Correlation of airbag fabric material mechanical failure...

Correlation of Airbag Fabric Material Mechanical Failure Characteristic for Out of Position Applications

C. Bastien1, M. V. Blundell

1, D. Stubbs

1, J. Christensen

1, J. Hoffmann

2, M. Freisinger

2, R. Van Der

Made3

1 Coventry University, Department of Engineering and Computing

Priory Street, Coventry CV1 5FB, UK

2 Toyoda Gosei Europe NV

Ernst-Abbe-Str. 3, D-66115 Saarbrücken

3 Tass

Einsteinlaan 6

2289 CC Rijswijk, The Netherlands

Abstract The current paper will discuss airbag fabric mechanical failure characterisation in order to improve

simulations of airbag behaviour for out-of position (OoP) occupant scenarios. This failure mechanism is

necessary to represent the controlled failure of airbag internal tethers when drivers of different mass and

stature are slumped over the steering wheel, thereby contacting the airbag module at the start of the airbag

deployment. The high deployment forces associated with these scenarios are caused by the increasing gas

pressure inside the airbag [1].

The methodology discussed involves the execution of physical static and dynamic tensile tests of an airbag

fabric sample in warp, weft and bias directions. The airbag fabric permeability is derived and airbag

module cover, tear seam rupture tests are performed. Mathematical simulations of airbag fabric with non-

linear fabric material characterisations are correlated to the test results. The physical airbag fabric failure

is compared to various computer simulation techniques. The correlated airbag fabric models are used in

simulations with 5th percentile female, 50

th and 95

th percentile male anthropomorphic test devices (ATD’s)

to demonstrate the pressure build up in the airbag chambers and provide additional knowledge of this

phenomenon [2].

If a driver occupant exerts an extreme breaking intervention existing of horizontal deceleration, a 50th

percentile ATD could end up in a sort of an OoP scenario with a posture close to the driver airbag module

at the moment of airbag ignition. The presented study prepares this combination of active and passive

safety (integrated safety) exploring the advanced driver airbag model behaviour by varying the occupant

types (size) during initial OoP deployment.

1 Introduction

Extensive studies have shown that the airbag deployment in OoP loadcases (based on FMVSS208

legislation) consists of two occupant loading phases: a punch-out effect where the airbag bursts out of its

container with the airbag and airbag module cover accelerating towards the occupant, and a second

loading phase during which the airbag is taking on its deployed shape and volume (membrane-loading

effect). The loading associated with the airbag deployment creates injuries mainly in the head, neck and

chest areas.

The mitigation of injuries associated with OoP scenarios have led to the low-risk development or “smart”

airbags. Some advanced airbags contain multi-chambers and sacrificial tear seams in efforts to reduce the

punch-out and membrane-loading effect [2], [3]. Mathematical simulations of smart airbags use very

complex techniques such as, fluid mechanics (Gas Flow) to describe the inflator gas flow (pressure

1679

gradient) and the initial airbag inflation, and improve the representation of the pressures within the airbag.

For in-position scenarios it is sufficient to represent the gas inside the airbag using a uniform pressure

approach since the airbag is fully deployed when the occupant interacts during the crash. It has been

shown that a driver slumped over the steering wheel and hence the driver airbag module, increases the gas

pressure inside the airbag leading to higher deployment forces [3], which creates a need to improve the

accuracy of the airbag representation, in particular the airbag fabric material characteristics and its ability

to accurately replicate the rupture of sacrificial tethers in order to simulate the airbag pressure

characteristics.

The advanced method used for simulating the sacrificial tear seam or “control chamber” failure presented

in this study is based on an improved mechanical failure model [4]. Whilst this provided relatively good

levels of correlation in terms of occupant injuries for particular scenarios, the method is considered to be a

predictive method.

The control chamber rupture does not occur at the same time for all applications: the airbag working

pressure is different in the case of single or dual stage inflator deployment and, different loading scenarios

lead to different airbag working pressures which in turn cause the control chamber failure to occur at

different times.

2 Airbag Fabric material characterization

The airbag fabric described in this report is uncoated, 350 decitex, for which the characteristics are

obtained by performing tensile test and shear panel test.

Figure 1: Typical deployed airbag after the sacrificial tether failure

Obtaining accurate airbag material characteristics is critical to the airbag deployment kinematics and

determination of the exact tether rupture load (figure 1).

2.1 Uni-axial tensile tests

To determine the characteristics of the airbag fabric material, a number of airbag fabric sample tensile

tests were carried out. The test data used in this study was taken from material tests with 100 mm by 50

mm fabric samples, with tests conducted at speeds of 100 mm/min and 1 m/s, with 100 mm/min

representing the “static” load case and 1 m/s representing a “dynamic” load case.

In order to take into account the strain rate properties of the fabric material, MADYMO has the possibility

to add a strain rate function to the material behaviour [9]. Two functions were investigated in this study:

Cowper-Symonds and Johnson-Cook strain rate functions. Both functions relate the yield stress to the

strain rate, and include a further amplification factor which indicates how strongly the strain rate effect

works on the yield stress hardening. The study showed that the Johnson Cook strain rate dependency

function provided the best level of correlation [4] (figure 2).

1680 PROCEEDINGS OF ISMA2010 INCLUDING USD2010

Warp tensile tests Weft tensile tests

Figure 2: Correlation of airbag fabric warp (left) and weft (right) stress-strain characteristics

2.2 Bi-axial tensile tests

A series of bi-axial tests, referred to as picture frame tests, were used to investigate the shear stiffness of

the airbag fabric. The shear stiffness is a function of shear stress against shear strain. The picture frame is

essentially a pin jointed frame, clamped around the airbag fabric, fixed at the bottom corner and loaded at

the top corner, as shown in below.

The airbag fabric is clamped in the frame so that the warp and weft run parallel with the sides of the

frame. In this way, displacing the top corner of the picture frame applies a shear force to the fabric (figure

3).

DYNAMIC TESTING: METHODS AND INSTRUMENTATION 1681

Frame dimensions The picture frame test set-up

Figure 3: Picture frame test schematic and shear panel representation of the picture frame

The airbag fabric shear stress-shear strain characteristic, derived from the static picture frame tests is

shown in figure 3. The derived data (equation 1) was therefore reflected to produce a symmetric function

(figure 4).

165.0

24cos165.0

cos22

2 1

l

F = applied load

d = depth of shear panel

t = thickness of panel

l = length increase

dt

F

cos

Shear strain derived equation Shear stress derived equation

Equation 1: Derived shear stress – strain equations

Figure 4: Correlation of airbag fabric shear stress-shear strain characteristic

The current computer models are using the MATERIAL.FABRIC_SHEAR [7] Madymo material model.

From the tests curved computed, it can be seen that these fabric models are non-linear. Looking closely to

the picture frame test, the response is initially linear and then parabolic, which is due to fabric blockage/

locking effect at higher forces

The main airbag material characteristics are now defined and correlated.

ß

165 mm

165 mm

F

z


2.3 Material Failure tests

Further static tests were carried out on 100 mm by 50 mm fabric samples with a “perforation” across the

width of the fabric at the mid length. The perforation is used to control the point at which the airbag fabric

fails and to reduce the load at which it fails. A perforation could be implemented in the airbag “control

chamber” to enable tuning of the airbag deployment (figure 5).

Figure 5: Test setup and computer correlation of cloth tearing (warp left, weft right)

Using MADYMO MATERIAL.INTERFACE [7] elements, it is possible to investigate the stress level

needed to fail the sample. This threshold is described as the ultimate normal traction force and is defined

as the force per unit area, where the area of the tear seam is defined as the length of the tear seam

multiplied by the element thickness [9].

3 Fabric Dynamic Permeability

The airbag fabric is a woven material and permeable, causing a certain amount of pressure loss to occur

during the airbag inflation process.

The MADYMO PERMEABILITY.MODEL2 [7] was used to represent the permeability characteristic

determined using the Textest FX3350 [8] airbag tester, in the airbag simulation with the newly correlated

material characteristics.

Figure 6: dtex350 Permeability correlation (pressure-time left, bulging-pressure right)

The pressures calculated by the computer simulation of the Textest test match the pressures from the test,

and the fabric bulge calculated is 10.07mm versus 10.26mm measured in the test. Therefore a good level

of correlation was achieved (figure 6).


4 Airbag module cover correlation

The influence of the airbag module cover stiffness is extremely important in obtaining accurate injury

predictions. This stiffness is understood as mass transfer, bending of the cover and the ability of the tear

seam to resist tearing.

The material characteristics used for this driver airbag cover model were initially derived from physical

tests with speeds ranging from 100mm/min to 4m/s and have been validated during this study.

Some improvements were needed in the modelling of the cover (including the cover thicknesses (figure 7)

and mesh boundary conditions) and the tear seam (including the tear seam geometry and the failure

method).

Figure 7: Mesh definition of airbag module cover plastic reinforcement (left) with varying tear seam thickness

(right)

Therefore, the opportunities of employing plate theory to predict the stress and strain states within the

airbag module cover [5], under the influence of the deploying airbag, was investigated. In order to do so, it

was necessary to significantly simplify the geometry of the cover, and model the loading scenario as linear

static. Initially, the areas of primary interest were in the vicinity of the tear seam. The Kirchoff and the

Von Kármán plate theories were adapted for the purpose of conducting these analyses.

The investigation concluded that utilising plate theory to obtain the stress and strain states of the cover is

not feasible, particularly in the vicinity of the tear seam. This is primarily due to violations of the basic

assumptions of plate theory and the relatively large deflections of the airbag module cover under the

influence of the deploying airbag.

The consequence of these findings is that the failure stresses in the tear seam areas can only be modelled

in hindsight after physical tests.

Modelling a varying thickness tear seam (figure 7) is performed using elemental strain deleted is used to

simulate the tearing in the improved model. To achieve this the DAMAGE.STRAIN_PLASTIC [7] model

is used using an equivalent plastic strain at material failure (EPSF [7]) to define initiation of material

damage, where each integration point in the element fails independently if the corresponding equivalent

plastic strain exceeds the failure strain specified by EPSF.

A 4m/s punch-out test using a 100mm rigid sphere, against the inner surface of the airbag module cover,

was carried out and from the correlation work undertaken, it can be seen that the opening of the cover is

comparable to test: the visible opening and the acceleration levels correlate well (figure 8).


Figure 8: Pad cover impact correlation visually at the moment of opening (left) and at acceleration correlation (right)

The improved cover model replicates the primary tear seam’s ability to resist opening. Unfortunately,

modelling a varying thickness tapered notch (3D detailed feature) cannot be properly captured using shell

elements using a set mesh size: a necessity for reasonable runtimes. Therefore the accuracy of the tear

seam failure propagation cannot be fully met everywhere in the model.

5 Implementation of correlated airbag in OoP testing

Two OoP test set-ups are specified in FMVSS208 (Code of Federal Regulation, chapter 49, part 571,

subpart 208, section S26), “procedure for low risk deployment tests of driver airbag”: Driver position 1

(chin on module) a 5th percentile female driver represented using a 5

th percentile Hybrid III ATD with chin

on airbag module and Driver position 2 (chest on module) a 5th percentile female driver represented using

a 5th percentile Hybrid III ATD with chest on airbag module.

The MADYMO simulations were improved to include the new airbag material characteristics and the

airbag tether rupture model created in this study.

5.1 OoP1 (Chin on Module)

Physical tests were conducted to assess the OoP1 scenario, chin on module. A comparison of the

simulation and test kinematics are shown in figure 9.

G101OF001 G101OF002 Simulation


Figure 9: OoP1 side view comparison of test and simulation for airbag module with the updated cover model

In both OoP1 tests, the control cloth ruptured at around 37 ms. The rupture does not occur instantaneously

and it is hardly possible to “see” the duration of the rupture from the test data. In the simulation the

rupture begins at 35 ms and finishes around 52 ms.

A comparison of the ATD output channel data for the simulation with the updated airbag cover model is

shown in figure 10. Looking at the injury traces, it has to be noted that the first 15ms are primary

influenced by the airbag blast phase (punch-out phase) and that past 15ms by the airbag working pressure

(membrane loading). In this OoP1(chin on module), the head x acceleration and Upper neck Fz forces are

at the beginning of the deployment heavily loaded in the punch out phase; the signal is flatter after 15ms.

Chest deflection of chest x accelerations are more loaded after 17ms in a gradual manner.


Figure 10: OoP1 test and simulation ATD outputs for airbag module with updated cover model

The peak injury values for the 5th percentile in the OoP1 test and simulation are shown in table 1.

AF05 injury criteria

OoP position 1 Test average

Simulation

Improved cover model

Head HIC (15 ms) [-] 26 7

Neck

Nij [-] 0.24 0.20

Tension force [N] 580 427

Compression force [N] 20 27

Flexion [Nm] 18 17

Extension [Nm] 5 8

Chest accel (3 ms) [g] 11 8

Deflection [mm] 9 9

Table 1: Comparison of 5th

percentile occupant injuries in OoP1 for test and simulation

In the OoP1 tests, the control cloth ruptured at around 37 ms. In the simulation the rupture begins at 35

ms and finishes around 52 ms. The simulation results obtained match the test curves and in jury levels.

From the movies, it can be seen that the airbag module cover opens fully as in the test. The improvements

in the airbag module cover computer model have yielded to improved injury predictions. OoP2 (Chest on

Module)

Physical tests have been conducted and simulated to assess the OoP2 scenario, chest on module.

5.2 OoP2 (Chest on Module)

In the OoP2 tests, the control cloth ruptured at around 35 and 37ms. The rupture does not occur

instantaneously and it is not possible to “see” the duration of the rupture from the test data. In the

simulation the rupture begins at 34 ms and finishes around 45ms.


The peak injury values for the 5th percentile in the OoP2 test and simulation are shown in table 2.

AF05 injury criteria

OoP position 2 Test average

Simulation

Improved cover model

Head HIC (15 ms) [-] 7 3

Neck

Nij [-] 0.18 0.30

Tension force [N] 430 508

Compression force [N] 25 104

Flexion [Nm] 5 9

Extension [Nm] 10 17

Chest accel (3 ms) [g] 12 8

Deflection [mm] 20 18

Table 2: Comparison of 5th

percentile occupant injuries in OoP2 for test and simulation

Comparing the OoP2 tests and simulation, it can be observed that the airbag module cover model is stiffer.

The simulated cover does not open as far, preventing the airbag from deploying between the steering

wheel hub and steering wheel rim and changing the position of the airbag against the occupant. The main

effect of this is on the neck Fz component, in both magnitude and timing.

6 Effect of ATD on airbag tether failure response

Using the improved airbag module model (folded cushion and cover) the 5th percentile dummy ATD

model has been replaced by a 50th and 95

th percentile ATD dummy in order to investigate the mechanical

airbag tether failure effect.

Airbag pressures and volumes have been plotted for each ATD for the two OoP scenarios (figure 11).

The pressure graph shows clearly the burst pressure in the punch-out phase at the start of the inflation

process and subsequently the airbag working pressure or “membrane loading phase” during the working

time of the airbag. Each pressure phase creates loads on the occupant.


Figure 11: Airbag chamber internal pressures

The burst pressure varies slightly in the first 20ms between the computer runs (figure 11) depending on

the ATD being impacted. As this even is very short in time, one may suggest that this pressure difference

may mainly be due to the ATD’s compliance. The secondary pressure (membrane-loading) builds up and

remains higher the heavier the occupant (inertia). The working pressure decays quicker for the 5th

percentile female than for the 95th percentile male, starting from 45 to 50ms. This information concurs

with the previous works ([6]) stating that heavier occupants slumped over the airbag increased the airbag

pressure during deployment.

It can be seen that the airbag volume increases in stages (figure 12), due to the gradual failure of the airbag

control chamber. The start of the tearing process is not always evident, however, the end of the tearing

process was observed from the sudden increase in airbag volume (timing comparable between figure 11

and figure 12).

Figure 12: Airbag chambers internal volumes


Figure 13: Tether rupture time vs. ATD

Figure 14: Airbag losses by permeability

Considering the tether rupture timing results from figure 13, it can be noted that the tether failure starts at

approximately 34ms, but ends at a different time for each deployment study. This proves that the

improved mechanical tether failure is better suitable for this application compared to e.g. a time switch

derived from 5th percentile dummy ATD OoP tests.

The permeability plot (figure 14) suggests that the losses due to permeability are comparable for the first

40ms for the different ATD sizes investigated. Nevertheless, the permeability losses increase the heavier

ATD, as more pressure is applied to the airbag, leading to airbag volume reduction.

7 Conclusions

The study investigated the effect of airbag deployment in OoP scenarios and has looked into the behaviour

of an airbag when restraining occupants of various masses and statures. It has shown that airbag punch-out

effect and injury peaks have been modelled correctly using the CFD Gas Flow tool.

Extensive correlation work to the involved airbag module components (chambered cushion and cover) has

been performed to simulate the OoP tests, with noticeable injury prediction improvements. The presented

advanced initially chambered driver airbag model performs far below the injury value limits required by

0

10

20

30

40

50

60

70

OoP1 OoP2 OoP1 OoP2 OoP1 OoP2

5th female 50th male 95th male

Tether rupture times vs ATD

Tether failurestarts (ms)

Tether failureends (ms)


FMVSS208. The airbag model with improved mechanical tether failure predicts reasonable well the major

injury values measured in real static FMVSS208-OoP tests. The study has shown that a mechanical tether

failure model has been derived successfully and enables the representation of airbag deployment

behaviour in any OoP scenario. The simulation of the loading phase has shown promising results but

further improvement to the tether rupture and cover model still need to be made.

From the airbag pressure results obtained, it can be concluded that the original punch-out effect is caused

primarily by the inflator characteristics and the ATD’s compliance. The second phase is characterised by

an increased airbag working pressure when a heavier ATD is slumped over it (inertia effect).

An important result of this study is therefore that the presented advanced airbag model equipped with an

initial control chamber rupturing during the initial deployment behaves stable and leads reasonable results

if it interacts with different occupant ATD sizes (5th, 50

th and 95

th).

The found stable behaving occupant-airbag-interaction allows a further look into effects of active safety

devices (like frontal full break) resulting occupant postures close to the steering wheel. Especially the 50th

percentile dummy ATD allows a practical replacement with the Madymo human model to further improve

the biofidelity and to potentially study the muscle tone influence.

Acknowledgement

Martin Tijssens, for his kind advice on the use of Madymo material modelling.

References

[1] C. R. Morris, Measuring Airbag Injury Risk To Out-Of-Position Occupants, SAE 986098

[2] M. Mahangare, Investigation Into The Effectiveness of Advanced Driver Airbag Modules Designed For OoP

Injury Mitigation, ESV 07-0319

[3] M. Blundell, M. Mahangare, Investigation of an Advanced Driver Airbag Out-of-Position (OoP) Injury

Prediction with Madymo Gasflow Simulations, Madymo User Meeting (2006)

[4] D. Stubbs, Development of a simulation failure method to represent airbag fabric failure, MSc thesis, Coventry

University, January 2010

[5] J. Christensen, Modelling of airbag pad cover by plate theory, MSc thesis, Coventry University/ Aalborg

University, January 2010

[6] J. Horsh, Assessment of Air Bag Deployment Loads, Paper 902324, The British Library

[7] Madymo 7.0, User Reference, Tass, 2010

[8] The TEXTEST Dynamic Air Permeability Tester. www.aticorporation.com/fx3350.pdf

[9] Madymo 7.0 Theory Manual


Correlation of airbag fabric material mechanical failure...

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