SEAT BACKRESTFRAMEA typical automotive seat is comprised of abackrest frame, a base frame, connectorcomponents connecting the two, headrests,armrests and foam components like cushionand seat back. The frame forms the skeletonof the seat which is completely wrapped bythe foam and thereafter, fabric or leathermaterials. The frame provides shape, strength,and rigidity to the seat under varied loadingconditions. The frame and foam also holdcertain control components of seatadjustments and comfort components withinthem. Seat adjustments are part of the skeletalframe with controls conveniently placed foreasy passenger access. The design of anautomotive seat is carried out according to fourimportant criteria: passenger safety, comfort,integration within the vehicle and aesthetics.The present work is concentrated toward theoptimization and analysis of the backrestframe along with the connectors. Functionalperformance of the base frame and seatadjustment mechanisms are not consideredin the present work. In National Highway TrafficSafety Administration, it is shown that similar

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overall deflection, stress levels and distributionare obtained in the backrest frame andbackrest when the connector is included in themodel. The backrest frame is constructed fromstamped low-carbon steel sheets and consistsof two vertical members and two horizontalcross members welded to the top and bottomof the vertical members.

TEST REQUIREMENTS OFUNECE REGULATION NO. 17A automotive seat for commercial usage mustconform to strength, deflection and energy testrequirements. These tests are carried outphysically for vehicle front end, rear end andside impact loads. Two of the important testrequirements are prescribed by the UnitedNations Economic Commission for Europe,Vehicle Regulations No. 17 or UNECE R-17.The regulations require the prevention of injuryand death due to structural and functionaldefects of the vehicle seat. There are two testsunder this regulation and they are the backrestmoment test, seat anchorage test. Backrestmoment test is a test of strength for the seatbackrest frame and its adjustments, Seatanchorage test is a test of strength for the seatanchorage and the adjustments, locking anddisplacement mechanisms.

Loading and Boundary ConditionsBackrest LoadsAs per the ECE R-17 backrest moment test,a moment of 530 N-m is to be appliedrearwards on the backrest about the H-point.With the absence of backrest support plate,the required moment is applied as adistributed pressure along the front surface ofthe top cross member. This is a conservativeassumption of the overall effect of the force.

The approximation is justified as the majorportion of the human load is applied throughthe upper half of the human torso in contactwith the seat backrest or at the furthest positionfrom the H-point.

The magnitude of the pressure isdetermined by the following equation, , where,is the pressure in , is the equivalent forceproducing a moment at a perpendiculardistance from the H-point, and is the surfacearea (in ) of the surface on headrest support(or top) cross member on which the pressureis applied. The distance is the perpendiculardistance measured between the center ofpressure on the applied surface and thereference seat H-point.

Boundary ConditionsFor the model with the backrest frame with theconnector, the boundary conditions are definedon the connectors, as they are the componentsin contact with the base frame, through thebackrest adjustment mechanisms. Couplingconstraints are defined on a slot on theconnector that accommodates the cross tube,that further connects it to the base frame. Thecenter node of the coupling constraint isallowed to rotate about the axis of the centerpoint, whereas other degrees of freedom arerestrained. A fixed boundary condition isdefined on the leading half of the bottom edgeof the connector, the region of the connectorthat meshes with the gear of the backrestadjustment mechanism. All degrees offreedom are restrained in that region; thisboundary condition is the replication of thenecessary restraint generated by locking theseat backrest at any particular position usingthe backrest adjustment mechanisms.

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Factor of Safety for the ReferenceSeat Backrest FrameThe factor of safety for the reference seatbackrest frame is computed. Three types offailure criteria are taken into consideration;Von Mises stress Max in-plane ultimate plasticstrain and deflection of the top most point onthe backrest frame. If any of the threeparameters in the backrest frame componentsexceeds the corresponding limiting value, it isconsidered to be failure. Failure is consideredat the point in time where one of the threecriterions is first exceeded.

SCOPE• Free-size optimization is performed on the

reference backrest frame sheet byconforming to the load and strengthrequirements of ECE R-17 backrestmoment. The backrest moment test isquasi-static strength tests that arepredicted during vehicle rear-end crash.

• This gives rise to a scope for betteroptimization of the reference backrestframe under various other load cases(offset, side and buckling loads) that arepredicted during vehicle front end and sideimpact during crash.

• Optimization encompassing multiple loadcases will be optimal to all the load casesand will be conforming to the testrequirements as well.

• The considered backrest frame isconstructed from low carbon ductile steelsheet metal components made of multiplesheets of steel with a number of stress relieffeatures. In the present work, free-sizeoptimization of the backrest frame isconsidered and a mass reduction is

obtained in the backrest frame by onlychanging the thickness of sheet metalcomponents.

• By using the suitable types of welding, thestrength of seat can be improved.

• In An evaluation properties of seat, speciallystress distribution in the contact zone,during side and rear impact very can playvery important role for the comport.

• By using FE model, it is possible to predictdynamic behavior of the automotive seatbefore the prototype is made and optimizingmechanical characteristics of used materialand optimizing comfort layer with differentlayer of seat cushion to reduce the effect ofimpact.

• After all this result, we can find the stressanalysis on backrest portion of seat in anyaccidental condition.

REFERENCES1. Andrzejak D V, Hering W E and Viano D

C (1995), “Seatback Load ApplyingDevice”, US Patent No. 5,379,646,January 10.

2. Bertholon N, Robin S, Le-Coz J-Y, PotierP, Lassau J-P and Skalli W (2000),“Human Head and Cervical SpineBehavior During Low-Speed Rear-EndImpacts: PMHS Sled Tests with a RigidSeat”, Proceedings IRCOBI Conference.

3. Federal Motor Vehicle Safety Standards:571.207, Department of Transportation,National Highway Traffic SafetyAdministration.

4. Federal Motor Vehicle SafetyStandards: 571.301, Fuel System

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Integrity, Department of Transportation,National Highway Traff ic SafetyAdministration.

5. Laboratory Test Procedure for FMVSS301R, Fuel System Integrity – RearImpact (2007), Department ofTransportation, National Highway TrafficSafety Administration.

6. Herbst B R, Clarke C C, Meyer S E, OliverA A, Hock D A, Hayden J D and Forrest SM (2008), “Quasi-Static and DynamicTesting as a Basis for Determining SeatBack Strength”, ASME.

7. MLIT/JAMA/JARI (2004), “Proposals forActivities on International HarmonizationAdvanced Frontal Impact Dummies”,

July 16, IHRA BIO-WG in Washington DC,USA.

8. States J D, Balcerak J C Williams J S,Morris A T, Babock W, Polvino R, Riger Pand Dawley R E (1972), “Injury Frequencyand Head Restraint Effectiveness in RearEnd Impact Accidents”, Proc. of 16th StappCar Crash Conference, pp. 228-245, LC67-22372, Detroit, MI.

9. Van Der Horst M J (2002), “Human HeadNeck Response in Frontal, Lateral andRear End Impact Loading Modeling andValidation”, Ph.D Dissertation, Tech. Univ.of Eindhoven, Eindhoven, North Brabant.

10. Viano D C (2002), “Role of the Seat inRear Crash Safety”, SAE International,ISBN 0-7680-0847-6.