Improved Energy Absorber and Vehicle Design Strategies for Pedestrian Protection

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2005-01-1872

Improved Energy Absorber and Vehicle Design

Strategies for Pedestrian Protection

Stephen Shuler and Frank MooijmanGE Advanced Materials

Alok Nanda and Gopi SurisettyGE India Technology Center

Reprinted From: Vehicle Aggressivity and Compatibility in Automotive Crashesand Pedestrian Safety

(SP-1936)

2005 SAE World CongressDetroit, MichiganApril 11-14, 2005

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2005-01-1872

Improved Energy Absorber and Vehicle Design Strategies for

Pedestrian Protection

Stephen Shuler andFrank MooijmanGE Advanced Materials

Alok Nanda and Gopi SurisettyGE India Technology Cente

Copyright © 2005 SAE International

ABSTRACT

This paper presents the effect of finite element analysis(FEA) model improvements to better correlate predictiveanalyses to pedestrian protection lower leg impact tests.The FEA analysis model prediction is now within 10% ofthe tested values for tibia deceleration, knee bendingangle and knee shear. By using this improved FEAmodel, new, more efficient energy absorber and vehiclefront end design strategies can been developed. Anumerical approach to optimizing vehicle front endstructures is presented.

Figure 1. Computational analysis model of an injection molded energyabsorber, reinforcing beam and lower legform

INTRODUCTION

Pedestrian safety is now a high profile issue within theautomotive industry. Accident investigations show thatthere are three areas of the pedestrian’s body that aremost subject to injury, and each of these is associatedwith an area of the car. The knee and lower leg isusually injured through contact with the bumper.Investigations have shown that vehicle styling and

efficient front bumper design contribute a lot towards

lower leg impact protection. Schuster and Staines [11identified some of the styling factors and their likelycontribution in lower leg impact using a concept FiniteElement Lower Leg Form Impactor.

The European Union and the Japanese governmenhave both issued guidelines to assess the risk topedestrians from passenger cars during an accidentEuropean New Car Assessment Programme(EuroNCAP) has an ongoing program to test and ratemainstream vehicles available on the European markefor pedestrian safety [3]. It uses an approach that is

similar to the one being considered by the European

Union. Poor pedestrian impact results from testingconducted on mainstream vehicles that were notspecifically designed to meet the tests illustrate the needfor new design ideas that will meet the requirementswithout adversely affecting other performancerequirements.

Currently, front bumper systems on vehicles sold inNorth America are required to meet 4 km/hr FMVSS(Federal Motor Vehicle Safety Standard) pendulum andbarrier impacts [4] and 8 km/hr CMVSS (Canadian

Motor Vehicle Safety Standard) pendulum and barrierimpact requirements. In addition, most bumper systems

are also designed to meet 8 km/hr IIHS (the InsuranceInstitute for Highway Safety) 30° corner and flat barrieimpact. Front bumper systems on vehicles sold inEurope and Japan are typically designed to withstand 4km/hr ECE42 pendulum impact and 15 km/hr offseAllianz barrier impact. Future front bumper systems soldinto European markets will need to meet both ECE42and pedestrian impact requirements. In addition, manyglobal vehicle platforms will be sold in EuropeanJapanese and North American markets. This will requirea focus on vehicle structure and styling that is flexible



enough to meet all of the global legislative impactrequirements.

BACKGROUND

This paper describes bumper systems designed to meetthe European Enhanced Vehicle Safety Committee(EEVC) WG17 requirements for lower leg pedestrianimpact protection [1] (The EEVC was founded in 1970 in

response to the US Department of Transportation'sinitiative for an international program on ExperimentalSafety Vehicles. The EEVC steering committee,consisting of representatives from several EuropeanNations, initiates research work in a number ofautomotive working areas. These research tasks arecarried out by a number of specialist Working Groupswhich operate for over a period of several years givingadvice to the Steering Committee who then, incollaboration with other governmental bodies,recommends future courses of action designed to leadto improved safety in vehicles).

Previously [2], through impact test validation, a bumperenergy absorber injection molded inpolycarbonate/polybutyelene terephthalate, PC/PBT(XENOY® resin) was shown to be capable of providingWG17 level lower leg impact performance (Figure 1).The same energy absorber effectively managed theenergy from 4 km/hr barrier and pendulum impacts.This was achieved without any modifications to theexterior styling or base vehicle structure.

This dual performance car bumper system – 4 km/hrbarrier and pendulum combined with lower leg impactprotection was achieved through a combination ofmaterial properties and design. The difficulty indesigning such a system arises from conflicting bumpersystem requirements. In order to achieve lower legprotection, a relatively soft bumper system is requiredwhile a relatively stiff system is typically needed tomanage barrier and pendulum impacts.

LOWER LEG PROTECTION REQUIREMENTS

As illustrated in Figure 2, the current pedestrian safetyassessment procedure consists of several different teststhat represent the impact of the leg, upper leg and head.

The legform impact test typically involves the frontbumper, radiator grill, hood and headlights. In addition,some vehicle styles incorporate lower fascia structuresthat can have a significant effect on lower leg impact.

Figure 2: EuroNCAP pedestrian impact criteria tests

The legform impact test is carried out by impacting thelegform into the front of the vehicle as pictured in Figure3.

Figure 3: Lower leg impact test

Illustrated in Figure 4, the EEVC WG17 test has three

criteria to be met [5]:

1. Knee lateral bending angle <15°2. Lateral shear at the knee < 6 mm3. Peak deceleration at the proximal tibia < 150g



Figure 4: Leg impact criteria

ENERGY MANAGEMENT

Since a lower leg is likely to first contact the frontbumper, it is a key part of any discussion regardinglower leg impact, and – more specifically - the energymanagement capability of the bumper-energy absorbersystem. The faster the energy absorbing structureresponds to the impact event, the more efficient theenergy management and, therefore, the smaller thedepth of space needed to absorb the energy from theevent. Injection molded energy absorbers made fromPC/PBT have been shown to offer higher energyabsorption efficiency and a more consistent impactperformance over a range of temperatures, than manyconventional foam systems [6,7].

Figure 5 compares a typical load/displacement responseof an EPP foam and a PC/PBT energy absorber duringan 8 km/hr pendulum impact. The area under each

curve is the total amount of energy that is absorbedduring the impact. The same impact energy was appliedto both the PC/PBT and EPP foam absorbers. As figure5 depicts, the higher efficiency of the PC/PBT absorberallows the impact energy to be absorbed with less totalintrusion.

Figure 5: 8 km/hr, 1.315Kg Pendulum impact data for an EPP foam and

PC/PBT energy absorber.

To better understand how increased impact efficiencycan improve lower leg impact performance, the followingimpact energy balance equation can be applied tocalculate the impact efficiency of a bumper system:

(IILFLHQF\FH 'LV )RUFH&RPSOLDQFH 09 WDQ

=

Where M = vehicle mass and V = impact velocity. Thecompliance in this equation refers to the compliance ofthe vehicle during impact. The vehicle compliance is theamount of energy from the impact that is not imparted tothe bumper or energy absorber. This “lost” impacenergy is energy that is absorbed by tire roll, vehiclesuspension jounce, vehicle rebound, etc. From pasexperimental studies, vehicle compliance isapproximately 0.85 for barrier impact and 0.45 fopendulum impact [8]. PC/PBT energy absorbers have

been shown to reach efficiencies of up to 65% whilecurrent foam systems are approximately 45% to 50%efficient [6].

PREDICTIVE ENGINEERING ANALYSIS ANDCORRELATION

Utilizing finite element analysis (FEA) techniques formodeling the lower leg, pendulum and barrier impacts, aPC/PBT energy absorber design was developed for acurrent production 1800 Kg European sedan vehicleThe final design was then prototyped and impact testedThe goal was to design a bumper system that wouldmeet the dual requirements of lower energy leg impactsand higher energy pendulum and barrier impacts withinthe package spacing typical of today’s vehicle styling(70-100 mm).

The energy absorber design, depicted in Figure 6utilizes a “crush box” geometry. The crush boxes aredistributed along the length of the reinforcing beam. Bytailoring the length, width, height of the boxes and thewall thickness of the side walls, an efficient design wascreated that met a balance between acceptable tibiadeceleration, knee bending angle, and knee sheardisplacement with the ability to also manage 4 km/hbarrier and pendulum impacts.

Figure 6: PC/PBT “crush box” energy absorber.



A cut-away view of the FEA model used to predict theenergy absorber performance is shown in Figure 7. Theenergy absorber is shown positioned behind the frontvehicle fascia and in front of the reinforcing beam. Thefascia, grill, energy absorber and beam were all includedin the FEA model.

Figure 7: FEA model for lower leg impact

Commercially available LSDYNA finite elementcomputation code was used to analyze the energyabsorber performance during both the pedestrian legform impact as well as barrier and pendulum impact. AMAT_024 material model [12] (Mat-Piecewise-Linear-

Plasticity) including strain rate dependence was used torepresent the PC/PBT.

4 KM/HR ECE42/FMVSS IMPACT CORRELATION

The 4 Km/hr pendulum and barrier impacts wereconducted by mounting the beam/energy absorbersystem to an 1800 Kg test cart. The test setup for thependulum impact is shown in Figure 8. No fascia waspresent during these tests.

Figure 8: Test setup for 4 km/hr pendulum impact (1800Kg) on the

PC/PBT energy absorber.

Previous studies have shown correlations of more than95 percent between FEA predictions and FMVSSpendulum and barrier tests by incorporating appropriatematerial models for the PC/PBT resin blend [8]. Since

these type of impacts are well understood and havebeen previously studied, it follows that the FEApredictions for the 4 Km/hr pendulum and barrieimpacts are in close correlation to the tested impactsThe correlation is defined as accurately capturing no

only the peak loads and displacements, but also thegeneral shape of the Load-Displacement curve.

The energy absorber exhibited high efficiency inabsorbing the pendulum (62% efficient) and barrier (60%efficient) impacts. The predicted vs. test results aregraphed in Figures 9 and 10.

Figure 9: Predicted and tested results; 4 km/hr center pendulum impact

(1800Kg) on the PC/PBT energy absorber.

Figure 10: Predicted and tested results; 4 km/hr barrier impact (1800Kg

on the PC/PBT energy absorber.



LEGFORM IMPACT CORRELATION

In order to test lower leg pedestrian protectionperformance, the PC/PBT energy absorber was fittedonto the production sedan’s existing reinforcing beamand covered with the vehicle fascia. The impact testswere conducted using the industry standard WG17 legform. No modifications were made to the vehicleexterior or body structure.

Table 1 lists the impact test results and the FEApredictions for the legform tibia deceleration, knee shearand knee lateral bending angle. All tests andsimulations were conducted at the centerline of thevehicle, Y = 0mm position. Tests were conducted asdepicted in Figure 3, with the vehicle bumper coverattached and all grill and lighting in place on the vehicle.The vehicle was held stationary while the legform waslaunched at the vehicle front end.

PC/PBT Absorber

Y position = 0 (Center) Deceleration Shear AngleWG17 105G 1.9 mm 19.9 deg.

WG17_M1 101G 1.9 mm 15.0 deg.

WG17_M2 110G 3.5 mm 15.0 deg.

Test (A) 101G 3.6 mm 13.7 deg.

Table 1: Predicted and testing legform impact results using the PC/PBT

energy absorber.

The results listed in Table 1 are presented graphically inFigures 11, 12 and 13.

Figure 11: Predicted and tested results; Peak deceleration at the

proximal tibia.

Figure 12: Predicted and tested results; Knee lateral bending angle.

Figure 13: Predicted and tested results; Lateral knee shear.

FEA MODEL CORRELATION DEVELOPMENT

The FEA simulation model was initially analyzed utilizingthe purchased CAE legform model without any

modification (Denoted as “WG17” in Figures 11, 12, and13). It was found that while the predicted legdeceleration was accurate, the model significantlyunderpredicted the knee shear and overpredicted theknee bending angle. A main-effects design oexperiments was then utilized to investigate several ofthe key components that comprise the FEA model of thelegform and energy absorber. This Screening DOE [13

shows the effect on the response when a factor ischanged from its low level to its high level. Theseincluded the leg inner and outer foam cover, the kneeligaments, the shear springs and leg damper of the

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legform. Engineering analysis of the effect of modifyingthese components yielded 2 independent legformmodifications that improved the correlation. The kneeelements stiffness was modified to improve thecorrelation of knee lateral bending angle prediction(WG17_M1). The shear spring was modified to havethe additional effect of improving the lateral knee shearcorrelation (WG17_M2). The WG17_M2 modifiedlegform model provided the best balance of tibia

deceleration, bending angle and shear correlation to thetested data.

NEW VEHICLE DESIGN STRATEGIES

With the improved correlation between test data andFEA prediction now established, the new FEA modelcan now be used to investigate the effects of vehiclestyling on pedestrian protection performance. A designfor Six Sigma (DFSS) approach was used to study howthe height and shape of a vehicle affects the lower legimpact. DFSS is a statistical pro-active approach fordesigning products and processes to achieve Six Sigma

quality.

Some of the key steps adopted therein are shown inFigure 14.

Figure 14: Design for Six Sigma (DFSS) Methodology

In order to compress the analysis process, arepresentative but simplified vehicle front end wasdeveloped. This model included a beam, PC/PBTenergy absorber, fascia, upper spoiler (hood, grill) andlower spoiler (Figure 15).

Figure 15: Idealized vehicle front end.

Since the diameter of the lower leg impactor is only 130mm, the sweep of the beam was neglected. The beamwas modeled as rigid (beam deformation is not usuallysignificant during impact). These approximations helpedto reduce the required modeling time withoucompromising the accuracy significantly. The newlymodified (WG17_M2) leg form impactor was used fothe analysis.

Some initial runs were performed to decide the boundaryconditions on the representative fascia, limiting theenergy absorbed by fascia to 15-20% as has beenobserved in actual cases. In addition, the edge effectswere eliminated by using appropriate length of theenergy absorber. The energy absorber package spacewas set to a low level of 40 to 60 mm to better match theindustry’s desire for reduced bumper offsets.

Key vehicle styling variables (X1 to X4), representing theouter front geometry of the car were parameterized asshown in Figure 16. For the study described here

Factors B, C, E, F and G were taken as constant. ThePC/PBT energy absorber design was similar to the onedescribed above and depicted in figure 6. This energyabsorber design was parameterized by the energyabsorber design features X5 to X10. Suitable rangeswere chosen for these variables to utilize the uniqueproperty of injection molded EAs to have tailor madestiffness.



Figure 16: Factors and their range

NUMERICAL SIMULATION

To investigate the design space optimally, a Design ofExperiments (DOE) was employed. First, a screeningDOE (Resolution III DOE, 2 levels, 16 runs) [12] was set

up and impact simulations were performed to choose the

variables having a statistically significant effect onpedestrian protection requirements over the chosenranges. The screening DOE set up is shown in Table 2.

Table 2: Screening DOE Set-up

The FEA impact analysis was performed for each of thesixteen combinations of the DOE. The resulting trendsfrom the screening DOE are shown in Figure 17. Theup/down direction of the arrows indicates the effect ofincreasing the value of the X variable. For example, byincreasing the value of variable X4, the deceleration andshear decrease whereas the bending angle increases

Lateral arrows indicate that a change in the value of theX variable has no effect.

Figure 17: Trends plot from screening DOE

All ten factors turned out to be statistically significantTherefore, all of the vehicle’s styling variables wereretained for further analysis. An increase in the value othe two energy absorber design variables, X9 and X10resulted in either a reduction or no change in all threeparameters governing pedestrian protection (ie areduction in deceleration, shear and bending angle)



Therefore, to simplify the analyses, X9 and X10 weresubsequently fixed at the highest value of their range.Also, the operating range of the energy absorber designvariable X6 was increased to vary from 0.5 to 0.9 inorder to enlarge the design space.

Having obtained a new list of significant factors (X1, X2,X3, X4, X5, X6, X7 and X8), a higher resolution optimalDOE was designed with these variables (Q-optimal, 50

runs, varying levels). Previous investigations haveshown that the relationship between variables andPedestrian protection requirements is non-linear with avarying order of non-linearity [2]. Based on this

experience, different levels (within the chosen range)were chosen for different variables such as, three levelsfor packaging space X1 (40, 50 and 60 mm) and fivelevels for vehicle height with respect to the energyabsorber centerline X3 (-75, -50, -25, 0 and 25 mm) etc.Impact analyses were performed for each of the DOEsetting and the values of tibia deceleration, knee shearand knee bending angle were captured. A main effectschart is plotted in Figure 18 - depicting the effect on the

response when a factor is changed from it’s low level toits high level. The values therein, have been normalizedto depict the trends.

Figure 18: Main effect plots

Regression is the science of curve and surface fittingRegression tools provides an approximate means ofobtaining the relationship between the response andfactors that influence it when true relationship isunknown or cannot be extracted from physicaprinciples. Statistical regression tools were used togenerate a transfer function between the pedestrianprotection requirements and the variables i.e.

Deceleration = f(X1, X2, X3, X4, X5, X6, X7, X8) Shear = f(X1, X2, X3, X4, X5, X6, X7, X8) Bending angle = f(X1, X2, X3, X4, X5, X6, X7, X8)

Several FEA validation runs were also performed. Amarginal error of 5-15% was observed for decelerationand slightly more for shear and bending angle.

With these transfer functions in hand, variousoptimization strategies are available depending on the

need; e.g. one can find the energy absorber designparameters for minimum deceleration, shear andbending angle keeping car geometry factors at a desiredconstant value. Another strategy could be to keep anyone or all-styling factors also as variables, then find theconfiguration for best pedestrian protection performanceIn this way, we can assign specific values to the cageometry variables as defined by the stylingrequirements, then carry out an optimization to find thebest energy absorber geometry. One such strategy andthe results obtained from it are shown in Figure 19Here, an optimized energy absorber design wasobtained for a car with packaging space X1 of 60 mmand impact height with respect to the energy absorbecenterline X3 of 25 mm. The other car styling variables,namely, upper and lower spoiler locations, and, theenergy absorber design variables were treated asvariables.

Figure 19: Optimum energy absorber design for 60 mm packaging spa



RESULTS

Previously, an injection molded PC/PBT energyabsorber was designed and manufactured todemonstrate that it is possible to meet, and exceed, theWG 17 directives within a packaging space typical oftoday’s vehicle styling. In addition, the same energyabsorber design was shown to effectively handle 4 km/hrpendulum and barrier impacts. In order to have

confidence in designing future energy absorbers,accurate FEA models must exist. Through previousinvestigations [8], pendulum and barrier impact FEAmodeling correlation has been well established.Through this most recent work, the correlation betweenFEA modeling of a PC/PBT energy absorber and testeddata from pedestrian legform impact on a vehicle hasbeen improved to within 10% for the tibia deceleration,knee bending angle and knee shear.

Utilizing the newly correlated FEA modeling technique, aDFSS process was followed to assess the contributionof car styling parameters on pedestrian protection

performance. This method can provide vehicle stylistand engineers with a way to make an initial assessmentregarding the car styling design direction and the energyabsorber capabilities. The process can be used topotentially reduce design cycle time by assisting early ina vehicle’s development cycle.

Study of the main effect yields several useful guidelines.The location of a lower spoiler front face with respect tothe energy absorber front face plays a significant role incontrolling knee bending angle. If the leg form impactorhits the lower spoiler early in the event, almost at thesame time as the energy absorber, then the lowerportion of the leg does not rotate inwards and finalbending angle values are significantly lower. On theother hand, the simultaneous contact with the lowerspoiler and energy absorber introduces higherresistance for leg and hence an increased deceleration.The balance of energy absorber and lower spoilerstiffness becomes extremely critical to meet both kneebending angle and deceleration requirements. The effectbecomes more pronounced if the package space isreduced, since the impact energy needs to be absorbedwith less leg intrusion. Tailor made stiffness can beachieved for injection molded energy absorbers by using

appropriate design features as shown by the spread inthe main effects of the energy absorber designparameters. This unique ability of these EAs can providecar stylists greater design freedom.

As the vehicle height increases, the knee bending anglegenerally increases. By choosing to do the analysis atmany levels, including a case where the impact happens

exactly when knee is located in front of the energyabsorber centerline, an interesting pattern emerges asshown in the main effect plot of variable X3. The steepshift in deceleration, shear and bending angle in caseswhere impact happens when knee is located in front ofthe energy absorber centerline suggests that we need tobe careful when operating in this region.

CONCLUSIONS

The current vehicle design trend is to minimize bumpeoffset. This requires a reduction in the overall energyabsorber package space between the reinforcing beamand fascia. A higher efficiency energy absorber canserve to minimize this space and still meet bumpeimpact and pedestrian protection requirementsInjection molded PC/PBT energy absorbers have beenshown to have higher impact efficiencies than traditionafoam absorbers and can help enable the design of loweoffset bumper systems.

Often, to assist in the up-front evaluation of such a

bumper system, predictive engineering FEA models areused to assess the how the bumper system will performunder impact loads. Therefore, accurate FEA modelingtechniques are needed to achieve the most optimizedbumper systems and energy absorbers. By modifying acommercially available pedestrian impact legform FEAmodel, the predicted leg impact performance for thePC/PBT energy absorber system is now within 10%agreement with the tested values for tibia decelerationknee bending angle and knee shear.

Utilizing this improved FEA model, a strategy for vehiclefront end design was described. Through the use of anumerical design of experiments investigation, mainvehicle styling parameters of energy absorber packagingspace, vehicle ride height, positon of upper hood or griland lower spoiler position were assessed for their effectson pedestrian protection performance. Each of thesewas shown to be important and interrelated indetermining the overall pedestrian protection capabilityof a vehicle.

Finally, the energy absorber design parameters andmain vehicle styIing cues were parameterized andtransfer function equations were developed between

them and the pedestrian protection lower legperformance. Following from this, we now have apredictive tool that can quickly generate potential energyabsorber designs for different car geometries. Theenergy absorber design generated by this method givesa good starting point for energy absorber developmentfor an actual car and significantly reduces the iterationsrequired to come up with a final design

Future work will include additional investigations on theeffects of whole-vehicle body structure on pedestrianperformance. This will include the study of upper leg



and head impact events. In addition, there have beenseveral recent efforts focused on how mechanicallegforms such as the one described above relate toreal-life leg injury during impact [9] [10]. This paper has

focused on lowering the deceleration, bending anglesand knee shear on the EEVC WG17 legform since theseare the current measurements imposed on Europeanautomobile manufactures. In the future, the authorsexpect updated injury test protocols and measures

based on an increased understanding of how to modelreal-life injury.

XENOY is a registered trademark of General Electric

Company.

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REFERENCES

1. “Improved Test Methods to Evaluate PedestrianProtection Afforded by Passenger Cars” 1998 EEVCWorking Group WG17 report

2. S. Shuler, F. Mooijman and A. Nanda. “BumperSystems Designed for Both Pedestrian Protectionand FMVSS Requirements: Part Design andTesting,” SAE paper 2004-01-1610.

3. More information can be found at: www.etsc.be4. More information regarding bumper test procedures

can be found at the NHTSA web site at:http://www.nhtsa.dot.gov/cars/testing/procedures/TP-581-01.pdf

5. More information can be found at the EEVC web site(www.eevc.org) “EEVC Working Group 17 Report -Improved Test Methods To Evaluate PedestrianProtection Afforded By Passenger Cars (December1998 with September 2002 updates)”

6. D. Evans and T. Morgan. “EngineeringThermoplastic Energy Absorbers for Bumpers,” SAEpaper 1999-01-1011.

7. D. Evans, S. Shuler, S. Santhanam, “Predicting theBumper System Response of Engineering

Thermoplastic Energy Absorbers with Steel Beams,”SAE paper 2002-01-1228.

8. D. Evans, “Correlation Study on Different BumperImpact Test Methods and Predicted Results” SAEPaper 2003-01-0211.

9. D. Bose, K, Bhalla, L. VanRooij, S. Millington, AStudley, J. Crandall “Response of the Knee Joint tothe Pedestrian Impact Loading Environment,” SAEPaper 2004-01-1608.

10. A. Konosu, M. Tanahashi, “Development of an FEFlexible Pedestrian Leg-form Impactor (Flex-PL2003R) Model and Evaluation of its Biofidelity.” SAEPaper 2004-01-1609.

11. P. Schuster, B. Staines, “ Determination of BumpeStyling and Engineering Parameters to ReducePedestrian Leg Injuries,” SAE 980361.

12. LS-DYNA Keyword User’s Manual, April 2003Version 970.

13. M. Kiemele, S. Schmidt, R. Berdine, “BasicStatistics Tools for Continuous Improvement”, BookAir Academy Press



14.Definitions, Acronyms, Abbreviations

EA: Energy Absorber

CAE: Computer Aided Engineering

FEA: Finite element Analysis

PC/PBT: Polycarbonate/Polybutylene Terephthalate

DOE: Design of Experiments

EPP: Expanded Polypropylene

ETP: Engineering Thermoplastic

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Improved Energy Absorber and Vehicle Design Strategies for Pedestrian Protection