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
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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).
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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).
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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
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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).
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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).
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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
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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.
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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.
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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.
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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
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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)
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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
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