Increasing fuel economy has been a principal issue in the development of new cars. Decrease in weight of vehicle components is one of the important approaches to solving the problem. In order to obtain the goal, researches sought for ways to design lighter and stronger bumper impact beams relinquishing neither impact absorbing capacity of bumpers nor the safety of vehicles. In this paper, a possible weight reduction of impact beam is examined by substituting conventional type of steel with high-strength steel (boron steel having tensile strength 1.5 GPa) with regard to variables such as frontal, offset and corner impact crash capability of a bumper impact beam. In addition, the effects of variation in structural differences in bumper impact beams (open section, closed section, open section with 5 stays) were carefully considered. Arguably, this analysis presents a good guide for an optimum design and development of bumper impact beams

Jun-Hyub Park

Kee Joo Kim

English

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K.J. Kim, J.-H. Park: Crash design and simulation of automotive bumper impact beam for weight reductionENGINEERING MODELLING 26 (2013) 1-4, 53-58 53SUMMARYIncreasing fuel economy has been a principal issue in the development of new cars. Decrease in weight of vehiclecomponents is one of the important approaches to solving the problem. In order to obtain the goal, researches soughtfor ways to design lighter and stronger bumper impact beams relinquishing neither impact absorbing capacity ofbumpers nor the safety of vehicles. In this paper, a possible weight reduction of impact beam is examined bysubstituting conventional type of steel with high-strength steel (boron steel having tensile strength 1.5 GPa) withregard to variables such as frontal, offset and corner impact crash capability of a bumper impact beam. In addition,the effects of variation in structural differences in bumper impact beams (open section, closed section, open sectionwith 5 stays) were carefully considered. Arguably, this analysis presents a good guide for an optimum design anddevelopment of bumper impact beams.Key words: automotive body, bumper, open section, closed section, impact beam, weight reduction.UDC 629.1.015:625.03:539.375:519.61Original scientific paperReceived: 02.09.2012.Crash simulation and development ofautomotive bumper impact beam designfor weight reductionKee Joo Kim(1) and Jun-Hyub Park(2)(1)Department of Automobile Engineering, Seojeong College University,1049-56 Whahap-ro, Eunhyeon-Myun, Yangjoo-City, Gyeonggi-do 482-777, KOREA,e-mail: kjkim@seojeong.ac.kr(2)Department of Mechatronics Engineering, Tongmyong University, Busan 608-711, KOREA1. INTRODUCTIONBumpers are automotive parts situated at the frontand the rear of a vehicle to ensure the protection of avehicle body in the case of a low-speed crash. This isenabled via the formability of bumpers and energyabsorption during an impact. The bumper primarilyconsists of an impact beam, bumper stays, impactabsorbing materials in the form of structure and acover. Among these, the majority of energy during animpact is absorbed by the bumper impact beam built ina body side member [1, 2]. The crash capability of thebumper should also meet worldwide regulationsregarding the safety of passengers. In order to developbumpers that might help reduce fuel consumption butstill remain safe for passangers, the ways to designlighter, better energy impact absorbing bumpers hasbeen investigated. To evaluate the structural capabilityof bumpers, finite element simulation (FE-simulation),whose scope gradually widened, has been adopted forthe design [2-5]. There are two methods of weightreduction for the improvement of a vehicle. The firstrequires the substitution of conventional steelreinforcements with thinner, high-strength steelreinforcements and the second implies thedevelopment of substitution materials (Al, Mg, etc.)instead of the steels. However, aluminum alloys or otherlighter metal materials have their shortcomings too.High cost, low press formability, adhesion (welding)problems and surface treatment difficulty are some ofthe disadvantages of using these materials.In order to apply high-strength steels to vehiclecomponents, high formability of material is required.In the case of high-strength steels, the formabilityK.J. Kim, J.-H. Park: Crash design and simulation of automotive bumper impact beam for weight reduction54 ENGINEERING MODELLING 26 (2013) 1-4, 53-58mostly decreases with the increase of the steel strength.This problem has been solved by hot press forming(or press hardening) process thus ensuring thehardenibility increase of boron steel. This processinvolves rapid cooling after high-temperature formingprocess for steels containing boron and having bakehardening properties. After the formation process, theshot blasting was performed to remove a thin oxidelayer without any particular tempering.This paper shows the effects of varying the type ofmaterial (high-strength steel with tensile strength of1,500 MPa employed instead of conventional steel) inthe design of impact beams, as well as a comparativeanalysis of frontal, offset and corner impact crashcapabilities of such beams with the scope of increasingthe possibility of bumper weight reduction. An FE-simulation has been adopted for the analysis.2. ANALYSIS OF STRUCTURALPARAMETERS BY FE-SIMULATIONFigure 1 presents an FE-simulation model of abumper impact beam including load and boundaryconditions. To calculate the torsional stiffness invarious cases, one side of the impact beam was fixedusing rigid body element (RBE) and a unit moment of100 Nmm was applied to the end of the other side ofthe beam, as shown in Figure 1.Figure 2 shows the predicted displacement resultsof the bumper impact beam with a maximumdisplacement of 1.07 E-01 mm. Lower displacementindicates higher torsional stiffness.Fig. 1  Model of a bumper impact beam includingboundary conditionsThe effect of section height (with an initial heightof 109 mm), as yet another variable having an effecton the torsional stiffness of bumper impact beam, wasanalysed. The initial section height of the impact beamwas magnified to 165.5 mm (1.5 times), 218 mm (2times) and 272.5 mm (2.5 times), respectively. Inaddition, the effect of variation in structural design (thenumber of stays) on the torsional stiffness of bumperimpact beam was analysed. The initial open sectiondesign of the impact beam was varied according to thenumber of stays in the section (from 3 to 7). Moreover,the effect of varying thickness of the beam (from initialthickness of 2.0 mm to 3.5 mm) on the torsionalstiffness were investigated as well.Fig. 3  Effect of the varying number of stays on torsionalstiffnessFigure 4 shows the results of simulation taking intoaccount parameters such as section height, thicknessand change in design of an impact beam and singlingout the most effective method for the increase oftorsional stiffness of the beam. Among theseparameters, the increase in section height of a beamproved as the most effective parameter. In addition,the design change (adding on the number of stays)proved to be the least effective method in obtaining thedesired goal. Finally, the section height should bemaximized to effectively increase the torsional stiffnessof the bumper impact beam.Fig. 2  Predicted displacement of bumper impact beam in caseof 5 stays (max.displ.: 1.07 E-01 mm)Figure 3 shows the results of the effects thatstructural design differences have on the torsionalstiffness. Larger number of stays contributed to thehigher torsional stiffness of the impact beam.Additionally, if the back of the impact beam had fullyclosed section, the torsional stiffness of impact beamwas 500 times higher than that of open section [6].K.J. Kim, J.-H. Park: Crash design and simulation of automotive bumper impact beam for weight reductionENGINEERING MODELLING 26 (2013) 1-4, 53-58 55Fig. 4  Effect of section variations on torsional stiffness [6]3. ANALYSIS OF WEIGHT REDUCTIONAND IMPACT CHARACTERISTICS ASA CONSEQUENCE OF MATERIALCHANGEA comparative study was conducted on twodifferent types of material for the production of bumperimpact beams. The first was SPFC780 (tensile strengthof 780 MPa), commercially available material, and theother was boron steel (tensile strength of 1,500 MPa).Table 1 shows the mechanical properties of the twotypes of steel, while Figure 5 presents experimentallymeasured tensile curve of boron steel. Weight reductionwas tested for cases in which bumper impact beamsmade of high-strength boron steel varied in design.Several models were tested, from the one having closedsection structure to the modified one containing opensection without deterioration of impact capability.Figures 6(a), 6(b) and 6(c) show simulation of loadingconditions for frontal, offset and corner impacts,respectively. The impact simulation was done by usingcommercial software LS-DYNA3D.Table 1. Mechanical properties of beam materialsFig. 6  Impact conditions (speed and loading regions)Figures 7(a), 7(b) and 7(c) show the FE-modelsfor frontal, offset and corner impact simulations,respectively.Fig. 5  Experimentally obtained tensile curve for boron steel Fig. 7.  FE-model for frontal impact simulationMaterials Yield strength (MPa) Tensile strength (MPa) Elongatin (%) SPFC780 534.6 816.6 15.4 Boron steel 1,048.1 1467.8 7.9  K.J. Kim, J.-H. Park: Crash design and simulation of automotive bumper impact beam for weight reduction56 ENGINEERING MODELLING 26 (2013) 1-4, 53-58Figure 8 shows the simulation of an impact with anintrusion displacement in the longitudinal direction of avehicle (X-displacement) versus time after frontalimpact simulation for material SPFC780 and in the caseof a closed section. In this case, the maximum intrusiondisplacement was 21.3 mm.Figure 9(a) shows the simulation of an impact witha resulting intrusion displacement in the longitudinaldirection of a vehicle (X-displacement) versus timeafter frontal impact in the case of boron steel beamswith an open section structure. In this case, themaximum intrusion displacement was 24.6 mm. Figure9(b) shows an intrusion displacement in the longitudinaldirection of a vehicle (X-displacement) versus timeafter an offset impact for beam made of boron steelwith an open section structure. In this case, themaximum intrusion displacement was 31.0 mm. Figure9(c) illustrates an intrusion displacement in thelongitudinal direction of a vehicle (X-displacement)versus time after a corner impact for a beam made ofboron steel and having an open section structure. Inthis case, the maximum intrusion displacement was9.8 mm.As shown in Figure 9, the intrusion displacementof the boron modified beam was higher than that ofthe beam made of SPFC780. The improvement of thisresult was obtained by adding 5 stays in the designedstructure of the beam. The improved design structureis shown in Figure 10.Therefore, Figures 11(a), 11(b) and 11(c) displayintrusion displacements in the longitudinal direction ofa vehicle (X-displacement) versus time after frontal,offset and corner impact simulations, respectively. TheFigure 11 is valid for beams made of boron steel andhaving open section structures with 5 stays. Themaximum intrusion displacements obtained in thesesimulations were 21.2 mm, 28.6 mm and 9.8 mm,respectively. Compared to the results of the maximumintrusion displacement of SPFC780 impact beam witha closed section, the maximum intrusion displacementof the improved impact beam was reduced by 1% inthe case of frontal impact and by 21.6% in the case ofa corner impact.Table 2 shows summarized results of the variousconditions discussed above. Accordingly, as shown inTable 2, the weight of the bumper impact beam couldbe reduced by employing adapted boron steel and inthe case in which section structure contains 5 stays,as modelled in Figure 10.Furthermore, additional changes in the design ofbumper structure with respect to distribution andallocation of bumper stays are also possible and worthconsidering for the future research in finding theoptimum structure design; changes are shown inFigure 12.Fig. 8  Impact intrusion displacement of SPFC780  adaptedbeam (max.displ.: 21.3 mm)(a)  BL=0 (max.displ.: 24.6 mm)(b)  BL=300 (max.displ.: 31.0 mm)(c)  Corner impact (max.displ.: 9.8 mm)Fig. 9  Impact intrusion displacement of boron steeladapted beamK.J. Kim, J.-H. Park: Crash design and simulation of automotive bumper impact beam for weight reductionENGINEERING MODELLING 26 (2013) 1-4, 53-58 57Fig. 10  Modified bumper impact beam including 5 staysMaterials (Section) Maximum intrusion (mm) Weight (kg) Time (sec) SPFC780 21.3 (100%) 6.5 (100%) 0.03 SPFC780 31.0 (146%) 4 .3 (66.2%) 0.05 Boron steel 24.6 (115%) 4 .3 (66.2%) 0.04 Boron steel (with 5 stays) 21.2 (99%) 4 .6 (70.8%) 0.03 Materials (Section) BL = 0 BL = 300 Corner impact SPFC780 21.3 (100%) 27.9 (100%) 12.5 (100%) SPFC780 31.0 (146%) - -  Boron steel 24.6 (115%) 31.0 (111%) 9.8 (78.4%) Boron steel (with 5 stays) 21.2 (99%) 28.6 (103%) 9.8 (78.4%) 4. CONCLUSIONThe effect of structural variables on crashcharacteristics was analysed on the basis of thechanges and modifications made on both structure andmaterial seeking out the optimal solution for weightreduction. To analyse the possibility of weight reductionby substituting conventional types of steel with high-strength boron steel, the frontal, offset and cornerimpact crash simulations were performed. The changein design was also taken as one of the parameters inthe crash simulations. The results indicate that if 5 staysare added on the impact beam made of boron steel, themaximum displacement of frontal and offset impactintrusion amounts to 21.2 mm and 28.6 mm,respectively; and these are very similar to the initial,reference values for impact beam made of SPFC780.In addition, displacement of corner impact intrusionadding 5 stays for boron steel was 9.8 mm and itsbumper crash safety was improved by 21.6% withrespect to that of the SPFC780 impact beam.Finally, the results obtained make this analysis agood reference for the future advancement of bumperimpact beams. It can contribute in the development ofan optimal design and the reduction of bumper weight.(c)  Corner impact (max.displ.: 9.8 mm)Fig. 11  Impact intrusion displacement of boron steeladapted beam containing 5 staysTable 2(a)  Summary of the results of the analysis andobtained weight values (BL = 0)Table 2(b)  Summary of the results indicating intrusiondisplacements values (mm)(a)  BL=0 (max.displ.: 21.2 mm)(b)  BL=300 (max.displ.: 28.6 mm)K.J. Kim, J.-H. Park: Crash design and simulation of automotive bumper impact beam for weight reduction58 ENGINEERING MODELLING 26 (2013) 1-4, 53-58SIMULACIJA AUTOMOBILSKOG UDARA I PROJEKTIRANJE UDARNE GREDEODBOJNIKA S CILJEM SMANJENJA TE@INE GREDESA@ETAKPoticanje energetske u~inkovitosti danas predstavlja jedno od klju~nih pitanja pri razvoju automobila, a redukcijate`ine njegovih sastavnih dijelova jedan je od va`nijih ciljeva u rješavanju tog problema. Znanstvenici su stogaistra`ivali optimalna rješenja kojima bi se postiglo da udarna greda odbojnika bude istovremeno sna`nija i laganija,a da se pritom njezino svojstvo apsorpcije energije ne smanjuje te posljedi~no ne umanjuje sigurnost vozila. U ovomeistra`ivanju razmatrana je mogu}nost redukcije te`ine udarnih greda odbojnika variranjem parametara sposobnostiapsorpcije energije pri potpuno frontalnom, djelomi~no-frontalnom (40% prednjeg dijela automobila) i kutnomudaru automobila u slu~aju odabira druga~ije vrste materijala, odnosno koriste}i ~elik izrazito velike ~vrsto}e(boron ~elik vla~ne ~vrsto}e 1.5 GPa) umjesto klasi~nog ~elika. Nadalje, prednosti s obzirom na vrstu struktureudarne grede odbojnika (otvorenog profila, zatvorenog profila te otvorenog profila s pet pri~vrš}iva~a) tako|er supomno razmotrene. Na koncu, mo`e se tvrditi da ova analiza predstavlja dobar temelj i smjernice pri optimalnomprojektiranju udarne grede odbojnika.Klju~ne rije~i: tijelo automobila, odbojnik, otvoreni profil, zatvoreni profil, udarna greda, reduciranje te`ine.Fig. 12  Example of a structural variation in allocation ofstays; a suitable ground for further studies5. REFERENCES[1] S.J. Lee, J.S. Park, D.H. Koo and B.H. Jung, Thedevelopment of material technology applied tobumper beam, Transactions of KSAE, Vol. 10,No. 4, pp. 206-215, 2002. (in Korean)[2] J.W. Lee, K.A. Yoon, Y.S. Kang, K.T. Park andG.J. Park, Hood and bumper structure designmethodology for pedestrian regulation,Transactions of KSAE, Vol. 13, No. 3, pp. 162-170, 2005. (in Korean)[3] K.H. Lee and W.S. Joo, Optimum design of anautomobile front bumper using orthogonal array,Transactions of KSAE, Vol. 10, No. 6, pp. 125-132, 2002. (in Korean)[4] J. Suh, J.-H. Lee, H.-S. Kang, K.-T. Park, J.-S.Kim, M.-Y. Lee and B.-H. Jung, Optimalprocessing and system manufacturing of a laserwelded tube for an automobile bumper beam, Int.J. of Automotive Technology, Vol. 7, No. 2, pp.209-216, 2006.[5] K.J. Kim, C.W. Sung, Y.N. Baik, Y.H. Lee, D.S.Bae, K.-H. Kim and S.-T. Won, Hydroformingsimulation of high-strength steel cross-membersin an automotive rear subframe, Int. J. ofPrecision Engineering and Manufacturing, Vol.9, No. 3, pp. 55-58, 2008.[6] K.J. Kim and S.T. Won, Effect of structuralvariables on automotive body bumper impactbeam, Int. J. of Automotive Technology, Vol. 9,No. 6, pp. 713-717, 2008.6. ACKNOWLEDGEMENTThis research was supported by the Korea ScienceResearch Program through the National ResearchFoundation of Korea funded by the Ministry ofEducation, Science and Technology (No. KOSEF2009-0085880).

Crash simulation and development of automotive bumper impact beam design for weight reduction

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