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Modelling and validation of motorcyclist helmet with composite shell
Article in International Journal of Crashworthiness · April 2012
DOI: 10.1080/13588265.2011.648465
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Modelling and validation of motorcyclist helmet with
composite shell
a
a
V. Tinard , C. Deck & R. Willinger
a
a
Institut de Mécanique des Fluides et des Solides, Université de Strasbourg, Strasbourg,
France
Available online: 09 Jan 2012
To cite this article: V. Tinard, C. Deck & R. Willinger (2012): Modelling and validation of motorcyclist helmet with composite
shell, International Journal of Crashworthiness, DOI:10.1080/13588265.2011.648465
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International Journal of Crashworthiness
iFirst 2012, 1–7
Modelling and validation of motorcyclist helmet with composite shell
V. Tinard, C. Deck and R. Willinger∗
Institut de Mécanique des Fluides et des Solides, Université de Strasbourg, Strasbourg, France
Downloaded by [Université de Strasbourg, SCD ] at 01:39 10 January 2012
(Received 23 May 2011; final version received 7 December 2011)
The aim of this paper is to propose a new helmet finite element model of a commercial helmet with a composite outer
shell and to validate it under normative conditions against experimental tests. After a meshing of the helmet based on the
computer-aided design (CAD) provided by the manufacturer, the mechanical properties of each component of the helmet (the
outer shell and the foam) have been implemented under LS-DYNA FE code. The composite outer shell has been modelled
with a non-homogeneous law by taking into account characteristics of each ply composing the laminate in terms of elastic
behaviour as well as rupture behaviour. The foam characteristics are based on literature data. After coupling of the helmet
model with the headform model, the novel helmet model has then been validated against experimental data under normative
conditions as prescribed by standard ECE 22.05.
Keywords: composite helmet; finite element modelling; standard validation
Introduction
Motorcyclists’ helmets for motorcyclists are basically made
from two main parts, the outer shell and the foam liner.
The main function of the outer shell is to distribute impact loads over a large area and to protect the head from
the penetration of sharp objects. There are essentially two
different types of outer shells: thermoplastic and composite outer shells. Thermoplastic outer shells can be made
either of ABS (acrylonitrile butadiene styrene) or polycarbonate. Composite outer shells are usually made of fibrereinforced plastics. The type of plastic commonly used is
epoxy resin and the type of reinforcement is glass fibre due
to its relatively low cost and fairly good mechanical performance. Carbon and Kevlar are also used, but only for the
most advanced or competition helmets. The main advantage of using composite outer shells lies in their capability
of absorbing more energy by rupture in comparison with
thermoplastic outer shells.
The main function of the energy-absorbing liner is to
provide a stopping distance for the head during an impact. The material commonly used for the foam liner is expanded polystyrene (EPS) as it has excellent performances,
lightweight characteristics and low cost.
In order to better understand the helmet behaviour in
case of impact, a number of authors have proposed helmet
models. The first helmet models that can be found in the
literature are lumped-mass models. Köstner and Stöcker
in 1987 [7] and Mills and Gilchrist in 1988 [9] proposed
lumped-mass models of helmet with a symmetrical outer
shell and whose foam was based on springs and dampers.
∗
Corresponding author. Email: remy.willinger@imfs.u-strasbg.fr
ISSN: 1358-8265 print / ISSN: 1754-2111 online
C 2012 Taylor & Francis
http://dx.doi.org/10.1080/13588265.2011.648465
http://www.tandfonline.com
The main disadvantage of such models is that they did not
take into account the helmet geometry and thus are not reliable for quantitative studies. Furthermore, they allow describing the deformation only in a specific direction, which
involves recalibrating the model for each impact configuration. Finally, these models can be used only for linear
impacts, excluding any possibility of analysing tangential
ones.
In order to compensate for these limitations, finite element models of motorcyclist helmets have been developed. These models can be divided into two categories
according to the helmet outer shell material (homogeneous
thermoplastic or composite outer shell). Focusing on helmet finite element models with composite outer shell, only
five models are reported in the literature. The first one has
been proposed by Brands in 1997 [1] and aimed to explain the dynamical behaviour of a helmet during impact.
The author validated this model with regard to standard
ECE 22.04. However, the outer shell was modelled with
an elastic law as the author has considered that there was
no delamination during impact and that the fibres of the
composite material were randomly oriented. The model developed by Kostopoulos et al. in 2002 [8] aimed to evaluate
the absorption capability of three different fibres included
in the composite materials (glass, carbon and Kevlar) of
the outer shell of a motorcyclist helmet in case of impacts.
The authors concluded that the materials with the lowest
shear modulus lead to additional absorption mechanisms
and thus allow a better helmet behaviour in case of impact. The model proposed by Kostopoulos et al. is the most
2
V. Tinard et al.
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Figure 1. Illustration of foam meshing.
advanced helmet finite element model with composite outer
shell as the authors used realistic theoretical properties for
the composite material and took into account delamination
in the modelling of the rupture. Nevertheless, their model
remains a simplified modelling of a laminate and the mechanical properties come from literature data and were not
obtained through experimental tests. Pinnoji and Mahajan
[12], Cernicchi et al. [2] and Ghajari et al. [5] proposed
models of helmets with composite outer shell and coupled
them with an anatomical head model in order to evaluate
the injury risk sustained by the head in case of normative
impacts. The authors have concluded that wearing helmet
allows a decrease of the injury risk sustained by head in
case of normative impact, even if normative requirements
seem to be insufficient to really protect a human head. Mills
et al. [10] also proposed a helmet finite element model with
composite outer shell and EPS foam. As the authors have
focused their study on the friction coefficients between the
different parts of the helmet, they have modelled the composite outer shell by considering a simple elastic model.
Considering the limitations of the existing helmet finite element models found in the literature, the aim of the
present work is to propose a new helmet finite element
model with composite outer shell whose characteristics will
be based on experimental tests of the material and structure under consideration and which will finally be validated
against experimental tests under normative conditions.
(CAD) file. The outer shell was meshed with 7542 shell
elements, with a thickness varying from 2.7 mm to 3.5 mm
according to the considered helmet area. The foam was
meshed with 13,145 brick elements and 398 tetrahedron
elements. Its thickness ranges from 13 mm for the chin to 46
mm for the vertex area. Specific attention has been applied
to the foam meshing as its geometry is quite complicated,
as illustrated in Figure 1.
The headform used to perform the normative impacts
was meshed by means of 3280 shell elements and considered as rigid with a mass of 5.7 kg (corresponding to the M
headform of ECE 22.05 standard).
The two test anvils were meshed with solid elements
(2820 elements for kerbstone anvil and 306 for flat anvil)
and were considered as rigid.
An illustration of the headform helmet and anvils meshing is given in Figure 2.
Materials
For the helmet under consideration, foam is made of EPS of
two different densities. As illustrated in Figure 3, front and
cheeks have an 85 kg·m−3 specific mass, whereas for other
parts, this parameter is 55 kg·m−3. The material constitutive
Model description
Geometry and meshing
The finite element modelling of the considered helmet under impact consists of four parts: the helmet outer shell,
the helmet foam, the headform and the anvils (kerbstone
and flat). The comfort liner has not been included in the
model as it has been considered that it has no influence on
headform response during an impact.
The geometry of the helmet outer shell and the foam was
provided by the manufacturer in a computer-aided design
Figure 2. Illustration of headform helmet and anvils meshing
(kerbstone anvil on the left and flat anvil on the right).
International Journal of Crashworthiness
3
Table 1. Mechanical properties of foam for the two considered expanded polystyrenes (specific mass of 55 and 85 kg·m−3).
Foam + Chin [12]
Front + Cheek [3]
55
15
0.01
85
28
0.01
4
4
3
3
2
1
0
0.0
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Stress (MPa)
Stress (MPa)
ρ (kg·m−3)
E (MPa)
υ
0.2
0.4
0.6
0.8
2
1
0
0.0
1.0
0.2
law model was implemented under LS-DYNA by means of
law ∗ MAT CRUSHABLE FOAM. Thus, the stress versus
volumetric strain functions for both types of polystyrene
materials were introduced. The mechanical characteristics
of the considered EPSs come from experiments reported in
the literature [3, 12] and are summarised in Table 1.
Regarding the outer shell mechanical properties, a detailed presentation of the methodology used to identify the
constitutive law of the heterogeneous composite structure
can be found in Tinard et al. [13] in terms of elastic behaviour as well as rupture behaviour. The results of this
study are synthesised hereafter. The helmet outer shell is
divided into four areas corresponding to different laminates
of glass fibres-reinforced epoxy resin. These laminates are
combinations of four plies: one biaxial ply and three mat
plies. The composition of the outer shell is illustrated in
Figure 4.
Elastic and rupture properties of each ply composing
these laminates have been obtained through experimental tests coupled with a combined experimental–numerical
0.4
0.6
0.8
1.0
Volumetric strain
Volumetric strain
method. Elastic characterisation of each layer composing the outer shell has been based on experimental
modal analysis on samples combined with a coupled
experimental–numerical method. Starting with an initial
estimation of parameters to be identified, the coupled
experimental–numerical method has iteratively updated
these parameters until the difference between values obtained from numerical calculation and experiments is minimised. An error function containing the deviation between
experimental and calculated results has been established.
Rupture characterisation has been performed by impact
tests on the entire outer shell. Tests have been performed for
two different impact velocities in the vertex area. The characterisation has been done by identifying composite outer
shell fracture and in terms of force–displacement curves of
the impact point.
All these characteristics have been implemented under
LS-DYNA by means of a ∗ MAT ENHANCED COMPOSITE DAMAGE law.
The elastic properties of each ply composing the outer
shell are summarised in Table 2 and rupture properties are
reported in Table 3.
Interfaces and boundary conditions
As the meshes of the outer shell and the foam were not
continuous, an interface has been introduced to model the
Table 2. Elastic mechanical properties of each ply composing
the helmet outer shell.
Ply
Figure 3. Illustration of the two areas composing the foam. Front
and cheeks are made of expanded polystyrene of 85 kg·m−3
specific mass, whereas foam and chin are made of expanded
polystyrene of 55 kg·m−3 specific mass.
Biaxial
Mat 1
Mat 2
Mat 3
EL
(GPa)
ET
(GPa)
GLT
(GPa)
υ LT
ρ
(kg·m−3)
14
13
9
16
14
13
9
16
11
9
6
13
0.29
0.32
0.31
0.3
2000
2300
2100
2000
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4
V. Tinard et al.
Figure 4. Illustration of the four areas of the outer shell and their ply organisation.
interactions between these two components. A tied interface
(∗ CONTACT TIED SHELL EDGE TO SURFACE
CONSTRAINED OFFSET under LS-DYNA) has been
used for this purpose to prevent any translation between
the foam and the outer shell but node rotation is nevertheless allowed.
Sliding
interfaces
(∗ CONTACT SURFACE TO
SURFACE) have also been assumed to model contact
between the outer shell and the anvil as well as between
the headform and the foam. Friction coefficients of 0.1 and
0.2 were used, respectively, for these two interfaces.
Helmet model validation procedure
For the validation step of the developed helmet model, simulations have been performed based on ECE 22.05 standard
impacts [4]. The ECE 22.05 requires a series of tests, the
severest of which are impact absorption tests. The impact
absorption capacity is determined by recording the acceleration of the headform fitted with the helmet when it is
dropped in a guided free fall at a specific impact velocity
upon a fixed steel anvil. The sites of the helmet to be imTable 3. Rupture parameters of each ply composing the helmet
outer shell.
Strength (MPa)
Longitudinal
compressive (XC )
Longitudinal tensile (XT )
Transverse compressive
(YC )
Transverse tensile (YT )
Shear (SC )
pacted sequentially are point B on the front, point X on
either side, point R on the rear, point P on the crown and
an optional point S on the chin bar. The specified impact
velocity is 7.5 m·s−1 for each impact point, except for point
S, where it is 5.5 m s−1. The two types of anvils required by
the ECE 22.05 standard have been used for the simulations:
a flat circular anvil and a kerbstone anvil.
As approval limits, ECE 22.05 standard requires that
the linear resultant acceleration peak does not exceed 275 g
and that HIC (Head Injury Criteria) does not exceed 2400.
HIC is a head injury evaluation criterion widely used in the
automotive industry [11] and is given by:
Biaxial
Mat 1
Mat 2
Mat 3
1100
600
900
1400
260
1100
150
600
150
900
670
1400
260
100
150
100
150
90
670
100
HIC = (t1 − t2 )
1
t1 − t2
2.5
t2
a(t) dt
,
(1)
t1
where a(t) is the measured linear acceleration of the headform’s centre of gravity in multiples of g, and t1 and t2 are
any two instants during the impact.
Results and discussion
The validation of the helmet model is performed with regard
to experimental data provided by the manufacturer in terms
of linear acceleration of the headform recorded during the
impact and HIC. Impacts have been carried out for four
impact locations (B, P, R and X) and for both anvils required
in ECE 22.05 standard.
Results obtained in terms of headform response for
impacts on the kerbstone anvil for all considered impact
points are presented in Figure 5. As it can be observed, a
good correlation has been obtained between experimental
and numerical results in terms of acceleration peak as well
as time evolution in all considered configurations.
International Journal of Crashworthiness
Vertex impact
Frontal impact
experimental
simulation
200
150
100
50
0
250
Acceleration (g)
Acceleration (g)
250
2
4
6
8
10
12
experimental
simulation
200
150
100
50
0
0
14
0
2
4
Acceleration (g)
Acceleration (g)
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200
experimental
simulation
150
100
50
0
0
2
4
6
8
8
10
12
14
Lateral impact
Rear impact
200
6
Time (ms)
Time (ms)
250
5
10
12
14
experimental
simulation
150
100
50
0
0
2
Time (ms)
4
6
8
10
12
14
Time (ms)
Figure 5. Comparison between experimental (black) and numerical (grey) accelerations for impacts on kerbstone anvil for the four
considered impact points.
Results in terms of HIC are also coherent for all impact
configurations, as shown in Table 4. The obtained values
show a very good correlation between experimental results
and numerical ones for frontal (B), vertex (P) and rear (R)
impacts. For lateral impacts (X), a higher difference can be
observed between experimental and numerical values but
results remain acceptable (Table 5).
Regarding results obtained for impacts on a flat anvil,
a good correlation between experimental and numerical
results can also be observed in terms of acceleration peak
as well as peak duration for all considered impact points, as
illustrated in Figure 6. Results are also coherent in terms of
HIC, except for lateral impacts (X), where the experimental
value is twice of the numerical one.
The proposed helmet model with composite outer shell
has thus been validated under normative conditions against
experimental data in terms of headform linear acceleration
and HIC. However, some aspects have not been dealt with
and they require further considerations.
The first aspect to be considered is the influence of the
temperature on the helmet response. Indeed, all experimental tests and simulations have been performed under ambient conditions (+20◦ C) and it could be interesting to carry
out experimental and numerical tests at extreme temperature (+50◦ C and –20◦ C). Moreover, in order to complete
the validation of the helmet model, impacts have to be performed on the chin area (point S), as required by Directive
ECE 22.05.
Table 4. Results obtained in terms of HIC during impacts on
kerbstone anvil.
Table 5. Results obtained in terms of HIC during impacts on flat
anvil.
HIC
Experimental
Simulation
Frontal
impact
Vertex
impact
Rear
impact
Lateral
impact
1613
1618
1871
1851
1585
1505
1251
992
HIC
Experimental
Simulation
Frontal
impact
Vertex
impact
Rear
impact
Lateral
impact
1868
1862
1989
2141
2020
1972
1972
968
6
V. Tinard et al.
Frontal impact
Vertex impact
250
experimental
simulation
200
Acceleration (g)
Acceleration (g)
250
150
100
200
150
100
50
50
0
experimental
simulation
0
0
2
4
6
8
10
12
0
2
8
10
250
experimental
simulation
200
Acceleration (g)
Acceleration (g)
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6
12
Lateral impact
Rear impact
250
4
Time (ms)
Time (ms)
150
100
experimental
simulation
200
150
100
50
50
0
0
0
2
4
6
8
10
Time (ms)
0
2
4
6
8
10
12
Time (ms)
Figure 6. Comparison between experimental (black) and numerical (grey) accelerations for impacts on flat anvil for the four considered
impact points.
A second aspect concerns the modelling of the helmet itself and especially the outer shell modelling. Regarding the
geometry of the outer shell, some simplifications have been
performed as lapping between the different areas which
have not been taken into account in the model. Finally,
regarding energy absorption mechanisms of the composite outer shell, only damage has been modelled using the
Tsai–Wu criteria. Delamination has indeed not been considered for this model. However, some authors have shown
that this mechanism is of huge importance for crash of
composite materials. Kostopoulos et al., in 2002, showed
that delamination plays an important part in energy absorption process, as it represents up to 12% of impact energy
[8]. Another study led by Iannucci in 2006 [6] proposed a
method for modelling delamination in composite laminates
and compared results of experimental impacts and simulation impacts on laminates with and without modelling of
delamination. Results obtained showed a better correlation
with experimental results when delamination is considered.
This aspect has thus to be further developed in order to propose a more complete composite outer shell model.
Conclusion
The aim of the present study is to propose a new finite
elements model of a commercial helmet with composite
outer shell and to validate it against experimental tests under
normative conditions.
The first step consisted in meshing the helmet model
from a CAD provided by the manufacturer. The final meshing counts a total of 21,085 elements (13,543 brick elements
and 7542 shell elements). The helmet meshing has then
been transferred under LS-DYNA and mechanical properties of the foam and the outer shell have been implemented
into the model. The foam mechanical properties came from
literature data and those of the outer shell (in terms of elastic and rupture characteristics) have been obtained through
experimental tests.
The second step consisted in validating the helmet
model against experimental tests under normative conditions by superposing experimental and numerical headform
acceleration versus time. Simulations performed on both
anvils (kerbstone and flat anvils) have shown a good
agreement with experimental results in terms of linear
International Journal of Crashworthiness
acceleration of the headform’s centre of gravity as well
as HIC. The proposed helmet finite elements model has
thus been validated for the four impact points and on both
anvils, as required by the European standard ECE 22.05.
The proposed approach constitutes an important step
towards numerical tools for new helmet design and optimisation.
Acknowledgements
This work has been developed within the ANR PREDIT project
BioCASQ (French Department of Transport). The authors wish
to thank Shark Helmets for their collaboration.
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