8/31/2015 Development process of new bumper beam for passenger car: A review

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doi:10.1016/j.matdes.2012.03.060

Materials & DesignVolume 40, September 2012, Pages 304–313

Development process of new bumper beam for passenger car:A review

M.M. Davoodia, b, , , S.M. Sapuanb, A. Aidyc, N.A. Abu Osmana, A.A. Oshkoura,W.A.B. Wan Abasa

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Abstract

Bumper beam absorbs the accidental kinetic energy by deflection in lowspeed impactand by deformation in highspeed impact. The safety regulations “low, and highspeed,and pedestrian impacts” along with new environmental restrictions “endoflife vehicles”increased the complexity level of bumper system design. The new bumper design mustbe flexible enough to reduce the passenger and occupant injury and stay intact in lowspeed impact besides being stiff enough to dissipate the kinetic energy in highspeedimpact. The reinforcement beam plays a vital role in safety and it must be validatedthrough finiteelement analysis (FEA) and experimental tests before mass production.The careful design and analysis of bumper beam effective parameters can optimize thestrength, reduce the weight, and increase the possibility of utilizing biodegradable andrecyclable materials to reduce the environmental pollution. Developing the correctdesign and analysis procedures prevents design remodification. On the other hand,analysis of the most effective parameters conducive to high bumper beam strengthincreases the efficiency of product development. Cross section, longitudinal curvature,fixing method, rib thickness, and strength are some of the significant design parametersin bumper beam production. This study critically reviews the related literature on bumperdesign to come up with the optimal bumper beam design process. It particularly focuseson the effective parameters in the design of bumper beam and their most suitable valuesor ranges of values. The results can help designers and researchers in performingfunctional analysis of the bumper beam determinant variables.

Highlights

The process of new bumper beam development for passenger car is discussed. Anew bumper system has been added to the previous developed bumper systems. Theflow chart of design and analysis of bumper beam is shown. Different analysis fordeveloping new bumper beam before production is discussed. The process ofmaterial selection in bumper beam is discussed.

Keywords

Developing process; Bumper beam; Design parameters

1. Introduction

Design is the preliminary stage of product development and analysis. The embodiment

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stage of the design process fairly predicts the failure(s), if any, before mass production.Passenger vehicles make up over 90% of the fleet of registered vehicles. In 2009 it wasestimated that 9,640,000 vehicles were involved in policereported crashes, 95%(9,161,000) of which were passenger vehicles. Furthermore, there, 45,435 vehicles ofthese were involved in fatal crashes and eighty percent of which (36,252) werepassenger vehicles. More than 23,000 passenger vehicle travelers lost their lives intraffic crashes in 2009 and an estimated 1.97 million persons were injured [1]. Therefore,vehicle safety requirements forced by Governments and insurance companies increasefrequently [2]. In most of the accidents, the bumper system is the first vehicle part thatreceives the collision and which may to some extent protect the car body andpassengers. This system comprises three main parts: fascia, energy absorber, andbumper beam [3]. The fascia is a nonstructural aesthetics component that reduces theaerodynamic drag force while the energy absorber dissipates part of the kinetic energyduring collision. The bumper beam is a structural component which absorbs the lowimpact energy by bending resistance and dissipates the highimpact energy by collision[4].

There are some investigations of new material development, property improvement, andFEA of bumper beam structures by researchers and car manufacturers. These partiesare mainly interested in substituting the conventional material with lighter and strongermaterial [5]. Renault used SMC in a passenger car bumper in 1972 instead of steel [6]and General Motors (GMs), used the sheet molding compound (SMC) beam in PontiacBonneville Cadillac Seville and Cadillac Eldorado instead of steel which was used inprevious models [7]. Cheon et al. [8] found that the polymer composite bumper beamoffers 30% less weight than steel without scarifying the bumper beam’s bending strength.Wakeman et al. [9] found that holding time pressure is the most effective parameteramong five processing parameters in microstructure and macrostructure properties ofglass mat thermoplastic (GMT) in a bumper beam. Peterson et al. [10] from Azdelcompany developed the GMT with a high surface finish for aesthetic components.Raghavendran and Haque [11] also developed a lightweight GMT composite containinglongchopped fiber strands to be used in headliner and other automotive interiorapplications. Suddin et al. [12] used the weight analysis method to select fascia for adesired vehicle. He used the knowledgebased system (KBS) approach to select thematerial for bumper beam development [13]. Sapuan et al. [3] studied the conceptualdesign and material selection for development of a polymericbased compositeautomotive bumper system. Hosseinzadeh et al. [14] studied the shape, material, andimpact conditions of the bumper beam and compared the results with conventionalmetals like steel and aluminium. He found that GMT can replace SMC as a recyclablematerial. Kokkula et al. [15] experimentally studied bumper beam performance at 40%offset impact crashworthiness and concluded that materials with moderate strainhardening properties are preferable over the higher strainhardening materials for hisstudied system. Hambali et al. [16] studied employed the analytical hierarchy process(AHP) in concept selection of bumper beam during the conceptual design stage ofproduct development. Marzbanrad et al. [17] studied bumper beam crashworthinessimprovement by analyzing bumper beam material, thickness, and shape as well asimpact condition parameters. He found that a modified SMC bumper beam is preferableto the ribbed GMT bumper beam as the former has the potential to minimize the bumperbeam deflection, impact force, and stress distribution and to maximize the elastic strainenergy while exhibiting almost the same energy absorption of the unribbed SMC bumperbeam. Park et al. [18] developed an optimized bumper beam cross section that satisfiesboth the safety requirements for a front rigidwall impact and lower leg injuries in apedestrian impact test. Most of the abovementioned research emphasizes on materialand concept selection for, and numerical analysis of, bumper beam. However, no articlesregarding procedure(s) for new bumper beam development could be found in the openliterature. This study therefore focuses on the process of bumper beam development andsummarizes the method of design and analysis of the new bumper beam in new vehicledevelopment based on the previous research and the authors’ personal experiences. Inconsequence, this article helps the designer to follow the right procedure for bumper

8/31/2015 Development process of new bumper beam for passenger car: A review

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beam development. It emphasizes on the parameters that have to be considered in thedesign of bumper beams and illustrates the procedure for FE analysis the bumpersystem.

2. Bumper system

2.1. Bumper system definition

A bumper system is a set of components in the front and rear parts of the vehicledesigned for damping the kinetic energy without any damage to the vehicle in lowspeedimpact and for energy dissipation in highspeed impact conditions besides servingaesthetic and aerodynamic purposes [19] and [20]. A bumper system mainly comprisesthree components: fascia, energy absorber, and beam [3]. The bumper system haschanged over the last three decades due to new government safety regulations andstyling concepts. The ability to maintain the vehicle intact at highspeed impactconditions and to damp the kinetic energy are the most important factors in bumpersystem selection besides its weight, manufacturability, cost, reparability, and formabilityof materials [21] (Fig. 1).

Fig. 1. Common bumper systems.

The American Iron and Steel Institute [22] offered four proposals for bumper systems: (1)metal face bar, (2) plastic fascia and reinforcing beam, (3) plastic fascia reinforcing beamand mechanical energy absorbers, and (4) plastic fascia reinforcing beam and foam, orhoneycomb, energy absorbers. According to the new regulation, the pedestrian legimpact test was due to be enacted and implemented starting from 2010. Some researchhas been carried out to offer methods for complying with the pedestrian impact test. Theenergy absorption density in the lowimpact test approximately doubled in comparisonwith the pedestrian impact [23]. Choi et al. [24] came up with the concept of locating theenergy absorber between the bumper fascia and the reinforcement beam to absorb theimpact energy when the second energy absorber is subjected to an impact greater thanits critical elastic force. Therefore, this concept (to be referred to hereafter as conceptnumber 5 or concept No. 5) can be added to the four bumper system components whichthe American Iron and Steel Institute (AISI) offered in 2003 (Fig. 2) which is a schematicview of a concept No. 5 system modified from AISI for car bumper system. In this method,two types of energy absorbers are considered: firstly, a low stiffener absorber, which iscalled the reversible absorber, is designed for protection against low and pedestrianimpact; and secondly, the irreversible energy absorber, which comprises the beam andthe crushable energy absorber and is usually located at the back of the beam andattached to the main face bar.

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8/31/2015 Development process of new bumper beam for passenger car: A review

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Fig. 2. Pedestrian, low impact and crash impact system.

2.2. Bumper beam definition and function

The bumper beam is the backbone of the energyabsorption mechanism of the bumpersystem [8]. It is usually located in the front, and sometimes in the rear, sides of thevehicles. However, the testing process for both sides is almost the same; the forwardsystem should be stronger than the backward one for driver safety. On the other hand,the current trends in bumper design focus on aerodynamic efficiency where the designedcurve should be embraced with the same style in other parts of the bumper system [22].So, the conformable composite material solves this dilemma by providing the requiredcurvature and lowering the manufacturing cost, e.g., by multistage stamping of themetallic bumper beam, and decreases the beam weight [25].

Dissipation of energy by the bumper beam can be determined both by material andstructural energy absorption [26]. The effective parameters in energy absorption ofcomposite materials depend on type of fibre [27], matrix [28], fibre orientation [29],fabricating conditions [30], interlaminar bond quality [31], and toughness [32]. Theeffective parameters of structural energy absorption are longitudinal curvature, crosssection profile [33], strengthening ribs [34], thickness [35], and the overall dimensions ofthe crosssection [36]. The energy absorption of material and structure was investigatedby [37] and [38]. The crashworthiness of the vehicle and bumper system, which identifiesthe safety and performance of the vehicle in response to impact load, is a challengingissue. The enhanced performance of crashworthiness presents low damage to thevehicle and to occupants [39]. The impact energy in the bumper system can bedissipated reversibly (low impact) or irreversibly (crashworthiness) [40]. If the magnitudeof the load does not exceed the elastic region “lowimpact condition,” then the structurereturns to its previous position after releasing the load [4]. However, if the impact loadgoes beyond the elastic region “crashworthiness,” then most of the collision load isabsorbed by plastic deformation (irreversible energy absorption). The bumper systemshould overcome both scenarios and sustain the intense load which results in largedeformation, strain hardening, and various interactions between different deformationmodes such as bending and stretching [41] (Fig. 3).

Fig. 3. GMT bumper beam of Samand [42].

The proportion of energy reversibly absorbed by the bumper beam should be confinedand the high kinetic energy should be preferably dissipated by plastic deformation.Otherwise, the collision energy maximizes the structural strain energy and release thesame kinetic energy in return, which causes subsequent damage to the occupants oradjacent vehicles. Accordingly, the structural strain energy of the bumper beam shouldbe optimized during the design process. Besides, ductility of material improves theplastic energy absorption. Within this context, plastic composites, polymer foams, andaluminium alloys are commonly used in the bumper systems when plastic energydamping and weight are critical design and performance criteria [41] and [43].

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8/31/2015 Development process of new bumper beam for passenger car: A review

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2.3. Bumper beam design parameters

The stream of new materials, products, and process development has enforced arethinking of the role of structural design and of the effective parameters for theirimprovement. The bumper beam can be improved by adjusting a number of effectiveparameters. The efficiency of the parameters can be identified by any of a number ofmethods such as design of experiments (DOE) [44], reliabilitybased design optimization(RBDO) [45], and design sensitivity analysis. However, the current study is not intendedto identify the viability of the parameters. Variables such as thickness, bumper beamcurvature, rib strength, and crosssection profile are some of the most importantparameters which can improve the energy absorption of the bumper beam and sustainthe desired deflection of the bumper system as defined in the product designspecifications (PDSs). The optimal thickness of a bumper beam can construct a balancebetween the weight and strength of the structure in order to provide further effectiveenergy absorption [46]. The nominal thickness of the bumper beam is 4 mm. However, itis not completely constant in all beam parts. Surplus thickness of the polymer productshas some manufacturing constraints. As an illustration, it increases the cooling time andmakes warps in the flat surfaces and sink marks on the surface of the ribs’ interface,which is not suitable in visible products.

Strengthened ribs increases distortion resistance, rigidity, and structural stiffnessthrough using little material in the slender walls [47] and providing the required impactseverity [48]. Pattern, thickness, tip, and end fillet of the ribs should be designedaccording to load direction, impact position, material, and the manufacturing processavailable. Since the material thickness is high at the rib‘s contact area, it causes sinkmarks, but this is not much important a consideration, as a nonaesthetic part, for thebumper beam. It has been reported that the strengthened ribs increase the impactenergy by 7% and decrease elongation by 19% [14], [17] and [49]. Zhang et al. [20]showed that the optimized reinforced ribs have higherenergy absorption performancethan the empty and foamfilled beams.

Optimizing the crosssection of a bumper beam magnifies its strength, dimensionalstability, and damping capability. It has significant effects on the energy damping rateand bending resistance compared with other parameters [27] and [37]. The right crosssection can increase bumper beam strength and dimensional stability. Kim and Won [50]found that the section height is the most effective variable in torsional stiffness of thebumper beam. Additional strength permits more energy absorption with less consequentbumper beam distortion [51].

Frontal curvature increases the room between fixing points and top extremity beamcurvature and increases the stability of the beam and the energy absorption. It enhancesthe beam stability and extends the required collision displacement. Besides the aestheticpurposes, the curve facilitates additional load impact distribution through the frontalbeam and fixing points during the energy damping process. When an impact load isapplied to the bumper, the beam initial curvature tends to restore its original shape. So,some designers mounted a bar link between the beam fixing points in order to strengthenthe outward motion and the energy absorption tendency [51] and [52]. The bumper beamis an offset of the front bumper fascia that is intended to provide a consistent level ofprotection across the vehicle [53].

3. Material selection steps

Selecting a suitable material in bumper beam development is crucial and bad selectionmay cause poor performance, frequent maintenance or failure. Proper material selectionfor bumper beam requires information about type of loading (axial, bending, torsion ortheir combination), mode of loading (static, dynamic, fatigue, impact), operatingenvironment (temperature, humidity, chemical conditions), manufacturing process, cost(raw material, manufacturing, assembly) [54].

Environmental constraints, economical demands, and performance enhancement aremain issues for material selection [55]. Material usually should be finalized in preliminary

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design stage, while the material properties requirements are coupled with main structuralfunction [56]. The product function requirements usually identify through product designspecification (PDS) prior to development process to guide the designer for preciseselection of design parameters and material selection. Then based on the translated ofproduct design specification, constraints, objectives, geometry and process, which haveinteraction together the list of material should be narrow down to the best candidate tocomply with the defined properties [57].

Physical, chemical, and mechanical properties along with manufacturing and economicissues should be considered in selection of a favorite material for a bumper beam [58].Proper material selection can be achieved by constructing a balance or compromisebetween function, material, shape, and process [57]. The general properties, processing,and performance of materials are considered in the conceptual design phase and arerefined into specific requirements in the subsequent steps to ensure the performance ofthe final product. Material selection of a bumper system usually considers newenvironmental constraints, safety regulations, cost reduction, reliability improvement,and performance enhancement. Normally, the results of the failure analysis of previousproducts enable the designer to be more aware of material selection for the next product(Fig. 4).

Fig. 4. Material selection in bumper beam design process [57], [59] and [60].

There are two approaches for material selection of the bumper beam. Sincemanufacturing of the bumper beam is costly, the designers usually attempt to find themost consistent material for the available process that offers the desired properties.Otherwise, the material is selected initially and the optimized favorable manufacturingprocess is developed to meet the desired performance. Incorrect material selection andmanufacturing method may lead to product failure, performance reduction, and costincrease.

The material selection process requires knowledge about structural mechanics, materialstrength, material thermal properties, economics, and market demand. It entailsknowledge of the types of loading (axial, bending, torsion, or a combination of all or anytwo), mode of loading (static, fatigue, or impact), environmental conditions (temperatureand humidity), manufacturing processes (structure or components) and cost ofmanufacturing and assembly [54].

Review of the literature points out that there are some studies where discussion of

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material selection of automotive products has been be provided. For instance, Sapuan[61] selected the appropriate material for polymericbased composite automotive pedalbox system using an expert system with a comfortable graphical user interface. Hedeveloped a prototype KBS for material selection of ceramic matrix composites forengine components such as the piston, connecting rod, and piston ring [62]. In anotherexample, Sudin et al. [13] used the KBS for material selection in bumper beam design.

4. Design process of bumper beam

The design process is the solution steps for developing new products in a very specificway to prevent rework and reduce the production time and cost. It compromises betweenmarket requirements and production and converts the customer requirements into aproduct within the optimum economic conditions. The engineering design is combinationof knowledge for generating new ideas, evaluating these ideas, and selecting the bestconcept. A successful bumper beam design needs interdisciplinary team work, mainlyinvolving product design engineer, manufacturing engineer, materials specialist, qualityassurance specialist, and an analyst [63].

A successful bumper beam design starts with market research and customerrequirement investigation so as to develop an idea about the available products and theirmanufacturing technologies. These requirements are then finalized by converting thecustomer’s requirements into technical demands, i.e., PDSs. The design process usuallystarts with conceptual design generation followed by parameter identification in order forconcept evaluation to come up with reliable and feasible concept [64]. In a later stage; thedesign embodiment design stage, the separated components, configuration ofsubassemblies [65], and layout of the new bumper beam must be refined and evaluatedagainst the related technical and economic criteria. In the last stage, detailed design,precise dimensioning, tolerancing, material specification, configuration, and weight mustbe finalized [66] and [67]. This means that by this stage the final drawing of the wholeparts with the specified materials and dimensions is obtained.

Since the bumper beam surface should follow the style of the fascia (inner surface), thenew concept of the bumper beam will be released when the final version of fascia surfaceis issued. At the final stage of vehicle development, the latest clay model of the vehicle(fullscale) is released and the outer surface is scanned with a digitizer and a separationline for every visible component is identified as well. Then, the 3D model of everyseparated part, which is extracted from the clay model after reconstruction, is transferredto the appointed original equipment manufacturer (OEM) [68]. The appointed OEMmakes the inner surface (Bsurface) and designs other components such as bumperbeam, absorbers, brackets, and bumper grills in addition to specifying the method offixing them to the body in white (BIW), mainframe, or elements together. The positionsand fixing method(s) of the fog lamps, toe cap, and rubbing strip’s too should beconsidered during the bumper design stage. Besides, design for assembly (DFA), designfor manufacturing (DFM), maintenance, and other parameters have to be considered[69]. To release the final product, finiteelement analysis should be employed to virtuallyvalidate the structural strength of the developed product under working conditions.However, it is essential to make the softmold for the main parts to fabricate somesamples under similar production conditions with the selected material so as to obtain theexact shrinkage and conduct the actual structural test (Fig. 5).

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Fig. 5. Bumper beam design process flowchart.

4.1. Product design specifications (PDSs)

The design validation of the bumper beam is subject to material and structuralacceptance and should be presented in functional approval under working conditions.The design criteria compromise between the margins of safety value, damage criteria,environmental issues, manufacturing process, and maintenance [70]. The newcomposite structure should be precisely analyzed before going through the fabrication. Itmust be assured that all criteria and customer requirements (that’s, PDSs) are satisfied.The finalized designed structure should be analyzed by FEA software followed byproduction of a prototype for real test and analysis before mass production.

The PDSs are a list of the product specifications corresponding to customer’srequirements or expectations compiled in a detailed technical document [71]. The PDSsmust present the margin or exact value of each property clearly. It is quite difficult toprepare perfect PDSs in the early stage of product development wherein knowledge ofdesign requirements is imprecise and incomplete [72]. The PDSs are prepared by adisorganized brainstorming team with various specialities and experiences, e.g.,manufacturing, design, sales, assembly, and maintenance. Nonetheless, the PDSs canbe modified in response to unforeseen changes in product or manufacturingspecifications or constraints. In the case of the bumper beam, the PDSs address safety,performance, weight, size, cost, environmental issues, and appearance (Fig. 6). In otherrespects, the PDS parameters in general can be classified into three main subdivisions:material, manufacturing, and design. Since energy absorption of different bumper beamideas and designs is the core competency of this study, it is emphasized in the safetyparameters of the PDSs. Some of the specifications of the mechanical and physicalproperties presented here has been drawn from experimental results while others weremanaged from existing PDS data.

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Fig. 6. Product design specification of bumper beam [12] and [73].

Safety: A number of different bumper safety regulations for passenger’s cars have beenissued by governmental safety organizations, insurance companies, or OEMs [74].Insurance companies usually offer more rigorous conditions than other engagedorganizations in order to decrease their own monetary losses. Examples on safetyregulations for the passenger’s cars bumper systems include those issued by The UnitedNations Economic Commission for Europe (ECE) Regulation No. 42 [75]; theInternational Highway Traffic Safety Administration (NHTSA) [1]; and the CanadianMotor Vehicle Safety Regulation (CMVSR) [76]. Size: Dimensions of the bumper beamdepend on the size and weight of the vehicle and on the target value of energyabsorption. Maintenance: Design for assembly and DFM should be considered duringproduct design. Performance: The defined goal of the product should be attainable [77].Installation: Design for manufacturing and assembly (DFMA) helps in minimizing thebumper components in the product or assembly to allow for easy assembling withoptimized fixing point. The material should be selected according to the preset propertiesor desired problem solution. Materials of the bumper should be light, costcompetitive,accessible, producible, recyclable, and rather biodegradable.

4.2. Conceptual design of the bumper beam

The preliminary stage of every product development starts with conceptual design, whichis derived from the customer requirements “voice of the customer” [65] and [78], in orderto find a solution that satisfies the design criteria [64]. Inaccurate engineering calculationin bumper beam design and material selection can lead to an increase of up to 70% of thetotal product cost for redesigning, selecting material, or changing the manufacturingequipment [79]. In other words, the designer has to select the most suitable idea fromdifferent possible solutions to meet the desired PDSs in each design stage, i.e. totaldesign, subsystem or component’s level total design, and subsystem level [77] and [80],to decrease the rework expenses to less than twice the design and manufacturing costs[80], [81] and [82]. Therefore, bumper beam design concept selection (DCS) has recentlybeen under focus of the designers. Many tools have been developed for DCS to evaluateand compromise different effective factors such as customer requirements, designerintentions, and market desire to eventually propose the best conceptual design.

4.2.1. Concept design selection

Developing different concepts drives decision makers to narrow down the selectedconcepts to the best feasible solution and to cut the redesign cost and production delay in

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the early stage of the design process. The previous research presented differentapproaches to selection of the best concepts. The decision matrixbased methods (e.g.,Pugh’s method [77]) offer a qualitative comparison and quality function deployment(QFD) to perform the best concept selection [83]. Selecting the most suitable concept byanalysis and manipulation of experimental tests along with comparisons with the productstandards help the designer in selecting the optimal solution with the minimum risk [84].The fuzzy ANPbased approach is useful for valuation of a set of conceptual designalternatives to satisfy both customer’s satisfaction and the engineering specifications[85]. The AHP developed a mathematicallybased technique for analyzing complexsituations [86]. Multicriteria decisionmaking (MCDM) is an effective method for singleselection when mixed criteria are involved. The multiattribute decisionmakingtechnique (MADM) was developed to solve conflicting preferences among criteria forsingle decision makers. The TOPSIS is a technique well suited to dealing with multiattribute or multicriteria decisionmaking (MADM/MCDM) problems in real world idealsolutions [87]. Its methodology is based on the principle of selecting an alternative havingthe shortest distance from the positive ideal solution and the farthest distance from thenegative ideal solution. It helps in organizing problems and in comparing and rankingalternatives to search for better options [88]. This method has been used to select thebest concept in this research.

4.2.2. Embodiment

In the embodiment design stage, the bumper beam is investigated with reference totechnical and economic criteria. It makes highly accurate modeling of the values of forceswhen analyzing the final dimensions of components. The appropriate function and safetyrequirements must be approved before evaluating other parameters such as ease ofproduction; low investment; ease of assembly, transportation, maintenance; materialrecyclability; and cost.

The best shape, size, arrangements, and layouts stem from the combined ideas andsolutions from other professions’ points of view. This can eliminate the weak spot orcombine the appropriate solutions to propose the best layout. It is often crucial that thedesigner produces several preliminary methods of bumper system layout with wholecomponents to analyze and compare their advantages or disadvantages and select themost proper one. The best layout is the one that can fulfill the required function with theminimum parts, process, and optimum possible standard parts in order to decrease theproduction cost. The selected material should have the desired strength, corrosionresistance, energy absorption, service life, availability, and recyclability.

4.2.3. Detailed design

In the detailed design phase, all the information about the parts must be placed on thedrawing. All the individual component drawings of the bumper system must have therequired manufacturing dimensions, material specifications, tolerance, surfaceroughness, part list, and bill of material (BOM) [89]. Use of 3D CAD software facilitatesimplementation of the various processes involved in the detailed design phase since bythis stage all parts have been modeled with the real dimensions in their position and withthe required fitting tolerance. Hence, a 2D drawing can be extracted easily from theassembly model and fully dimensioned. Moreover, it allows for using the data formanufacturing, production planning, and toll making. The detailed design must becritically checked and approved by an experienced designer to validate the fit andfunction of the final design and prevent any error in the fabrication phase. However, theperfect detailed design cannot cover the poor conceptual design, if any, and investigationshowed that most of the errors during production are caused by flawed conceptualdesign [66] and [90].

Shrinkage (mold shrinkage, postmolding shrinkage, and enduse shrinkage) [91] of thepolymer material depends on a variety of parameters (thickness, geometry, temperature,flowing speed) and cannot be exactly estimated in complicated parts. The moldingoperation could not make a uniform temperature in the fabricated products. A smallscale

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deviation in shrinkage percentage in huge parts such as bumper results in unfitness inthe adjacent parts after production. Consequently, it is essential to make a real samplefrom the big parts by softmold with the same material and production conditions in orderto find the exact value of shrinkage, provide a proofofconcept, and implement amechanical test as well [92].

5. Analyzing the bumper beam

Two main analyses of a bumper beam should be carried out in. First, the designer mustbe confident about that the selected material fulfills the defined PDS properties (materialproperties). Then, it must be assured that the structure developed with the selectedmaterial can achieve the main functions, safety parameters, and the desiredperformance.

5.1. Analysis of material

Based on the desired product design specifications (PDSs) the plan for testing thematerial should be written to confirm that the test method addresses loading sequence,gauge placement and acceptance criteria of the material [69]. The test plan is usuallyprepared by a material developer and approved by an authorized certifying agency.

Since the performance of the material is closely related to the manufacturing conditions,the manufacturing parameters should be set to achieve the anticipated specification.However, the final approval of the bumper beam can be issued after the structural test.Sometimes a manufacturer modifies the production parameters for different applicationsor various conditions. Totally, the test may include mechanical tests such as tensile andcompressive strength, yield strength, and toughness [93] with a consideration of theenvironmental conditions, like humidity (20%–95%) and temperature (−30 °C to 85 °C)[94], to which the bumper beam may be exposed. Besides, advances in numericalmethods for product analysis and material processing analysis made it possible tooptimize the product, manufacturing process, and structural components.

5.2. Structural analysis

The next step in bumper beam analysis is to ensure the proper structural performance ofthe selected material. The most reliable method is to fabricate the samples from selectedmaterials and appointed production methods and to carryout the test under realconditions, which is cost, and timeconsuming. The FE analysis does not offer 100%reliable results, but its cost is considerably less than that of the experimental test.

Numerical analysis of the developed bumper beam is the preliminary stage of approvingthe bumper beam. The final geometry with defined longitudinal curvature, thickness,cross section profile, material, rib strength, and overall dimensions should be performedto obtain the final geometry of the bumper beam FE model. A reasonable prediction ofthe bumper beam performance depends on the accuracy of the simulated geometry, i.e.,the longitudinal curvature, cross section, thickness, ribs, and material model. However,the realscaled bumper beam with defined material and geometry should be tested underactual conditions for final verification. The value of numerical analysis is stronglydependent on a validated modeling technology with accurate material models andfracture criteria.

5.2.1. Low speed impact test

The procedure of low speed impact test in European countries is different compare withAmerican countries. Three low speed impact tests with their criteria explained as follows.The criteria for the lowimpact test in European countries is a pendulum test at 4.0 km/h(2.5 mph) with no damage to the bumper, and in the USA and North America, it is thesame test at 8 km/h (5 mph), but damage to the fascia is not considered [1], [75] and [95].

The United Nations Economic Commission for Europe (ECE) Regulation No. 42:Requires that a car’s safety systems continue to operate normally after the car has beenimpacted by a pendulum or moving barrier on the front or rear longitudinally at 4 km/h(about 2.5 mph) and on the front and rear corner at 2.5 km/h (about 1.5 mph) at 455 mm

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(about 18 in.) above the ground under loaded and unloaded conditions, which calls for noserious damage (light bulbs may be changed) [75].

National Highway Traffic Safety Administration (NHTSA) – Code 49 Part 58: Longitudinalpendulum impact test 4 km/h at the curb position of bumper and 2.5 km/h bumper at thecorner [96].

Canadian Motor Vehicle Safety Regulation (CMVSR): Canadian safety regulation hasthe same limitation and safety damage as NHTSA (pendulum test4 km/h of bumper faceand 2.5 km/h bumper corner), but the speed is double [96].

After the test, any damage to bumper visual and functional should not occur. The lights,bonnet, boot, doors operate in the normal manner, and all the essential features for safeoperation of the vehicle must still be serviceable. EU regulation allows greater damage inthe car in lowimpact test compare with US regulation. Moreover, US regulation useslower speed, permits running lamps, fog lamps and equipment on the bumper face bar tobe remove if they are optional and requires no visual damage to all nonbumper parts,while Canadian uses higher regulation speed, and does not allow anything to beremoved, and it requires no damage to safety and functional items.

5.2.2. Highspeed impact test

Besides the lowimpact test, the bumper system has to be able to absorb enough energyin highspeed impact to meet the OEM‘s internal bumper standard in design stage. Thenew bumper systems are not design to overcome entire of the highspeed impact energy.However, systems are being developed that can damp about 15% of energy under thehighspeed impact. The design criterion for a highspeed impact for bumper system isdefined as follows [22]:

No bumper damage or yielding after 8 km/h (5 mph) frontal impact into a flat, rigidbarrier. This criterion does not apply to low speed bumpers, where controlledyielding and deformation are beneficial.

No intrusion by the bumper system rearward of the engine compartment rails for allimpact speeds less than15 km/h (9 mph).

Minimize the lateral loads during impacts in order to reduce the possibility of lateralbuckling of the rails.

Full collapse of the system during Danner (RCAR), NCAP, and IIHS high speedcrash without inducing buckling of the rails.

Absorb 1% of the total energy every millisecond and 15% of the total energy in theNCAP crash, including engine hit.

5.2.3. Pedestrian impact test

The pedestrian impact test needs lower stiffness in dissipation of impact energy over alonger time span. Bumper system requires elastic energy absorption before any plasticyielding of the bumper beam takes place.

The second phase of pedestrian impact test consists of three test procedures and eachusing different subsystem impactors.

A legform impactor representing the adult lower limb to indicate lateral kneejointshear displacement and bending angle, and tibia acceleration, caused by contact ofthe bumper.

An upper legform impactor representing the adult upper leg and pelvis to recordbending moments and forces caused by contact of the bonnet leading edge.

Child and adult headform impactors to record head accelerations caused by contactwith the bonnet top [23] and [97].

In pedestrian leg impact test, a ‘legform’ impactor is propelled toward a stationaryvehicle’s longitudinal velocity of 40 km/h parallel to the vehicle’s longitudinal axis. The

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test can be performed at any location across the face of the vehicle; between the 30°bumper corners (Fig. 7).

Fig. 7. Pedestrian ‘legform’ injury criteria [23].

The above regulation is defined for bumper system not specifically for bumper beam, butsince the bumper is the main components in energy absorption, so the regulation can beextended for bumper beam. It might be used another mechanism between bumper beamand fascia to fulfill the pedestrian criteria.

6. Future trend in automotive bumper

The automotive safety and environmental legislations became more stringent in last fewdecades. On the other hand, the high production demand besides raising the cost of thepetroleum resources encourage the automaker to exploit the natural resources in theirnew products for staying in the competitive edge. Other impact for future bumper is newsafety regulation. There are totally two parallel trends for future bumper safety system.Passive safety which emphasize on helping to the occupants during the crashes byimproving he structure of the vehicles, bumper damping system, seatbelts and primarilyairbags and active system to prevent of crash by using intelligent mechanism.

Development of new material to comply with safety and environmental regulations, anddevelopment of various intelligent precrash systems to prevent or decrease the injury(pedestrian impact test) by fully automated controlled of car safety mechanism (Fig. 8)[99]. In first approaches, based on the environmental regulation, automakersinvestigated to utilize the natural fiber in their new product, but poor mechanicalproperties of natural fibers do not allow to be used in automotive structural components.There is plenty of research to improve the mechanical properties of natural fibers in orderto improve their performance. Increasing the interfacial adhesion between fiber andmatrix [100], incorporating Zdirection fibers (3D composite) [101], improving theproperties of the matrix [102], and manufacturing method [103]. The investigation trendsshows that in early future the natural fiber will be used in automotive structuralcomponents.

Fig. 8. Automotive intelligent precollision system [98].

Figure options

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In second approaches based on the EU Parliament regulation, automakers should usethe advanced emergency braking systems from November 2015 for all new light andheavy commercial vehicles. Also National Highway Traffic Safety Administration(NHTSA) began studying “lane departure warning systems” and “frontal collision warningsystems” on vehicles [104]. These recent investigations are an intelligent preventingsystem (precrash system) with noncontact safety mechanism to reduce the severity ofan accident. However, it is not more sufficient to prevent injury or damage at higheroperating speeds [105].

7. Conclusions

The new safety regulations concerning bumper system specifications besides theautomaker environmental legislations (end of life vehicle) make it quite complicated andmuch costly for the design of this structure to fulfill all broad requirements. Finding thebest procedure for bumper beam development poses extra loads on the designer andmay influence his/her performance. The previous research discussed the designprocess, material selection, and analysis and verification of the developed productfunctionality by FEM tools, but there is limited information about the approaches tobumper system development. The present study concentrated on the bumper beamdevelopment process based on findings of previous studies. It analyzed comprehensiveinformation to determine the optimum method for bumper beam development and toidentify the settings for the effective structural variables (thickness, rib strength, crosssection, and frontal curvature) conducive to the optimal bumper beam strength.

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A. Miravete

Article outline Show full outline

AbstractKeywords1. Introduction2. Bumper system3. Material selection steps4. Design process of bumper beam5. Analyzing the bumper beam

6. Future trend in automotive bumper

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