- Email: [email protected]
Response of a helmet liner under biaxial loading
Accepted Manuscript Response of a helmet liner under biaxial loading S. Farajzadeh Khosroshahi, R. Olsson, M. Wysocki, M. Zaccariotto, U. Galvanetto
PII:
S0142-9418(18)31367-9
DOI:
10.1016/j.polymertesting.2018.10.012
Reference:
POTE 5643
To appear in:
Polymer Testing
Received Date: 28 August 2018 Accepted Date: 8 October 2018
Please cite this article as: S. Farajzadeh Khosroshahi, R. Olsson, M. Wysocki, M. Zaccariotto, U. Galvanetto, Response of a helmet liner under biaxial loading, Polymer Testing (2018), doi: https:// doi.org/10.1016/j.polymertesting.2018.10.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Product Performance
RI PT
Response of a helmet liner under biaxial loading S. Farajzadeh Khosroshahi1, R. Olsson2, M.Wysocki3, M. Zaccariotto1, U. Galvanetto1 Department of Industrial Engineering, University of Padova, Padova, Italy
2
Swerea SICOMP Co., Molndal, Gothenburg, Sweden
3
RISE, Borås, Sweden
M AN U
SC
1
Abstract
Helmets are the most effective protective item for motorcyclists. The liner of the helmet is the
TE D
part of the helmet which dissipates most of the impact energy and mitigates the risk of head injuries. It has been proposed that the helmet test standards should include assessment of the helmets for oblique impacts that is not currently addressed in the standards. A conventional
EP
uniaxial compression test method is still used for characterization of the helmet liner material. However, compressive tests of EPS foams provide reliable results for normal loading on EPS, but
AC C
do not provide a realistic result for oblique impacts. Therefore, we carried out experimental tests to measure the response of EPS foams, which are commonly used for helmet liners, under biaxial loading. The result of our experiments show that the shear response of EPS foams is a function of axial compression, and increasing the axial strain leads to increased shear stiffness, and thus higher levels of shear stress. We also showed that including shear-stiffening of EPS in the FE assessment of helmets may change the headform rotational acceleration by 25%.
ACCEPTED MANUSCRIPT
Therefore, such behavior of EPS foams should be included in FE analysis of helmets in the case of oblique impacts for a more realistic assessment of their performance.
Biaxial mechanical response, EPS, helmet, oblique impact
SC
1. Introduction
RI PT
Keywords
Expanded polystyrene foam (EPS) is one of the most widely used materials for crash energy
M AN U
dissipation purposes, in particular for personal protective equipment [1–3].
Moreover,
numerical simulation has been known as an effective tool for design and assessment of such devices [4].
For the purpose of FE modeling, typical behavior of cellular solids can be
characterized using compression tests [1,5]. However, in reality, energy absorbing materials
TE D
may experience load conditions different from uniaxial compression, e.g. an oblique impact on a helmet. In such an impact, the helmet liner will be under a multiaxial loading condition. Thus,
EP
constitutive material models based on uniaxial test results may not be able to represent the behavior of helmet liners accurately. Therefore, characterizing the response of the helmet
AC C
liners under a realistic loading condition is necessary for the development and use of appropriate constitutive material models. A high number of research papers has been dedicated to assessing the response of protective helmets and their level of protection by means of the finite element method [6–8]. Most of these research works assess the helmets under loading purely in the normal direction. However, it has been known for some time that oblique impacts on the head have a higher risk
ACCEPTED MANUSCRIPT
of inducing brain injuries such as concussion in comparison with normal impacts [9]. In addition, numerical simulations have been proposed to be a part of the next generation of helmet
RI PT
standards to assess the response of the helmet in the case of oblique impacts [10]. Therefore, appropriate test methods should be used for material characterization of the helmet liner for such a loading condition.
SC
Gdoutos et al. [11] used an Arcan test rig for material characterization of Divinycell foams under shear and tensile loads and used cylindrical specimens to apply multiaxial loading i.e. tension,
M AN U
torsion and internal pressure. Mechanical response of Rohacell-51WF under shear-compression was reported by Li et al. [12]. In another study, a modified Arcan test rig was used to determine the mechanical response of PVC foam [13]. Mills and Gilchrist [14,15] applied shear and compression simultaneously on elastic and crushable foams using different test rigs. The tests
TE D
were carried out on PU [14] and PP [15].They concluded that shear hardening affects the mechanical response of the foam in the case of oblique loading and should be considered in the finite element analysis of foams [15]. Recently, Y. Mosleh et al. [3] carried out biaxial tests on
EP
EPS and polyethersulfone (PES) foams to study the effect of anisotropy on the level of energy
AC C
absorption. They reported that the loading direction has a significant impact on the energy absorption capacity of anisotropic foams such as PES. Hitherto, to the best of knowledge of the authors, the only available paper which addresses both compressive and shear responses of the helmet liner for FEA was published by Fahlstedt et. al. [16]. They used two different stress-strain curves in order to define the behavior of the helmet liner and reconstructed real bicycle accidents by means of the finite element method. One was the conventional compressive stress-strain material data and the second one was a
ACCEPTED MANUSCRIPT
shear stress-strain material data. Their study considered both compressive and shear response of the helmet liner to obtain the response of the head and the brain more accurately.
RI PT
Unfortunately, the procedure for obtaining the shear stress-strain behavior of the liner material used was not described properly. Moreover, the shear stress-strain material data presented did not include the effect of normal stress on the shearing response.
SC
In this paper, we carried out biaxial experiments in order to characterize the response of the EPS foams under a realistic loading condition and then used the results of the tests for a helmet
M AN U
finite element analysis to show how the result of realistic material characterization can influence the result of the simulations. The next section describes the experimental tests and Section 3 reports the result of the tests. Section 4 shows the difference of the results from finite element simulations of a helmeted headform for an oblique impact with and without including
TE D
the experimental data presented in Section 3. Finally, the conclusions and suggestions for further research are presented in Section 5.
EP
2. Methods
A triaxial test rig was used to study the biaxial response of EPS foams with different densities.
AC C
The tests were carried out at Swerea SICOMP (a partner of the MOTORIST EU network) while Dainese S.p.A (another partner of the same network) provided samples. The test set-up was equipped with a system of transducers to measure the axial compression and two transverse forces. The machine works with displacement controlled actuators to generate axial and shear movement (see Figure 1).
ACCEPTED MANUSCRIPT
The samples were provided as circular molded specimens, as shown in Figure 2, with dimensions of D=90 mm and H=40 mm (D and H stand for Diameter and Height respectively) according to ASMT D1621-16. Three different densities of EPS were used for biaxial testing i.e.
RI PT
25, 40 and 55 kg/m3. Due to the limited capacity of the load cell, the samples with density of 40 and 55 kg/m3 were cut to D=40 mm and H=40 mm (See figures 3 and 4).
SC
The experimental procedure was as follows:
The specimens were compressed axially and then a shear load was applied, as shown in Figure
M AN U
3. Both compression and shear loads were applied at quasi-static rate. The tests were carried out at different levels of axial strain i.e. 20%, 30%, 40% and 50% to study the effect of axial compression on the shear response of the foam. For each level of the compression the shear load was applied when the specimen was under the maximum axial strain. A sheet of silicon
TE D
rubber with diameter of 90 mm was used between the specimen and the contacting surfaces of the machine to maximize the friction between the specimen and the test set-up (Figure 2). The
1000 Hz.
EP
results were filtered using a Butterworth four-pole filtering function with cut-off frequency of
AC C
Rigid metallic end plates were attached to the specimens to avoid rotation, as shown in Figure 4. The end plates were attached to the specimens by means of epoxy adhesive and covered by sandpaper to increase the friction. Furthermore, sheets of silicon rubber were placed between the sandpaper covered end plates and the load cell surface, as shown in Figure 4. By using the end plates, no rotation was observed during the tests but at a certain level of the shear load the moving part of the test rig started to slide, despite the use of sandpaper and silicon rubber,
ACCEPTED MANUSCRIPT
therefore the results obtained after initial sliding between end plates and testing machine were neglected.
RI PT
3. Experimental results results The experimental results are presented in Figures 5 to 7. The results show that the foam responds to the shear force in a similar way at different levels of compression but the shear
SC
stiffness increased as the compression increased. The results show that the shear response has two main parts, i.e. the linear part and the plateau region. Figure 7 shows slightly different
M AN U
response for the heaviest tested foam (55 kg/m3), however, it is clear that the axial compression affects the shear response of the foam.
4. Finite element simulations using LSLS-DYNA
TE D
A simplified FE model of an AGV-T2 helmet [6,17,18] was used for two impact configurations, i.e. normal and oblique impacts. Since the objective of the simulations presented in this section is just to compare the results of helmet FE analysis with and without considering shear-
EP
stiffening of the foam liner constitutive law, we assumed that the helmet liner is a single part
AC C
made of EPS foam with density of 40 kg/m3, and the outer shell behaves elastically. These simplifications were considered to reduce the computational cost. The axial stress-strain material data of EPS with density of 40 kg/m3, obtained from a pure compressive test (see Figure 8) in the same test rig described earlier, was used for FE simulations. The axial stressstrain data was used once with shear stress-strain material data and once without them. 4node quadrilateral shell elements and an elastic material model with Young’s modulus of 7250 MPa, Poisson’s ratio of 0.3 and density of 1200 kg/m3 was considered for the outer shell [19]. In
ACCEPTED MANUSCRIPT
addition, the chin strap was modeled as an elastic band using 4-node quadrilateral shell elements with a Young’s modulus of 1 GPa and a Poisson’s ratio of 0.3 [17].
RI PT
In the case of normal impact, the helmeted headform was launched towards a rigid horizontal anvil at the speed of 7.5 m/s according to ECE 22.05 [20]. For the oblique impact the helmeted headform with the same speed hit at an inclined rigid anvil forming a 45° angle with respect to
[20]. Both impact conditions are shown in Figure 9.
SC
the horizontal axis [10]. The headform was modelled as a rigid part and was not constrained
M AN U
For both normal and oblique impacts, the simulations were first carried out with the constitutive material model which only needs the compressive stress-strain curve of EPS (MAT63-CRUSHABLE-FOAM from LS-Dyna material library [21]), and has been used by other researchers [17,22]. Then, the simulations were repeated using the constitutive material model
TE D
which includes the shear stress strain material data of the EPS foam (MAT-26-HONEYCOMB from LS-Dyna material library [21]). In order to use MAT_26, the shearing yield stress should be defined as a function of volumetric strain, so that the curves shown in Figure 6 could be
EP
presented in a single curve, as shown in Figure 8-b. This curve has been fitted to the
AC C
experimental data in such a way that extrapolated values for negative volumetric strains do not lead to negative yield stresses, as explained in [21,23]. Peak linear acceleration (PLA) of the headform and head injury criterion (HIC) have been introduced by ECE 22.05 for the assessment of motorcycle helmets [20]. The resultant acceleration of the center of gravity of the headfrom is shown in Figure 10 for the case of
ACCEPTED MANUSCRIPT
normal impact. Moreover, head injury criterion (HIC) has been calculated and shown in Table 1 for normal impact according to the following equation [20]: .
(1)
is the linear acceleration observed at the center of mass of the headform
(measured in g) and
and
are two time points (measured in seconds) during the impact. We
used the time interval of 15 ms to calculate HIC [24].
SC
where,
−
RI PT
=
M AN U
Table 1. Comparison of two different constitutive material models for normal impact.
HIC
PLA [g]
Material model without considering shear stress-strain curves
1417
200
Material model with considering shear stress-strain curves
1439
207
TE D
Model
It is shown that considering the shear-stiffening of EPS liner did not have a considerable effect on the simulation in case of normal impact, however, including the shear-stiffening increased
EP
both peak linear acceleration (PLA) and HIC slightly.
AC C
Moreover, rotational acceleration is known as the main cause of brain injuries [9]. Therefore, the rotational acceleration of the center of gravity of the headform has been measured for the oblique impact and is illustrated in Figure 11. The maximum rotational acceleration was measured at 7.5 and 10 rad/s2 for the cases without and with the shear-stiffening effect, respectively. Therefore, including the shear-stiffening effect could increase the rotational
ACCEPTED MANUSCRIPT
acceleration to the center of gravity of the headform by a considerable amount in the case of an oblique impact.
RI PT
In the present work, the experimental tests were carried out to understand the response of the helmet liner under realistic loading conditions and to understand the shear properties of such materials for more accurate finite element simulations. Moreover, FE analyses of a helmet in
SC
case of normal and oblique impacts with and without considering shear-stiffening effect were carried out. The simulations showed that including the shear-stiffening of EPS foam may not
M AN U
influence the helmet liner response in the case of normal impact, but it can increase the peak rotational acceleration by 25% in oblique impacts.
5. Conclusions Conclusions
TE D
The results of the tests showed that the response of EPS under shear loading is a function of the compression level. Also, finite element simulation of a helmet in the case of normal and oblique impacts was carried out. The results of the simulations show that including shear-stiffening
EP
behavior of EPS can affect the result of simulations by 25% in the case of oblique impacts. Therefore, a shear-stiffening material model should be included in simulation of helmeted
AC C
headforms (head) in the case of oblique impacts to provide a realistic evaluation of helmets.
6. Acknowledgement The research leading to these results has received funding from the People Programme (Marie Sklodowska Curie Actions) of the European Union’s Seventh Framework FP7/2007-2013/ under REA grant agreement n° [FP7-PEOPLE-2013-ITN-608092] and from the ECCELLENZA programme
ACCEPTED MANUSCRIPT
of the CARIPARO foundation under the REDIPhE project. Moreover, the authors would like to acknowledge Mr. Peter Hellström from Swerea SICOMP for his assistance in preparing and
AC C
EP
TE D
M AN U
SC
RI PT
testing the specimens.
ACCEPTED MANUSCRIPT
7. References [1]
L.J. Gibson, M.F. Ashby, Cellular Solids, Structure and Properties, 2nd ed., Cambridge Solid State
[2]
RI PT
Science Series, 1999. F.A.O. Fernandes, R.J. Alves de Sousa, Motorcycle helmets—A state of the art review, Accid. Anal. Prev. 56 (2013) 1–21. doi:10.1016/j.aap.2013.03.011.
Y. Mosleh, K. Vanden Bosche, B. Depreitere, J. Vander Sloten, I. Verpoest, J. Ivens, Effect of
SC
[3]
polymer foam anisotropy on energy absorption during combined shear-compression loading, J.
[4]
M AN U
Cell. Plast. (2017). doi:10.1177/0021955X17720156.
R. Miralbes, Design of motorcycle rider protection systems using numerical techniques, Accid. Anal. Prev. 59 (2013) 94–108. doi:10.1016/j.aap.2013.04.016.
A. Castiglioni, L. Castellani, G. Cuder, S. Comba, Relevant materials parameters in cushioning for EPS
foams,
Colloids
Surfaces
A
Physicochem.
TE D
[5]
Eng.
Asp.
534
(2016)
71–77.
doi:10.1016/j.colsurfa.2017.03.049.
M. Ghajari, U. Galvanetto, L. Iannucci, R. Willinger, Influence of the body on the response of the helmeted
EP
[6]
head
during
impact,
Int.
J.
Crashworthiness.
16
(2011)
285–295.
[7]
AC C
doi:10.1080/13588265.2011.559798. A. Cernicchi, U. Galvanetto, R. Olsson, Virtual testing of composite motorcycle helmets, Int. J.
Mod. Phys. B. 22 (2008) 1705–1711. http://www.scopus.com/inward/record.url?eid=2-s2.045149109476&partnerID=tZOtx3y1.
[8]
N.J. Mills, S. Wilkes, S. Derler, A. Flisch, FEA of oblique impact tests on a motorcycle helmet, Int. J. Impact Eng. 36 (2009) 913–925. doi:10.1016/j.ijimpeng.2008.12.011.
ACCEPTED MANUSCRIPT
[9]
A.H.S. Holbourn, Mechanics of head injuries, Lancet. 242 (1943) 438–441. doi:10.1016/S01406736(00)87453-X. N. Bourdet, S. Mojumder, S. Piantini, C. Deck, M. Pierini, R. Willinger, Proposal of a new
RI PT
[10]
motorcycle helmet test method for tangential impact, in: Int. IRCOBI Conf. Biomech. Impacts, 2016: pp. 503–504.
E.E. Gdoutos, I.M. Daniel, K.A. Wang, Failure of cellular foams under multiaxial loading, Compos. -
SC
[11]
Part A Appl. Sci. Manuf. 33 (2002) 163–176. doi:10.1016/S1359-835X(01)00110-5. Q.M. Li, R.A.W. Mines, R.S. Birch, The crush behaviour of Rohacell-51WF structural foam, Int. J.
M AN U
[12]
Solids Struct. 37 (2000) 6321–6341. doi:10.1016/S0020-7683(99)00277-2. [13]
S.T. Taher, O.T. Thomsen, J.M. Dulieu-Barton, S. Zhang, Determination of mechanical properties of PVC foam using a modified Arcan fixture, Compos. Part A Appl. Sci. Manuf. 43 (2012) 1698–
[14]
N. Mills, A. Gilchrist, Modelling the indentation of low density polymer foams, Cell. Polym. 19 (2000) 389–412.
N. Mills, A. Gilchrist, Shear and compressive impact of PP bead foam, Cell. Polym. 18 (1999) 157–
[16]
AC C
174.
EP
[15]
TE D
1708. doi:10.1016/J.COMPOSITESA.2011.11.010.
M. Fahlstedt, P. Halldin, S. Kleiven, The protective effect of a helmet in three bicycle accidents - A
finite element study, Accid. Anal. Prev. 91 (2016) 135–143. doi:10.1016/j.aap.2016.02.025.
[17]
M. Ghajari, C. Deck, U. Galvanetto, L. Iannucci, R. Willinger, Development of numerical models
for the investigation of motorcyclists accidents, in: 7th Eur. LS-Dyna Conf., 2009. [18]
M. Ghajari, The influence of the body on the response of the helmeted head during impact,
ACCEPTED MANUSCRIPT
Imperial College London, 2011. M.A. Forero Rueda, L. Cui, M.D. Gilchrist, Optimisation of energy absorbing liner for equestrian helmets.
Part
I:
Layered
foam
liner,
Mater.
doi:10.1016/j.matdes.2009.03.037. [20]
Des.
30
(2009)
3405–3413.
RI PT
[19]
ECE 22.05, Uniform provisions concerning the approval of protective helmets and of their visors
SC
for drivers and passengers of motorcycles and mopeds, E/ECE/324 & E/ECE/TRANS/505 Addendum 21, Regulation No. 22, Revision 4, Geneva, 2002. LS-Dyna keyword user’s manual, 2014.
[22]
A. Cernicchi, U. Galvanetto, L. Iannucci, Virtual modelling of safety helmets: practical problems,
M AN U
[21]
Int. J. Crashworthiness. 13 (2008) 451–467. doi:10.1080/13588260802055460. J.O. Hallquist, LS-Dyna theory manual, (2006).
[24]
R.M. Greenwald, J.T. Gwin, J.J. Chu, J.J. Crisco, Head impact severity measures for evaluating mild
TE D
[23]
traumatic brain injury risk exposure., Neurosurgery. 62 (2008) 789–98; discussion 798.
AC C
EP
doi:10.1227/01.neu.0000318162.67472.ad.
ACCEPTED MANUSCRIPT
Figure captions: Figure 1. Left: general view of the test rig, Right: description of test rig parts.
RI PT
Figure 2. Samples with D=90 mm and H 40mm. Figure 3. Multiaxial test of foams, left: beginning of the test: middle: compressed specimen
SC
without shearing load, right: specimen under compression and shear.
Figure 4. Samples with D=40 mm and H 40mm, left: loose contact after applying shear of the
M AN U
specimens without end plates, right: specimen with end plates. Figure 5. Test results for EPS with density of 25 kg/m3. Figure 6. Test results for EPS with density of 40 kg/m3.
TE D
Figure 7. Test results for EPS with density of 55 kg/m3.
Figure 8. Compressive (a) and shearing (b) stress-strain curve for EPS with density of 40 kg/m3.
EP
Figure 9. Helmet test Left: Normal impact, Right: Oblique impacts. Figure 10. Result of the normal impact simulations.
AC C
Figure 11. Result of the oblique impact simulations.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
RI PT
SC M AN U TE D EP
• •
Three different EPS foams were tested under compression-shear loading condition. The shearing tests were carried out at different levels of compression. The test results showed that the shearing response of the foam is a function of compression level. The test results used for FE analysis of helmets. The FE simulations showed that including shear-stiffening of EPS foam can change the head rotational acceleration by 25% in case of an oblique impact.
AC C
• • •
PII:
S0142-9418(18)31367-9
DOI:
10.1016/j.polymertesting.2018.10.012
Reference:
POTE 5643
To appear in:
Polymer Testing
Received Date: 28 August 2018 Accepted Date: 8 October 2018
Please cite this article as: S. Farajzadeh Khosroshahi, R. Olsson, M. Wysocki, M. Zaccariotto, U. Galvanetto, Response of a helmet liner under biaxial loading, Polymer Testing (2018), doi: https:// doi.org/10.1016/j.polymertesting.2018.10.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Product Performance
RI PT
Response of a helmet liner under biaxial loading S. Farajzadeh Khosroshahi1, R. Olsson2, M.Wysocki3, M. Zaccariotto1, U. Galvanetto1 Department of Industrial Engineering, University of Padova, Padova, Italy
2
Swerea SICOMP Co., Molndal, Gothenburg, Sweden
3
RISE, Borås, Sweden
M AN U
SC
1
Abstract
Helmets are the most effective protective item for motorcyclists. The liner of the helmet is the
TE D
part of the helmet which dissipates most of the impact energy and mitigates the risk of head injuries. It has been proposed that the helmet test standards should include assessment of the helmets for oblique impacts that is not currently addressed in the standards. A conventional
EP
uniaxial compression test method is still used for characterization of the helmet liner material. However, compressive tests of EPS foams provide reliable results for normal loading on EPS, but
AC C
do not provide a realistic result for oblique impacts. Therefore, we carried out experimental tests to measure the response of EPS foams, which are commonly used for helmet liners, under biaxial loading. The result of our experiments show that the shear response of EPS foams is a function of axial compression, and increasing the axial strain leads to increased shear stiffness, and thus higher levels of shear stress. We also showed that including shear-stiffening of EPS in the FE assessment of helmets may change the headform rotational acceleration by 25%.
ACCEPTED MANUSCRIPT
Therefore, such behavior of EPS foams should be included in FE analysis of helmets in the case of oblique impacts for a more realistic assessment of their performance.
Biaxial mechanical response, EPS, helmet, oblique impact
SC
1. Introduction
RI PT
Keywords
Expanded polystyrene foam (EPS) is one of the most widely used materials for crash energy
M AN U
dissipation purposes, in particular for personal protective equipment [1–3].
Moreover,
numerical simulation has been known as an effective tool for design and assessment of such devices [4].
For the purpose of FE modeling, typical behavior of cellular solids can be
characterized using compression tests [1,5]. However, in reality, energy absorbing materials
TE D
may experience load conditions different from uniaxial compression, e.g. an oblique impact on a helmet. In such an impact, the helmet liner will be under a multiaxial loading condition. Thus,
EP
constitutive material models based on uniaxial test results may not be able to represent the behavior of helmet liners accurately. Therefore, characterizing the response of the helmet
AC C
liners under a realistic loading condition is necessary for the development and use of appropriate constitutive material models. A high number of research papers has been dedicated to assessing the response of protective helmets and their level of protection by means of the finite element method [6–8]. Most of these research works assess the helmets under loading purely in the normal direction. However, it has been known for some time that oblique impacts on the head have a higher risk
ACCEPTED MANUSCRIPT
of inducing brain injuries such as concussion in comparison with normal impacts [9]. In addition, numerical simulations have been proposed to be a part of the next generation of helmet
RI PT
standards to assess the response of the helmet in the case of oblique impacts [10]. Therefore, appropriate test methods should be used for material characterization of the helmet liner for such a loading condition.
SC
Gdoutos et al. [11] used an Arcan test rig for material characterization of Divinycell foams under shear and tensile loads and used cylindrical specimens to apply multiaxial loading i.e. tension,
M AN U
torsion and internal pressure. Mechanical response of Rohacell-51WF under shear-compression was reported by Li et al. [12]. In another study, a modified Arcan test rig was used to determine the mechanical response of PVC foam [13]. Mills and Gilchrist [14,15] applied shear and compression simultaneously on elastic and crushable foams using different test rigs. The tests
TE D
were carried out on PU [14] and PP [15].They concluded that shear hardening affects the mechanical response of the foam in the case of oblique loading and should be considered in the finite element analysis of foams [15]. Recently, Y. Mosleh et al. [3] carried out biaxial tests on
EP
EPS and polyethersulfone (PES) foams to study the effect of anisotropy on the level of energy
AC C
absorption. They reported that the loading direction has a significant impact on the energy absorption capacity of anisotropic foams such as PES. Hitherto, to the best of knowledge of the authors, the only available paper which addresses both compressive and shear responses of the helmet liner for FEA was published by Fahlstedt et. al. [16]. They used two different stress-strain curves in order to define the behavior of the helmet liner and reconstructed real bicycle accidents by means of the finite element method. One was the conventional compressive stress-strain material data and the second one was a
ACCEPTED MANUSCRIPT
shear stress-strain material data. Their study considered both compressive and shear response of the helmet liner to obtain the response of the head and the brain more accurately.
RI PT
Unfortunately, the procedure for obtaining the shear stress-strain behavior of the liner material used was not described properly. Moreover, the shear stress-strain material data presented did not include the effect of normal stress on the shearing response.
SC
In this paper, we carried out biaxial experiments in order to characterize the response of the EPS foams under a realistic loading condition and then used the results of the tests for a helmet
M AN U
finite element analysis to show how the result of realistic material characterization can influence the result of the simulations. The next section describes the experimental tests and Section 3 reports the result of the tests. Section 4 shows the difference of the results from finite element simulations of a helmeted headform for an oblique impact with and without including
TE D
the experimental data presented in Section 3. Finally, the conclusions and suggestions for further research are presented in Section 5.
EP
2. Methods
A triaxial test rig was used to study the biaxial response of EPS foams with different densities.
AC C
The tests were carried out at Swerea SICOMP (a partner of the MOTORIST EU network) while Dainese S.p.A (another partner of the same network) provided samples. The test set-up was equipped with a system of transducers to measure the axial compression and two transverse forces. The machine works with displacement controlled actuators to generate axial and shear movement (see Figure 1).
ACCEPTED MANUSCRIPT
The samples were provided as circular molded specimens, as shown in Figure 2, with dimensions of D=90 mm and H=40 mm (D and H stand for Diameter and Height respectively) according to ASMT D1621-16. Three different densities of EPS were used for biaxial testing i.e.
RI PT
25, 40 and 55 kg/m3. Due to the limited capacity of the load cell, the samples with density of 40 and 55 kg/m3 were cut to D=40 mm and H=40 mm (See figures 3 and 4).
SC
The experimental procedure was as follows:
The specimens were compressed axially and then a shear load was applied, as shown in Figure
M AN U
3. Both compression and shear loads were applied at quasi-static rate. The tests were carried out at different levels of axial strain i.e. 20%, 30%, 40% and 50% to study the effect of axial compression on the shear response of the foam. For each level of the compression the shear load was applied when the specimen was under the maximum axial strain. A sheet of silicon
TE D
rubber with diameter of 90 mm was used between the specimen and the contacting surfaces of the machine to maximize the friction between the specimen and the test set-up (Figure 2). The
1000 Hz.
EP
results were filtered using a Butterworth four-pole filtering function with cut-off frequency of
AC C
Rigid metallic end plates were attached to the specimens to avoid rotation, as shown in Figure 4. The end plates were attached to the specimens by means of epoxy adhesive and covered by sandpaper to increase the friction. Furthermore, sheets of silicon rubber were placed between the sandpaper covered end plates and the load cell surface, as shown in Figure 4. By using the end plates, no rotation was observed during the tests but at a certain level of the shear load the moving part of the test rig started to slide, despite the use of sandpaper and silicon rubber,
ACCEPTED MANUSCRIPT
therefore the results obtained after initial sliding between end plates and testing machine were neglected.
RI PT
3. Experimental results results The experimental results are presented in Figures 5 to 7. The results show that the foam responds to the shear force in a similar way at different levels of compression but the shear
SC
stiffness increased as the compression increased. The results show that the shear response has two main parts, i.e. the linear part and the plateau region. Figure 7 shows slightly different
M AN U
response for the heaviest tested foam (55 kg/m3), however, it is clear that the axial compression affects the shear response of the foam.
4. Finite element simulations using LSLS-DYNA
TE D
A simplified FE model of an AGV-T2 helmet [6,17,18] was used for two impact configurations, i.e. normal and oblique impacts. Since the objective of the simulations presented in this section is just to compare the results of helmet FE analysis with and without considering shear-
EP
stiffening of the foam liner constitutive law, we assumed that the helmet liner is a single part
AC C
made of EPS foam with density of 40 kg/m3, and the outer shell behaves elastically. These simplifications were considered to reduce the computational cost. The axial stress-strain material data of EPS with density of 40 kg/m3, obtained from a pure compressive test (see Figure 8) in the same test rig described earlier, was used for FE simulations. The axial stressstrain data was used once with shear stress-strain material data and once without them. 4node quadrilateral shell elements and an elastic material model with Young’s modulus of 7250 MPa, Poisson’s ratio of 0.3 and density of 1200 kg/m3 was considered for the outer shell [19]. In
ACCEPTED MANUSCRIPT
addition, the chin strap was modeled as an elastic band using 4-node quadrilateral shell elements with a Young’s modulus of 1 GPa and a Poisson’s ratio of 0.3 [17].
RI PT
In the case of normal impact, the helmeted headform was launched towards a rigid horizontal anvil at the speed of 7.5 m/s according to ECE 22.05 [20]. For the oblique impact the helmeted headform with the same speed hit at an inclined rigid anvil forming a 45° angle with respect to
[20]. Both impact conditions are shown in Figure 9.
SC
the horizontal axis [10]. The headform was modelled as a rigid part and was not constrained
M AN U
For both normal and oblique impacts, the simulations were first carried out with the constitutive material model which only needs the compressive stress-strain curve of EPS (MAT63-CRUSHABLE-FOAM from LS-Dyna material library [21]), and has been used by other researchers [17,22]. Then, the simulations were repeated using the constitutive material model
TE D
which includes the shear stress strain material data of the EPS foam (MAT-26-HONEYCOMB from LS-Dyna material library [21]). In order to use MAT_26, the shearing yield stress should be defined as a function of volumetric strain, so that the curves shown in Figure 6 could be
EP
presented in a single curve, as shown in Figure 8-b. This curve has been fitted to the
AC C
experimental data in such a way that extrapolated values for negative volumetric strains do not lead to negative yield stresses, as explained in [21,23]. Peak linear acceleration (PLA) of the headform and head injury criterion (HIC) have been introduced by ECE 22.05 for the assessment of motorcycle helmets [20]. The resultant acceleration of the center of gravity of the headfrom is shown in Figure 10 for the case of
ACCEPTED MANUSCRIPT
normal impact. Moreover, head injury criterion (HIC) has been calculated and shown in Table 1 for normal impact according to the following equation [20]: .
(1)
is the linear acceleration observed at the center of mass of the headform
(measured in g) and
and
are two time points (measured in seconds) during the impact. We
used the time interval of 15 ms to calculate HIC [24].
SC
where,
−
RI PT
=
M AN U
Table 1. Comparison of two different constitutive material models for normal impact.
HIC
PLA [g]
Material model without considering shear stress-strain curves
1417
200
Material model with considering shear stress-strain curves
1439
207
TE D
Model
It is shown that considering the shear-stiffening of EPS liner did not have a considerable effect on the simulation in case of normal impact, however, including the shear-stiffening increased
EP
both peak linear acceleration (PLA) and HIC slightly.
AC C
Moreover, rotational acceleration is known as the main cause of brain injuries [9]. Therefore, the rotational acceleration of the center of gravity of the headform has been measured for the oblique impact and is illustrated in Figure 11. The maximum rotational acceleration was measured at 7.5 and 10 rad/s2 for the cases without and with the shear-stiffening effect, respectively. Therefore, including the shear-stiffening effect could increase the rotational
ACCEPTED MANUSCRIPT
acceleration to the center of gravity of the headform by a considerable amount in the case of an oblique impact.
RI PT
In the present work, the experimental tests were carried out to understand the response of the helmet liner under realistic loading conditions and to understand the shear properties of such materials for more accurate finite element simulations. Moreover, FE analyses of a helmet in
SC
case of normal and oblique impacts with and without considering shear-stiffening effect were carried out. The simulations showed that including the shear-stiffening of EPS foam may not
M AN U
influence the helmet liner response in the case of normal impact, but it can increase the peak rotational acceleration by 25% in oblique impacts.
5. Conclusions Conclusions
TE D
The results of the tests showed that the response of EPS under shear loading is a function of the compression level. Also, finite element simulation of a helmet in the case of normal and oblique impacts was carried out. The results of the simulations show that including shear-stiffening
EP
behavior of EPS can affect the result of simulations by 25% in the case of oblique impacts. Therefore, a shear-stiffening material model should be included in simulation of helmeted
AC C
headforms (head) in the case of oblique impacts to provide a realistic evaluation of helmets.
6. Acknowledgement The research leading to these results has received funding from the People Programme (Marie Sklodowska Curie Actions) of the European Union’s Seventh Framework FP7/2007-2013/ under REA grant agreement n° [FP7-PEOPLE-2013-ITN-608092] and from the ECCELLENZA programme
ACCEPTED MANUSCRIPT
of the CARIPARO foundation under the REDIPhE project. Moreover, the authors would like to acknowledge Mr. Peter Hellström from Swerea SICOMP for his assistance in preparing and
AC C
EP
TE D
M AN U
SC
RI PT
testing the specimens.
ACCEPTED MANUSCRIPT
7. References [1]
L.J. Gibson, M.F. Ashby, Cellular Solids, Structure and Properties, 2nd ed., Cambridge Solid State
[2]
RI PT
Science Series, 1999. F.A.O. Fernandes, R.J. Alves de Sousa, Motorcycle helmets—A state of the art review, Accid. Anal. Prev. 56 (2013) 1–21. doi:10.1016/j.aap.2013.03.011.
Y. Mosleh, K. Vanden Bosche, B. Depreitere, J. Vander Sloten, I. Verpoest, J. Ivens, Effect of
SC
[3]
polymer foam anisotropy on energy absorption during combined shear-compression loading, J.
[4]
M AN U
Cell. Plast. (2017). doi:10.1177/0021955X17720156.
R. Miralbes, Design of motorcycle rider protection systems using numerical techniques, Accid. Anal. Prev. 59 (2013) 94–108. doi:10.1016/j.aap.2013.04.016.
A. Castiglioni, L. Castellani, G. Cuder, S. Comba, Relevant materials parameters in cushioning for EPS
foams,
Colloids
Surfaces
A
Physicochem.
TE D
[5]
Eng.
Asp.
534
(2016)
71–77.
doi:10.1016/j.colsurfa.2017.03.049.
M. Ghajari, U. Galvanetto, L. Iannucci, R. Willinger, Influence of the body on the response of the helmeted
EP
[6]
head
during
impact,
Int.
J.
Crashworthiness.
16
(2011)
285–295.
[7]
AC C
doi:10.1080/13588265.2011.559798. A. Cernicchi, U. Galvanetto, R. Olsson, Virtual testing of composite motorcycle helmets, Int. J.
Mod. Phys. B. 22 (2008) 1705–1711. http://www.scopus.com/inward/record.url?eid=2-s2.045149109476&partnerID=tZOtx3y1.
[8]
N.J. Mills, S. Wilkes, S. Derler, A. Flisch, FEA of oblique impact tests on a motorcycle helmet, Int. J. Impact Eng. 36 (2009) 913–925. doi:10.1016/j.ijimpeng.2008.12.011.
ACCEPTED MANUSCRIPT
[9]
A.H.S. Holbourn, Mechanics of head injuries, Lancet. 242 (1943) 438–441. doi:10.1016/S01406736(00)87453-X. N. Bourdet, S. Mojumder, S. Piantini, C. Deck, M. Pierini, R. Willinger, Proposal of a new
RI PT
[10]
motorcycle helmet test method for tangential impact, in: Int. IRCOBI Conf. Biomech. Impacts, 2016: pp. 503–504.
E.E. Gdoutos, I.M. Daniel, K.A. Wang, Failure of cellular foams under multiaxial loading, Compos. -
SC
[11]
Part A Appl. Sci. Manuf. 33 (2002) 163–176. doi:10.1016/S1359-835X(01)00110-5. Q.M. Li, R.A.W. Mines, R.S. Birch, The crush behaviour of Rohacell-51WF structural foam, Int. J.
M AN U
[12]
Solids Struct. 37 (2000) 6321–6341. doi:10.1016/S0020-7683(99)00277-2. [13]
S.T. Taher, O.T. Thomsen, J.M. Dulieu-Barton, S. Zhang, Determination of mechanical properties of PVC foam using a modified Arcan fixture, Compos. Part A Appl. Sci. Manuf. 43 (2012) 1698–
[14]
N. Mills, A. Gilchrist, Modelling the indentation of low density polymer foams, Cell. Polym. 19 (2000) 389–412.
N. Mills, A. Gilchrist, Shear and compressive impact of PP bead foam, Cell. Polym. 18 (1999) 157–
[16]
AC C
174.
EP
[15]
TE D
1708. doi:10.1016/J.COMPOSITESA.2011.11.010.
M. Fahlstedt, P. Halldin, S. Kleiven, The protective effect of a helmet in three bicycle accidents - A
finite element study, Accid. Anal. Prev. 91 (2016) 135–143. doi:10.1016/j.aap.2016.02.025.
[17]
M. Ghajari, C. Deck, U. Galvanetto, L. Iannucci, R. Willinger, Development of numerical models
for the investigation of motorcyclists accidents, in: 7th Eur. LS-Dyna Conf., 2009. [18]
M. Ghajari, The influence of the body on the response of the helmeted head during impact,
ACCEPTED MANUSCRIPT
Imperial College London, 2011. M.A. Forero Rueda, L. Cui, M.D. Gilchrist, Optimisation of energy absorbing liner for equestrian helmets.
Part
I:
Layered
foam
liner,
Mater.
doi:10.1016/j.matdes.2009.03.037. [20]
Des.
30
(2009)
3405–3413.
RI PT
[19]
ECE 22.05, Uniform provisions concerning the approval of protective helmets and of their visors
SC
for drivers and passengers of motorcycles and mopeds, E/ECE/324 & E/ECE/TRANS/505 Addendum 21, Regulation No. 22, Revision 4, Geneva, 2002. LS-Dyna keyword user’s manual, 2014.
[22]
A. Cernicchi, U. Galvanetto, L. Iannucci, Virtual modelling of safety helmets: practical problems,
M AN U
[21]
Int. J. Crashworthiness. 13 (2008) 451–467. doi:10.1080/13588260802055460. J.O. Hallquist, LS-Dyna theory manual, (2006).
[24]
R.M. Greenwald, J.T. Gwin, J.J. Chu, J.J. Crisco, Head impact severity measures for evaluating mild
TE D
[23]
traumatic brain injury risk exposure., Neurosurgery. 62 (2008) 789–98; discussion 798.
AC C
EP
doi:10.1227/01.neu.0000318162.67472.ad.
ACCEPTED MANUSCRIPT
Figure captions: Figure 1. Left: general view of the test rig, Right: description of test rig parts.
RI PT
Figure 2. Samples with D=90 mm and H 40mm. Figure 3. Multiaxial test of foams, left: beginning of the test: middle: compressed specimen
SC
without shearing load, right: specimen under compression and shear.
Figure 4. Samples with D=40 mm and H 40mm, left: loose contact after applying shear of the
M AN U
specimens without end plates, right: specimen with end plates. Figure 5. Test results for EPS with density of 25 kg/m3. Figure 6. Test results for EPS with density of 40 kg/m3.
TE D
Figure 7. Test results for EPS with density of 55 kg/m3.
Figure 8. Compressive (a) and shearing (b) stress-strain curve for EPS with density of 40 kg/m3.
EP
Figure 9. Helmet test Left: Normal impact, Right: Oblique impacts. Figure 10. Result of the normal impact simulations.
AC C
Figure 11. Result of the oblique impact simulations.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
RI PT
SC M AN U TE D EP
• •
Three different EPS foams were tested under compression-shear loading condition. The shearing tests were carried out at different levels of compression. The test results showed that the shearing response of the foam is a function of compression level. The test results used for FE analysis of helmets. The FE simulations showed that including shear-stiffening of EPS foam can change the head rotational acceleration by 25% in case of an oblique impact.
AC C
• • •