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SAE TECHNICAL

PAPER SERIES 2004-01-0841

CAE-Based Side Curtain Airbag Design

Honglu Zhang, Deren Ma and Srini V. RamanDelphi Corporation

Reprinted From: Air Bags and Belt Restraints(SP-1876)

2004 SAE World CongressDetroit, MichiganMarch 8-11, 2004

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ABSTRACT

Since its invention in early 1990s, the side curtain airbaghas become an important part of the occupant restraintsystem for side impact and rollover protection. Computer

Aided Engineering (CAE) is often used to help sidecurtain airbag design. Because of the uniquecharacteristics of side curtain airbag systems, thesimulation of side curtain airbag systems faces differentchallenges in comparison to the simulation of driver andpassenger airbag systems.

The typical side curtain airbag CAE analysis includes, butis not limited to, cushion volume evaluation, cushioncoverage review, cushion shrinkage and tension forcereview, deployment timing review and seam shape andlocation review. The commonly used uniform pressureairbag models serve the purpose in most cases. But for analysis such as cushion deployment timing evaluation,Computational Fluid Dynamics (CFD) integrated airbag

models have to be employed because the uniformpressure airbag models are not able to simulate thedeployment time difference among various parts of theairbag cushion. In this paper, most types of the above-mentioned analysis are integrated into a typical sidecurtain airbag design cycle. Both uniform pressure airbagmodels and CFD integrated airbag models are usedcomplementarily to help the side curtain airbag design.The CAE-based side curtain airbag design procedurehelps to reduce the material cost and time duration of theairbag design process.

INTRODUCTION

Side impact represents one of the most important crashmodes in the field. In year 2001, 33.3% of fatalities and28.6% of injuries of passenger car occupants werecaused by side impacts; 19.7% of fatalities and 25.7% of injuries of light truck occupants were due to side impacts[1]. Side curtain airbag systems have been applied as acountermeasure in many vehicles. After deployment, theairbag covers the side windows of the vehicle like acurtain, as the name indicates, to avoid the direct contactof the occupant’s head to the window glazing and outsideobjects. It also helps retain the occupant from ejection ina rollover accident.

Compared with driver and passenger airbag systemsside curtain airbag systems have some uniquecharacteristics. First of all, unlike frontal crashes, sidecrashes involve considerably less crush space betweenthe point of impact on the striking vehicle and theoccupant. This limited crush space increases therequirements to the airbag deployment sensing systemsand inflator timing. Currently, side impact sensingsystems generally discriminate a crash condition within6-13 milliseconds as compared to 15-25 milliseconds fofrontal impact sensing systems. Airbag inflation time forside impact airbags is also less and ranges between 20and 30 milliseconds for side airbags as compared to 45milliseconds for frontal airbags [2]. Secondly, side curtainairbags have to cover one or more side windowstherefore they usually have a large coverage areaInflator gas has to travel a long way to fill all the airbagchambers. The inflation process causes unevendistribution of the inflator gas and different filling time fothe airbag chambers. To accelerate the inflation and

optimize the gas distribution, it is a common practice touse a diffuser tube in the top portion of the airbag. Thediffuser tube design is critical to the performance of theside curtain airbag systems.

Throughout the side curtain airbag design processnumerous tests are needed, including the airbag staticdeployment tests, free motion head form pole impacttests with linear impactor and system-level sled or barriertests with real vehicle structure and dummy, etcTraditionally, hardware design engineers think out aconcept, build hardware prototypes and conduct thetests, modify the design based on test results, then re-

test. The whole process is driven by tests. Hardwaretests are often expensive. Building prototypes can alsobe time consuming. To help shorten the design cycle andcut material costs, Computer Aided Engineering (CAE)especially mathematics-based simulation is proved to bea good alternative of the hardware tests [3,4,5]. In thispaper, a side curtain airbag design procedure based onCAE rather than hardware tests is demonstrated.

The most commonly used airbag simulation modeassumes uniform pressure and temperature everywhereinside the airbag. This is a close representative of theairbag after it is fully inflated and the gas flow inside the

2004-01-0841

CAE-Based Side Curtain Airbag Design

Honglu Zhang, Deren Ma and Srini V. RamanDelphi Corporation

Copyright © 2004 SAE International

8/8/2019 2004-01-0841


airbag stabilizes. For free motion head form impactsimulations, the head form usually impacts the sidecurtain airbag after its full inflation. The uniform pressureairbag models serve the simulation purpose adequately.However, for the studies such as side curtain airbagdeployment timing and inflation kinematics, the uniformpressure model is not a sufficient tool. ComputationalFluid Dynamics (CFD) integrated airbag model has to beemployed for this type of analysis. The drawbacks of theCFD integrated airbag models are the numerical

instability and expensive computer CPU time. CFDintegrated airbag models often take at least 10 timesmore CPU time than the uniform pressure airbag modelsdo. In this study, both uniform pressure airbag modelsand CFD integrated airbag models were usedcomplimentarily to help the side curtain airbag design.

CAE-BASED SIDE CURTAIN AIRBAG DESIGN

PROCEDURE

Figure 1 shows a flow chart of CAE-based side curtainairbag design procedure. The process starts from acushion design draft, then CAE is used extensively to

analyze and improve cushion design. The analysisincludes, but is not limited to, cushion volume evaluation,cushion coverage review, cushion shrinkage and tensionforce review, deployment timing review and seam shapeand location review. CFD integrated airbag models haveto be employed for cushion deployment timing analysisbecause the uniform pressure airbag models are notable to simulate the deployment time difference amongvarious parts of the airbag cushion.

Worthy of mention, the airbag fabric stress distributionduring the airbag deployment can be very different fromthat after the airbag fully stabilizes. In many cases a

seam breaks before the airbag is fully inflated. Under such conditions CFD integrated airbag models seem abetter analysis tool. However, due to the immaturity of CFD integrated airbag models, the correlation of thepressure distribution is often not satisfactory, thereforethe quality of stress analysis from CFD integratedmodels is not necessarily better than that from uniformpressure models. Here an analyst’s engineering

judgment is crucial.

If a cushion design passes all these simulationcheckpoints, prototypes can be built and tested. Thetests are mainly used to confirm the simulation results.

The design evolves based on CAE simulations rather than on numerous tests.

After successful cushion static deployment, a FreeMotion Head form (FMH) linear impact is simulated. Afew hardware tests may be needed to correlate themodel unless strong confidence has been built fromsimilar models. A parameter study of the linear impactmodel with different inflators and different impactlocations is often conducted to help select inflators andfurther improve cushion design. Upon the completion of CAE analysis, some tests are necessary to confirm the

simulation results. This concludes the component levedesign and analysis.

Figure 1 A Flow Chart of CAE-based Side Curtain AirbagDesign Procedure

N

Y

Cushion design draft

• Cushion volume evaluation

• Cushion coverage review

• Shrinkage & tension force review

• Deployment timing review (CFD analysis)

• Cushion seam shape & location review(stress analysis and/or CFD analysis)

Design change recommendations

FMH linear impact simulation

FMH impact test for designconfirmation

Build simulation model

Is design good

Cushion static deploymenttest for design confirmation

Inflator selection recommendation and further cushion design improvement by simulation

System-level side impact simulation

System optimization by CAE

Sled test for system model correlation

FMH im act test for model correlation

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From Figure 4, the general stress distribution of thisairbag cushion seems good. No severe stressconcentration was observed. However, there was somelevel of stress concentration at a few small areas.Recommendations about how to avoid them wereprovided to the design engineers.

To fully explore the potential of CAE-based side curtainairbag design, a CFD model was also set up to simulatethe gas flow inside the cushion. At this stage, no diffuser tube had been considered, so a tubeless airbag wasmodeled. Figure 5 and 6 represent the pressure profileswithin the airbag cushion at different time of the cushiondeployment.

Figure 5 Pressure Profile at Early Stage of the CushionDeployment

Figure 6 Pressure Profile at Late Stage of the CushionDeployment

From Figures 5 and 6, it is obvious that there waspressure concentration at different locationscorresponding to different stages of the cushiondeployment. Even though the stabilized stressconcentration was not severe, the airbag may burst outduring the deployment process due to transient stressconcentration as a result of uneven pressure distribution.

The physical test results confirmed the simulationprediction. The airbag cushion seams (shown in Figure7) were broken at the area corresponding to one of thehigh-pressure locations in Figure 5.

Figure 7 Cushion Torn Areas

Another important result from the CFD simulation is thedeployment timing. The simulation showed that without adiffuser tube to guide the inflator gas, the deploymenttiming cannot satisfy the requirement of side impacprotection. Because of module weight restriction, it is nopreferable to use a long diffuser tube to distribute theinflator gas in this case. The improvement of seampattern design, faster inflator mass flow rate and a shortdiffuser tube with non-uniform diameters and othestructure details were recommended.

Based on the results of the extensive CAE analysis, anew version of the cushion was designed and virtuallytested by simulation models. After a few iterations ofCAE analysis, the airbag cushion static deployment wassatisfactory. Figure 8 shows the coverage zone of thenew version bag, with all occupant sizes and seatingpositions considered. The airbag seems to cover theprotection zone of most cases sufficiently.

Figure 8 Coverage Zone of the New Version AirbagCushion

Figure 9 depicts the stress concentration pattern of thenew version airbag. The stress concentration factor iswithin a safe range.

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Figure 9 Stress Concentration Analysis of the NewVersion Airbag Cushion

The airbag deployment timing is also improved with thenew cushion design and a short diffuser tube. Theoverall static deployment performance of the new versionairbag meets the design requirements. Then thehardware prototype was built and physical tests wereconducted to confirm the simulation results. Throughoutthe process CAE is driving the design direction. Physicaltests were used only to confirm the design at the end of the CAE simulated iterations.

The next step is to simulate the FMH linear impact tests.The main purpose of the FMH linear impacts tests is to

provide recommendations for inflator selection andfurther cushion design improvement. It is required thatthe airbag can provide enough protections for theoccupant’s head at all possible impact locations.

The model setup of the FMH linear impact simulation isshown in Figure 10.

Figure 10 FMH Linear Impact Model

A parameter study with 3 different inflators, includingboth hot gas inflators and cold gas inflators, and 5different impact locations corresponding to variousoccupant sizes and seating positions was performed.

The 5 impact locations were marked in Figure 11.

The maximum head form accelerations of the variouscases are presented in Figure 12. Figure 13 is a two-dimensional projection of the three-dimensional surface.

Figure 11 Different Impact Locations for FMH Linear

Impact Simulations

Loc 1Loc 2 Loc 3

Loc 4Loc 5

I n f l a t o r 1

I n f l a t o r 2

I n f l a t o r 3

0

50

100

150

200

H e a d

A c c e l e r a t i o n ( g )

Maximum Head Acceleration

Figure 12 Maximum Head Accelerations at DifferentImpact Locations with Various Inflators

Loc 1 Loc 2 Loc 3 Loc 4 Loc 5Inflator 1

Inflator 2

Inflator 3

Maximum Head Acceleration Contour

Figure 13 Maximum Head Acceleration Contour (2-DProjection)

Besides the maximum head accelerations, the HeadInjury Criterion (HIC) [6] values were also evaluated(Figure 14 and 15)

14

5

32

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Loc 1Loc 2

Loc 3Loc 4

Loc 5

I n f l a t o r 1

I n f l a t o r 2

I n f l a t o r 3

0

500

1000

1500

H I C

HIC

Figure 14 HIC Values at Different Impact Locations withVarious Inflators

Loc 1 Loc 2 Loc 3 Loc 4 Loc 5

Inflator 1

Inflator 2

Inflator 3

HIC Contour

Figure 15 HIC Value Contour (2-D Projection)

In this model, both maximum head accelerations andHIC values show very similar patterns at different impactlocations with various inflators. From the figures, it isclear that impact locations 1 and 2 can provide the bestprotection for the head form, location 4 can also providesufficient protection in all the cases. Location 3 hasrelatively high head acceleration and HIC. Only inflator 1and 3 work well in location 3. Location 5 is the worstimpact location. High airbag pressure was required to

protect the head form at this impact location. With allcases considered, inflators 1 and 3 were recommendedbased on the simulation results. Suggestions aboutcushion design improvement on impact location 5 werealso proposed. Then a few more iterations of CAEanalysis were performed before a final satisfactoryairbag design was achieved and confirmed by thephysical tests.

Due to the complexity of the system-level side impactmodels, the procedure of system-level side impactsimulation and optimization is omitted from this paper.

DISCUSSIONS

The CAE-based side curtain airbag design procedurediscussed in this paper shows great potential to reducethe material cost and time duration of the airbag designprocess. Besides the cost savings, the CAE-basedapproach also provides the following benefits comparedwith the traditional test based approach:

1. The test based approach is a trial and erroprocess per se. With CAE models, especially theCFD simulations, deeper understanding andinsights to the physics of the design problemswhich are very hard to obtain through physicatests, can be achieved. In other words, CAEhelps to understand “how the design works andwhy it works”.

2. The test based approach often ends up with aworking design. But it is not always the optimadesign. With a CAE-based approach, Design ofExperiments (DOE) and other numericaoptimization tools can be used to optimize a

design.

3. The CAE experience learned from previousdesigns can be easily transferred to newproducts.

However, the CAE-based approach also faces somechallenges. The reliability of the numerical simulationresults remains the biggest concern for actual producdesign. So far moderate to high confidence level can beachieved for analysis such as cushion volume evaluationand FMH linear impact simulation. But the confidencelevel for the fluid dynamics related analysis such as

diffuser tube simulation is relatively low. More efforts arerequired to improve both the CAE simulation tools andthe numerical models.

REFERENCES

1. Traffic Safety Facts 2001: A Complication of Moto

Vehicle Crash Data from the Fatality Analysis

Reporting System and the General Estimates

System, National Highway Traffic Safety

Administration, National Center for Statistics an

Analysis, U.S. Department of Transportatio

December 2002.

2. An Innovative Inflator Designed to Reduce InflationTime for Roof Rail Airbags, Gregory J. Scaven

David Shilliday, Greg Mowry, Airbag 2002, 6t

International Symposium and Exhibition on

Sophisticated Car Occupant Safety Systems

December 2002.

3. Advanced Methods for Inflatable Curtain

Development – the Virtual Approach, Gerald

Krabbel, Kurt Fograscher, Autoliv GMbH, Airbag

2002, 6th

International Symposium and Exhibition on

Sophisticated Car Occupant Safety Systems

December 2002.

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4. Development of a Curtain Air Bag System for

Reduction of Head Injury During Side Impacts,

Deren Ma, Jeff Welch, Delphi internal report, 2000.

5. State-of-the-Art Side Airbag Modeling and Its

Application in Occupant Safety in Lateral Collisions,

S. Vaidyaraman, H. Khandelwal, C. Lee, J. Xu, A.

Nayef, SAE paper 980915.

6. U.S. Department of Transportation, Nationa

Highway Traffic Safety Administration, 49 CFR Parts

571, 572, and 589, [Docket No. 92-28; Notice 7]

[RIN No. 2127-AB85], Federal Motor Vehicle Safety

Standards; Head Impact Protection, p443, 1995

(http://www.nhtsa.dot.gov/cars/rules/rulings/12162.m

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