Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
Selection of Energy Absorbing Materials for Automotive
Head Impact Countermeasures
Rickard Juntikka and Stefan Hallström*
Department of Aeronautical and Vehicle Engineering, Royal Institute of Technology (KTH),
100 44, Stockholm, Sweden
Received: 5 August 2004 Accepted: 15 October 2004
ABSTRACT
Material candidates for energy absorption in head impact countermeasures for
automotive applications are evaluated using both quasi-static and dynamic test
methods. Ranking of different materials turns out to be difficult since the mechanical
response of a material could vary considerably with temperature, especially for
polymers. Twenty-eight selected materials, including foams, honeycombs and balsa
wood are tested and evaluated. The materials are subjected to a sequence of tests
in order to thin out the array systematically. Quasi-static uni-axial compression
is used for initial mapping of the selected materials, followed by quasi-static
shear and dynamic uni-axial compression. The quasi-static test results show that
balsa wood has by far the highest energy absorption capacity per unit weight but
the yield strength is too high to make it suitable for the current application. The
subsequent dynamic compression tests are performed for strain rates between
56 s-1 and 120 s-1 (impact velocities between 1.4 and 3 m/s) and temperatures
in the range -20 - 60 °C. The test results emphasize the necessity of including
both strain rate and temperature dependency to acquire reliable results from
computer simulations of the selected materials.
INTRODUCTION
In order to prevent head injuries to car passengers, the National Highway Traffic
Safety Administration (NHTSA) proposes a standard called Federal Motor
Vehicle Safety Standard (FMVSS) 201(1). The standard describes a procedure
for evaluation of head impact safety in road vehicles. It is used to evaluate
interior surfaces of a passenger compartment using a free motion headform
(FMH), that is a 4.54 kg aluminium skull covered with a layer of rubber flesh.
The target location typically consists of an energy absorbing material and a
*
Corresonding author, E-mail: [email protected]
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Rickard Juntikka and Stefan Hallström
covering plastic panel, both on top of the bearing chassis (body-in-white).
The purpose of the energy absorbing material is to decelerate the head in a
non-injurious manner to prevent serious head injuries in case of a car crash,
while keeping the contact force and acceleration sub-critical.
Practical evaluation of different energy absorbing concepts is costly and time
consuming. New materials and structures are developed at an increasing rate
while the lead-time for new car models constantly decreases. The current trend
in injury countermeasure design is therefore a shift from extensive testing to an
increasing use of computer simulations. Mechanical properties of the candidate
materials at specific intervals of environmental conditions and velocities are
however still required input for such simulations and extensive comparative
investigations of mechanical properties of energy absorbing materials are
difficult to find in the literature. Some reports on materials suitable for car
interior energy absorption do exist(2-5), but they mainly deal with a few materials
from within a specific material category.
The work presented in this paper addresses the process to define desired
characteristics of energy absorbing materials for vehicle interiors, and evaluate
candidates for such applications. The selection methodology is to initially
investigate the quasi-static uni-axial compression behavior of the potential
material candidates. Based on the quasi-static compression results the most
promising materials are selected for quasi-static shear tests. Finally, three
materials are chosen for dynamic uni-axial compression tests using a weight
balanced drop rig.
The selection of materials is based on maximum allowable load on the head,
which limits the yield strength of the materials assuming a certain maximum
head impact area. The limit for the maximum allowable load was set quite
high with respect to relevant experimental results(6,7). The reason for this
decision was the expected larger loaded area of the head at impact compared
with conducted experiments. Based on the experimental results found in the
literature, the maximum load was set to Fmax = 7 kN and the maximum loaded
area of the head at impact was defined as Amax = 0.01 m2. Assuming a mass of
the head of mhead = 4.54 kg(1) and engineering relations,
σ=
F
F = ma
A
(1)
give a target yield stress of the energy absorbing material of σy = 0.7 MPa and
a maximum acceleration level of about a = 160g. These guiding values for
the selection assume a fairly constant impact area throughout the head impact
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
and a rigid bearing chassis. The selection constraints will of course differ
depending on the geometry of the energy absorbing panel and impact case,
but the presented figures constitute an initial guidance for this work.
CELLULAR MATERIALS
The materials investigated in this study were foams, honeycombs and balsa,
materials common in energy absorbing applications. Since the materials are
porous they are able to absorb energy at an almost constant stress level by
elastic bending, plastic bending and/or crushing of the cell walls. An advantage
of foams over honeycombs is that they commonly exhibit isotropic behavior,
i.e. their mechanical properties are similar in all directions. On the other hand,
honeycombs often show a higher strength-to-weight ratio than foams in their
longitudinal direction. Honeycombs exist in a variety of cell shapes, and
could thereby be either transversely isotropic or orthotropic. The honeycombs
investigated in this study had hexagonal or circular cell cross-sections.
Foams and Honeycombs
Following the discussion of Gibson and Ashby(8), the properties of foams and
honeycombs depend on the properties of the cell wall material, the shape of
the cells, whether the cells are open or closed, and the relative density of the
material. The relative density is defined as ρ*/ρs, where ρ* is the density of the
cellular material and ρs is the density of the solid (raw) material in the cell walls.
Depending on the properties of the solid material, global deformation of foams
subject to compression is associated with local elastic or plastic deformation, or
fracture of cell walls. With increasing deformation, elastic bending of the cell
walls turns into collapse of the cells caused by elastic buckling in elastomeric
foams, development of plastic hinges in elastic-plastic foams and by brittle
crushing in brittle foams. From a macroscopic point-of-view, this could be
characterized as the yield strength of the foam. When the collapse-plateau is
reached, the irrecoverable deformation (potentially recoverable for elastomeric
raw materials) progresses at more or less constant load with increasing strain
until the cell walls and edges begin to stack up, causing compression of the
solid material and a corresponding steep rise in the load-deformation response,
see Figure 1. The latter mechanism is denoted densification and the strain at
which it initiates is called the lock-up strain.
The properties of the solid material have strong influence on the energy
absorption of the cell walls in foams and honeycombs. Depending on the
temperature, a polymer foam material exhibiting a non-linear elastic response
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Rickard Juntikka and Stefan Hallström
Figure 1. Uniaxial quasi-static compression stress-strain response for Rohacell
31 IG foam
at room temperature can demonstrate elastic-plastic or elastic-brittle behavior
at -20 ºC (e.g. polypropylene, PP). The response is often governed by the glass
transition temperature, Tg, of the polymer. Thus, Tg is a significant property
when considering the mechanical behavior over a wide temperature span, in
this case the temperature span a vehicle is expected to encounter.
Theoretical models for estimation of the mechanical properties of cellular
materials exist, but the fact that the models are based on idealizations makes
the predictions somewhat approximate. However, relatively simple models and
dimensional considerations provide relations between the sought properties
and their governing parameters (relative density, raw material properties, micro
structure etc.). Methods for such estimates have been derived thoroughly by
Gibson and Ashby(8) and are used herein for the investigated materials.
Balsa
Depending on the species of tree, the overall density of cellular wood structure
varies between 40 and 1400 kg/m3. The density of balsa is in the lowest end
of this spectrum varying between 40 and 380 kg/m3 and is commonly used
in sandwich constructions due to its remarkable combination of specific
strength, stiffness and energy absorbing capacity(8,9,10,11). Balsa has a cellular
microstructure similar to honeycomb in shape and has three orthogonal axes,
longitudinal (L, along the grain), radial (R, across the grain and along the rays)
and tangential (T, across the grain and transverse to rays)(9). The high specific
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
mechanical properties of balsa in its longitudinal direction originate from the
growth of the wood occurring dominantly under compressive forces. This has
created a cellular structure with multi-layered composite cell walls containing
cellulose microfibrils making balsa a remarkable nanocomposite(11). Vural and
Ravichandran(9,10,11) investigated the quasi-static and dynamic compressive
properties of balsa in its longitudinal direction for densities ranging between
55 and 380 kg/m3, derived formulas for the failure stress and found very good
correlation between formulas and experiments throughout the tested density
range. The dominating failure mode for low density balsa wood was reported
to be plastic buckling and an empirical formula for the failure stress, σpb, was
presented(9) as,
σpb = 2σys (ρ/ρs)5/3
(2)
where σys is the yield strength of the cell wall material, ρ is the balsa density
and ρs is the density of the cell wall material. Typical values recommended by
Vural and Ravichandran(9,10) are σys = 350 MPa and ρs = 1500 kg/m3.
ENERGY ABSORPTION
Energy absorbing materials convert kinetic energy into some other form
through the creation of fracture surfaces, plastic or viscoelastic deformation,
kinetic energy or friction while keeping the reaction load (acceleration or
deceleration) below some critical level. The properties of energy absorbing
materials of primary interest for the automotive industry are the ability to
absorb energy, the specific weight, and the cost. For the case of interior head
impact in passenger compartments the viability of a material is determined
partly by the energy absorbing properties but also strongly dependent on the
manufacturers general material requirements. Foams and honeycombs are in
general excellent energy absorbers. The energy absorbing capacity of a foamed
material compared with that of its raw material is illustrated in Figure 2. Since
there are usually restrictions on the maximum contact force, there are restrictions
on the maximum allowable stress, illustrated by the dotted line in Figure 2.
The energy absorption per unit volume of a material subject to compression
is readily seen as the area under a stress-strain curve. For optimal energy
absorption at a maximum allowed load level, the stress-strain curve would
obviously entirely enclose the allowed stress-strain area. In the ideal case of
a flat surface impact leading to uni-axial compression, with bounds on the
maximum allowable load, a favourable material should thus demonstrate
a stiff elastic regime, a horizontal plateau regime and a high densification
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Rickard Juntikka and Stefan Hallström
Compression
Foam
Stress
Fully dense
elastic solid
Energy in
dense solid
Energy
in foam
0
10
20
30
40 50 60
Strain (%)
70
80
90
100
Figure 2. Stress-strain curves for a dense and a cellular material (redrawn from(8),
not to scale)
strain. Consequently, a material can have an almost ideal load-deformation
response but still be of little use for a specific application due to the shape of
the impacting object or too high or too low a plateau-level. Concerning head
impact, a stiff elastic region also limits the accumulated elastic energy, which
in turn limits the rebound. A rebound increases the risk for injury since it is
associated with a longer duration of the contact force, and the risk for neck
injury is also increased.
DESCRIPTION OF THE TESTED MATERIALS
This section lists the properties and raw materials of the selected foams and
honeycombs (Young’s modulus, density, etc.), as given by the manufacturers.
The density of most of the materials was chosen aiming for a suitable
yield strength (or crush strength) for the present head impact application.
The properties presented in Tables 1-4 are predominantly achieved from
testing with various standardized test methods, in accordance with the given
references. Where references are left out, no information was accessible from
the manufacturer.
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
Table 1. Description of investigated polymer foams and properties given by
the supplier
No. Manufacturer
1
σy*
ρ∗
Cell
E*
G*
3
structure (kg/m ) (MPa) (MPa) (MPa)
Material/
Designation
ROEHM
PMI/Rohacell
51 IG **
2
ROEHM
PMI/Rohacell
31 IG **
3
DOW
PP/Strandfoam
EA1000 **
4
Kaneka
PE/EPE 25 *
5
BASF
PP/EPP 25 *
6
BASF
PP/EPP 55 *
7
BASF
PS/EPS 50 *
8
BASF
PS/EPS 100 *
9
Caligen
PU/XE70—H *
10 General Plastics PU/FR-6703 **
11 General Plastics PU/FR-67045 **
12 General Plastics PU/FR-6706 **
13 General Plastics PU/FR-6708 **
14 General Plastics PU/FP-8015 *
15 Bayer AG
PU/64if80 **
16 Bayer AG
PU/67if80 **
*
Elastic ** Elastic-plastic
Closed
50.0(12)
70.0(14) 0.80(16) 21.0(17)
Closed
30.0(12)
36.0(14) 0.40(16) 14.0(17)
Option
40.0(13)
9.64(15) 0.34(15)
Closed
Closed
Closed
Closed
Closed
Open
Closed
Closed
Closed
Closed
Closed
Closed
Closed
25.0
25.0
55.0
50.0
100.0
70.0
48.1(12)
72.1(12)
96.1(12)
128.1(12)
240.3(12)
55.0
110.0
0.077
12.2(16)
22.6(16)
35.0(16)
54.3(16)
-
0.04
0.48(16)
0.80(16)
1.10(16)
1.70(16)
0.30
1.50
4.20(17)
6.30(17)
11.2(17)
16.9(17)
-
Table 2. Description of investigated balsa wood and properties given by the
supplier
σy*
No. Manufacturer Material/
Cell
ρ∗
G*
E*
Designation structure (kg/m3) (MPa) (MPa) (MPa)
17
Drünert
Balsa/70
Closed
~70
-
-
-
Table 3. Description of investigated metal foams and properties given by the
supplier
No. Manufacturer Material/
Designation
18 Cymat
Al/3% **
19 Hydro
Al/6% **
*
Elastic ** Elastic-plastic
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σy*
Cell
ρ∗
E*
G*
3
structure (kg/m ) (MPa) (MPa) (MPa)
Closed
81.0
1.50
0.06
Closed
170
190
3.80
-
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Rickard Juntikka and Stefan Hallström
Table 4. Description of investigated honeycombs and properties given by the
supplier
No. Manuf.
20
Material/
Designation
Tubus Bauer PEI/PEI3.5-70 **
21
Tubus Bauer PEI/PEI6-70 **
22
Tubus Bauer PC/PC6-70 **
23
Tubus Bauer PP/PP8-80 **
24
Plascore
PP/PP30-5 **
25
Plascore
PP/PP40-4 **
26
EuroAramid paper/
Composites ECA 4.8-32 **
27
Hexcel
Al/1/
4-5052.001N-2.3 **
28 Plascore
Al/
2.3-3/8-P-3003 **
*
Elastic ** Elastic-plastic
σy*
E*
Cell
ρ∗
G*
3
structure (kg/m ) (MPa) (MPa) (MPa)
Circular/
70.0
2.00 28.5(18)
φ3.5 mm
Circular/
70.0
1.80 30.0(18)
φ6 mm
Circular/
70.0
95
1.90 19.0(18)
φ6 mm
Circular/
80.0
97
2.00 12.0(18)
φ8 mm
Circular/
80.1
72.4
1.62
13.8
φ7.6 mm
Circular/
64.1
49.6
0.97
9.00
φ10.2 mm
Hexagonal/ 32.0
1.15
31.0
4.8 mm
Hexagonal/
6.4 mm
Hexagonal/
9.5 mm
36.8
310
0.83
221
36.8
-
-
-
EXPERIMENTS
The experiments were divided into quasi-static uni-axial compression, quasistatic shear, and dynamic uni-axial compression. Quasi-static compression tests
were performed for all materials. The materials showing the most favorable
energy absorbing capability for use in head impact countermeasures were
subsequently tested in quasi-static shear. Finally three materials were tested
in dynamic compression. Some quasi-static compression tests were made on
only a few test-samples due to limited material access (Plascore PP30-5 and
Plascore PP40-4) and the results from these tests should be judged accordingly.
Density measurements were conducted according to ASTM D 1622-98(12)
and ASTM C 271-61(19). The deformation in the quasi-static experiments was
measured using the crosshead displacement of an Instron 5567 test machine
and the results were extracted with account taken for the compliance of the
machine.
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
Quasi-Static Compression
The compression tests on foams and balsa wood were carried out according to
ASTM D 1621-00(16) and the compression tests of honeycombs were performed
according to ASTM C 365-57(20). The measured properties were Young’s
modulus and yield strength and the measurements were performed at 23 °C
and 50% humidity. The specimens were cylindrical with diameter 50 mm and
height 25 mm. The strain rate in all compression tests was 1.7 × 10-3 s-1. When
conducting compression tests of honeycomb specimens a self-aligning loading
plate was used. No pre-conditioning of the materials was performed.
Quasi-Static Shear
Single lap shear tests were made according to ASTM standard C263-00(21) and
the evaluated properties were shear modulus and shear strength. The strain
rate was 0.67 × 10-3 s-1 in all shear experiments and the dimensions of the
rectangular specimens were 300 × 75 × 25 mm.
Additional Quasi-Static Compression Experiments
In the quasi-static compression experiments, relatively low values of the stiffness
were obtained for Rohacell 31 IG, compared to the tensile data supplied by the
manufacturer. To examine the difference between the tensile and compressive test
results, uni-axial quasi-static compression tests were conducted for a set of different
sample configurations with specimens prepared from a new manufacturing
batch. The difference in results was assumed to be due to localized deformation
at the loaded surfaces of the test samples. The surface layers primarily consist
of damaged cells due to the cutting of the foam specimens. This layer of cut
cells was assumed to have a certain thickness t1 and a reduced stiffness, E1, with
respect to the intact material, see Figure 3. To examine the proposed origin of
discrepancy, a simplified model utilizing constant stress through the thickness
was applied, see Figure 3, together with three different sample configurations.
Configuration 1 was similar to the original specimens used in the quasi-static
compression experiments and used as a reference sample to investigate if material
from the different batches had different mechanical properties. Configurations
2 and 3 were primed with epoxy resin on the loaded surfaces of the samples.
Each configuration consisted of three cylindrical samples with diameter 65 mm,
which were conditioned in 50 °C for 48 hours. The conditioning was performed
for curing of the epoxy resin. The specimen height in configurations 1 and 2
was 25 mm. The height in configuration 3 was 13 mm. The compression tests
were conducted according to ASTM D 1621-00(16) and the conditions were the
same as for the quasi-static compression experiments.
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Rickard Juntikka and Stefan Hallström
Figure 3. Foam sample with presumed damaged cells at the free surfaces
The reason for testing different configurations of the foam was to estimate t1
and E2 through linear stress vs. strain relations, assuming that the two phases
could be treated as separate layers with constant stress through the thickness
of the foam. These assumptions give the following expression for the resultant
Young’s modulus of the foam
E=
tE1 E2
2t1 E2 + (t − 2t1 ) E1
(3)
where t represents the total height of the foam sample. Combination of the
results from configurations 2 and 3 with knowledge about the Young’s modulus
of epoxy allowed for determination of E2 and t1, through use of the linear
property of Eq. (3).
Dynamic Compression Experiments
Based on the results from the quasi-static experiments and other non-structural
aspects, three different foams were selected for further investigation; Rohacell
31 IG, Strandfoam EA1000 and BASF EPP55. These foams all consisted of
closed cells and two of them were produced from PP but except from that the
three materials were quite dissimilar. The reasons for choosing these particular
foams were their relative cost, Tg, behaviour in compression, and degree of
unrecoverable deformation, the latter related to the rebound at impact. The cost
of materials seemed to be a delicate matter depending on several aspects, and
suppliers were in general reluctant to specify material costs precisely. However,
in qualitative terms the Rohacell foam is a high-end product and relatively
expensive. The BASF foam offers lower specific structural performance and is
relatively inexpensive. The Strandfoam material is of intermediate mechanical
performance, and cost. Quantitatively, the cost is usually in the range of
approximately $1 - $30 per kilo of cellular material (November, 2000).
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
Tests were performed with foam samples 40 × 40 × 25 mm, at three different
temperatures (-20, 20 and 60 ºC) and two velocities (1.4 and 3 m/s), and thereby
corresponding global strain rates (56 and 120 s-1). The tests were conducted
using a weight balanced drop rig as described in(22), and the yield strength was
extracted in accordance with ASTM D 1621-00(16).
RESULTS
Quasi-Static Experiments
The results are illustrated in graphs showing comparisons between manufacturers’
data, predicted values and measured values. Unfortunately, some materials only
existed as foams (for example some polyurethanes) and some manufacturers
would not give information about their products. For materials for which
information of the solid material properties could not be obtained, data is given
as intervals estimated from typical numbers found in the literature(8,23,24), see
Table 5. Due to this the analytical estimates are also presented as intervals,
taking into account the uncertainties of the raw material data. The foams are
assumed to be isotropic (ν = 0.3) and at atmospheric internal pressure ρ0. The
analytical predictions were made using relations and recommended values
of constants from Gibson and Ashby(8), the raw material and cell structure
according to Tables 1-5 together with results from density measurements
(Appendix 1). The cellular materials are assumed to yield by either elastic
(σ*el) or plastic (σ*pl) collapse(8). The mechanism assumed to be prevailing for
each material is listed in Tables 1 – 4. For balsa (longitudinal direction), the
Young’s modulus calculation was performed according to Gibson and Ashby(8)
and the yield stress calculation according to Eq. (2).
Compression
It should be noted that the value of the Young’s modulus from ROEHM
(Rohacell) is determined through tensile tests and that the Bayer 67if80
material was tested without any skin-layer on the surface. For the low-density
aluminium foams and the Caligen E7000H foam no data was available from
the suppliers and the analytical predictions were also very poor. Due to this,
graphs are left out for these materials. Although Rohacell 31 IG and 51 IG are
not truly thermoplastics, they are presented among the thermoplastic materials.
Balsa is presented among the honeycomb materials. The results are illustrated
in Figures 4-9 and numerical results with standard deviations are presented in
Appendix 1. The presented data are mean values from the tests.
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Rickard Juntikka and Stefan Hallström
Table 5. Mechanical properties of raw materials
Tg
Tm
Material Density, ρS Young’s modulus, Yield strength,
3
(°C)
(°C)
σys (MPa)
(kg/m )
ES (GPa)
PMI
1200
3.6
120
185
PP
900 – 915
1.1 – 1.6
31 – 42
-20 – -5 165 – 175
LD PE
910 – 940
0.15 – 0.24
6 – 10
-5 – 0 170 – 240
HD PE
950 – 970
0.55 – 1.0
20 – 28
25 – 30 170 – 240
PS
1050
1.4 – 3.0
30 – 35
100
180 – 250
PU
1200
1.6
130
130
130
(rigid)
PU
1200
0.045
25 – 50
20 – 30
130
(flexible)
AL 5052
2700
69
90 – 290
650
660
AL 3003
2700
69
40 – 190
650
660
PEI
1270
3.0
105
215
PC
1200
~2
~60
~140 310 – 350
Balsa
1500
35
350
*
Nomex
740*
3.0 – 5.0*
150 – 250*
~350*
*
Derived from Nomex data-sheet
Young's modulus (MPa)
120
Manufacturer
Test
Theory
100
80
60
40
20
0
5
51 l 31
25
50 100
25
am
A SF SF 5 SF
F
el ndfo
K
c
A
A
E
AS
B
B
ha tra
N
BA
B
o
A
S
R
K
ell
c
ha
Ro
Figure 4. Young’s modulus, thermoplastics
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
Compression strength (MPa)
1.4
Manufacturer
Test
Theory
1.2
1
0.8
0.6
0.4
0.2
0
1
5
1
50 100
25
25
m
A SF SF 5 SF
ll 5 cell 3 dfoa
e
K
SF
c
BA BA
BA BA
ha oha tran
NE
A
S
R
Ro
K
Figure 5. Compression strength, thermoplastics
Young's modulus (MPa)
120
Manufacturer
Test
Theory
100
80
60
40
20
0
3
70
6
GP
45
70
6
GP
6
70
6
GP
8
70
6
GP
67
64
ER YER
Y
BA
BA
5
01
8
GP
Figure 6. Young’s modulus, thermosets
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Rickard Juntikka and Stefan Hallström
Compression strength (MPa)
3
Manufacturer
Test
Theory
2.5
2
1.5
1
0.5
0
03
GP
67
G
5
04
7
P6
6
70
6
GP
8
70
6
GP
67
64
R
R
E
E
Y
Y
BA
BA
5
01
8
GP
Figure 7. Compression strength, thermosets
1800
Young's modulus (MPa)
1600
Manufacturer
Test
Theory
1400
1200
1000
800
600
400
200
0
p .3
.3
-4
-5
lsa 70
70 70
80
Ba 135- 16- C6- P8- E 30 E 40 -Com cel 2 RE 2
E
P BP
x
o
R
R
E
P
O
e
r
P
TB T -CO -CO
Eu H P-C
TB
TB
P
P
Figure 8. Young’s modulus, balsa and honeycombs
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
Compression strength (MPa)
1800
Manufacturer
Test
Theory
1600
1400
1200
1000
800
600
400
200
0
p .3
.3
-4
-5
lsa 70
70 70
80
Ba 135- 16- C6- P8- E 30 E 40 -Com cel 2 RE 2
E
P
P
x
PE B P TB TB COR COR Euro He -CO
T
P
TB
PP-
Figure 9. Compression strength, balsa and honeycombs
Shear
Due to poor or unwanted results from the compression tests some materials were
rejected before the shear tests were performed. The remaining materials were
• Rohacell 31 IG
• Strandfoam EA1000
• General Plastics FR-6703
• Tubus Bauer PEI6-70
• Tubus Bauer PC6-70
• Plascore PP30-5
• Plascore PP40-4
• Hexcel 1/4-5052-.001N-2.3
• Plascore 2.3-3/8-P-3003
The reasons for leaving the other materials out were too low densification
strain, the plateau stress varying strongly with strain, too high plateau stress,
or poor specific properties (with respect to weight).
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The out-of-plane shear properties of the aluminum honeycombs (Hexcel and
Plascore) were measured in the direction in which the cell walls had double
thickness (the stiffest direction). The results from the shear tests are illustrated
in Figures 10-13 and the numerical values are presented in Appendix 1.
Shear modulus (MPa)
25
Manufacturer
Test
Theory
20
15
10
5
0
ell
31
c
ha
am
dfo
n
tra
Ro
S
03
GP
67
Figure 10. Shear modulus, foam
1
Manufacturer
Test
Theory
Shear strength (MPa)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
ell
31
c
ha
Ro
m
foa
St
d
ran
3
70
6
GP
Figure 11. Shear strength, foam
278
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
Young's modulus (MPa)
400
Manufacturer
Test
Theory
350
300
250
200
150
100
50
0
0
-70
6-7
1
PE
TB
TB
6
PC
0-5
3
RE
CO
P-
0-4
4
RE
L
CE
X
CO
HE
P-
2.3
E
OR
2.3
C
P-
Figure 12. Shear modulus, honeycombs
Shear strength (MPa)
2.5
Manufacturer
Test
Theory
2
1.5
1
0.5
0
0
6-7
TB
1
PE
TB
-70
6
PC
CO
P-
0-5
3
RE
CO
P-
0-4
4
RE
HE
X
L
CE
2.3
E
OR
2.3
C
P-
Figure 13. Shear strength, honeycombs
Cellular Polymers, Vol. 23, No. 5, 2004
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Rickard Juntikka and Stefan Hallström
Additional Compression Experiments
The results from the additional compression tests are presented in Table 6.
Standard deviations are given within parentheses. Stress vs. strain graphs are
presented in Appendix 2.
Examining the results from the unprepared samples (configuration 1), the
difference compared with previous compression results (Figure 4) is significant.
The tests were performed under identical conditions, which indicates that the
difference is related to the foam samples being extracted from different batches.
To confirm this result, the density of the foams was measured. The density of
the Rohacell foam used in the original compression tests was 31.3 kg/m3, and
the density of the foam used for the additional compression experiments was
35.3 kg/m3. The results from configuration 1 in Table 6 are still 25% lower
than the data given by the manufacturer (Table 1). Using the test results for
configurations 2 and 3 and assuming the Young’s modulus for the epoxy primed
surfaces E1 = 3 GPa, Eq. (3) gives
E2 = 36.0 MPa
t1 = 2.93 mm
Thus, the Young’s modulus matches the tensile data of Table 1.
Energy Absorption
The energy absorption of the different materials was evaluated simply by
plotting the stress-strain curves with the stress normalized with the density.
Figures 14-19 show the specific stress versus strain for all the materials.
Observe the different scales in the figures.
Dynamic Compression Experiments
The results from the experiments using the constant velocity impact rig(22)
were initially influenced by noise. Due to this, some results were filtered and
Table 6. Results from uni-axial compression tests of Rohacell 31 IG
Configuration
no.
1
2
3
280
Cell Poly 5 04.indb 280
Primed with
epoxy
Resultant Young’s modulus
(MPa)
Yield stress
(MPa)
No
Both ends
Both ends
26.6 (1.07)
46.6 (3.14)
65.2 (4.95)
0.450 (0.0004)
0.524 (0.02)
0.577 (0.017)
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
Figure 14. Stress vs strain normalized with density, materials 1-5
Figure 15. Stress vs strain normalized with density, materials 6-10
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Rickard Juntikka and Stefan Hallström
Figure 16. Stress vs strain normalized with density, materials 11-15
Figure 17. Stress vs strain normalized with density, materials 16-20
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
Figure 18. Stress vs strain normalized with density, materials 21-25
Figure 19. Stress vs strain normalized with density, materials 26-28
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Rickard Juntikka and Stefan Hallström
processed using algorithms in MATLAB. Filtering of the results was performed
with a cut-off frequency chosen so that the load during the rise-time would
not be distorted. For illustration, curves from unprocessed and processed data
from the test with BASF epp55 foam at 3 m/s (strain rate 120 s-1) and 20 ºC
are presented in Figure 20.
The scatter of the test results was very small. The yield stress and the plateau
level differed less than 5% between the samples. Due to this, the results are
presented as the average from three specimens. For more detailed tabulated
values and test graphs, see Appendix 3. Test results from the quasi-static uni-axial
compression tests are included as a reference in the graphs. The dependence
of both velocity and temperature are illustrated in Figures 21 and 22.
DISCUSSION
Quasi-Static Experiments
The test results were for most materials in reasonable agreement with data
from the supplier. Quite large discrepancies were found for the Young’s
modulus of the Rohacell IG-grade foam. For Rohacell 31 IG, the results
from the extended compression tests matched the manufacturer’s tensile data
perfectly. The result supports the assumption that localized deformation on
the loaded surfaces explains the measured difference in Young’s modulus in
Figure 20. Stress-strain curve, BASF epp55, 3 m/s (120 s-1), 20 °C. Original
(irregular) and processed (smooth)(22)
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
Compression strength (MPa)
1
0.9
0.8
Quasi-static 20 °C
-20 °C
20 °C
60 °C
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Rohacell 31 IG
Strandfoam
BASF EPP55
Figure 21. Compression strength as function of temperature, 1.4 m/s (56 s-1)
Compression strength (MPa)
1
0.9
0.8
Quasi-static 20 °C
-20 °C
20 °C
60 °C
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Rohacell 31 IG
Strandfoam
BASF EPP55
Figure 22. Compression strength as function of temperature, 3 m/s (120 s-1)
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Rickard Juntikka and Stefan Hallström
tension and compression. However, the obtained value for t1 is several times
larger than the characteristic cell size of the material and it is unlikely that
the epoxy resin penetrated the foam surfaces to that depth. A secondary effect
from the applied epoxy is the constrained lateral expansion of the foam at the
loaded surfaces. The transition from a condition of plane strain at the surfaces
to a more uni-axial stress state deeper into the specimen, could explain the
relatively high value of t1.
The analytical estimates are approximate and should be treated as such.
Raw material properties are in general uncertain and difficult to measure.
For foams the analytical estimates are fairly accurate, which indicates that
it is possible to estimate foam properties when reliable raw material data is
available. For honeycombs the analytical results were generally poor. The shear
test results were in good agreement with manufacturers’ data, but again, the
analytical predictions were not very accurate. An aspect concerning aluminium
honeycombs is that the plateau stress is generally high unless the honeycomb
has very low density. Neither the stress-strain behaviour at higher strain rates
nor the directional dependence of honeycombs (for oblique impacts) were
addressed in this work. The stress-strain response of foams is smoother than for
honeycombs, i.e. when load peaks occur they are considerably lower than for
honeycombs, for which the amplitude of the peaks could be of the same order
as the plateau stress. Another benefit with the foams is that they are isotropic
and their response is thereby independent of the direction of the applied load.
However, an advantage of honeycombs with respect to foams is the overall
larger densification strain, which allows for energy absorption at an almost
constant load over a greater strain interval.
The tests show that foams and honeycombs made of brittle or elastic-plastic raw
materials are capable of higher specific energy absorption than corresponding
structures made of hyper-elastic materials. Unfortunately it seems like the
specific energy absorption increases with increasing density, see for example
Rohacell 51 IG and 31 IG in Figure 14, while for the current application the
specified maximum force on the head indirectly sets a limit for the densities
that could be used.
Another factor involved in the choice of an energy absorbing material is
actually noise. The material is not allowed to generate annoying sounds at
small deformations, i.e. when for instance a car is driven at normal in-service
conditions. This is presumably a disadvantage for aluminium foams and
honeycombs.
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
As pointed out in the work by Gibson and Ashby(8) and Vural and
Ravichandran(9,10,11) balsa possesses remarkable specific stiffness and strength
in the longitudinal direction. This is confirmed in this work. However, the
analytical estimate of Young’s modulus of balsa was rather poor, in contrary
to the failure stress prediction. There are two possible explanations for the
modulus discrepancy; firstly, the cutting of the specimens might have introduced
deformed cells on the loaded surfaces influencing the elastic portion of the
compression behaviour. Secondly, the formula used for estimating the wood
Young’s modulus (Gibson and Ashby(8)) is derived for considerably higher
densities, ranging from approximately 200 to 1000 kg/m3.
The plateau level of the balsa material tested in this work was approximately
six times larger than the desired yield stress, which disqualifies balsa under the
given circumstances. According to Vural and Ravichandran(9,10,11) balsa exists
in densities ranging from 40 kg/m3, but even the lowest density of balsa would
still produce loads two to three times higher than allowed(9).
Constant Velocity Impact
It is very difficult to make definite conclusions about Rohacell 31 IG based
on fluctuations in the range of a few percents and only three samples at each
temperature and velocity. However, some observations useful for future material
modeling can be made:
• Temperature has a very small influence on compressive yield stress in the
strain rate range 56 to 120 s-1.
• The compressive yield stress increases (30 %) with strain rate in the range
1.7 × 10-3 to 56 s-1.
• The compressive yield stress at 20 °C increases slightly with increasing
strain rate (56 to 120 s-1).
• The lock-up strain decreases with increasing temperature.
The following observations were made in(25,26) from experiments on Rohacell
51 WF (a slightly heavier PMI-based foam compared to the foam used in this
study) and are here quoted for comparison.
• The compressive yield stress increased with increasing strain rate up to 100 s-1.
• The compressive yield stress was independent of strain rate from 100 to
102 s-1.
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• The collapse stress increased with temperature for all tested strain rates
(0.7 - 103 s-1) and temperatures from -20 to 0 °C.
• Between -20 and 0 °C, the compressive collapse stress decreased with
increasing strain rate.
• Above room temperature (20 to 80 °C) a strain rate hardening phenomenon
was observed.
• For all strain rates (0.7 - 103 s-1) and temperatures (-20 to 80 °C), the lock-up
strain decreased with increasing temperature and increased with increasing
strain rate.
• The strain rate sensitivity of the lock-up strain decreased with increasing
temperature.
The present results for Rohacell 31 IG are in good agreement with the results
from(25,26). The yield strength showed strain rate dependence up to a certain
level of strain rate and above this level the strain rate dependence seemed to
diminish. Although the elevated levels of strain rate in this investigation were
all of the same order of magnitude, thus limiting the range over which the
strain rate dependence was examined, the results support the observations made
in(26). The reason for the decreasing lock-up strain with increasing temperature
is difficult to determine without closer examination of the crushed foam. The
thermal expansion of the raw material is not high enough to solely explain
the observed behaviour. A plausible explanation could be that the crushing of
the cell members is more brittle at lower temperatures possibly enabling more
closed packing of the cell wall fragments.
For DOW Strandfoam ea1000 with a Tg of approximately -15 °C, the response
varied considerably over the studied temperature interval. Not only the
compressive yield stress and the lock-up strain, but also the overall stressstrain response changed with temperature. At -20 °C the behavior resembled
the elastic-brittle (crushing) response typical for Rohacell, but at 60 °C the
behaviour was closer to the hyper-elastic response found for e.g. flexible
polyurethane foams at room temperature.
BASF epp55 showed a behavior similar to that of Strandfoam except that
BASF epp55 did not change the overall stress-strain behaviour as significantly
with temperature as Strandfoam did. As for Strandfoam, Tg of BASF epp55
is approximately -15 °C, which makes it sensitive to temperature changes in
the studied temperature range.
288
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
CONCLUSIONS
The compression tests showed that balsa wood absorbed by far most energy per
unit weight. Unfortunately balsa is only available in densities above ~40 kg/m3
which means that it is impossible to use in head impact applications in solid
form because of too large forces being generated at head impact. Honeycombs
are generally good energy absorbers, but the specific energy absorption is still
only about 20 to 40% of that of balsa wood. Both balsa and honeycombs suffer
from anisotropy, resulting in large directional variations of the response. Some
foams are good energy absorbers, like Rohacell, Strandfoam and a couple of
General Plastic foams. These foams have in common that they demonstrate
relatively high specific energy absorption and a fairly horizontal plateau level.
Material properties for foams seem to be possible to estimate accurately when
reliable raw material data is available, but for honeycombs the analytical models
used in this study consistently gave rather poor results.
A comparison of the Young’s modulus extracted for Rohacell 31 IG from the
quasi-static compression tests and data supplied by the manufacturer showed
fairly large differences. The differences were ascribed to the different test
methods used. A general conclusion is that compressive tests are not suitable
for Young’s modulus measurements of cellular materials without use of
extensometers.
The behaviour of the studied foams differs considerably under dynamic
conditions. Relating the results to head impact and normal driving conditions in
a car, BASF epp55 is the foam that would give the most varying load response
with varying strain rate and temperature. It would be difficult to optimise a
preferred response based on FMVSS 201(1) and maximum allowed load over
the spans in strain rate and temperature, which have to be considered for
automotive applications. On the other hand, it is inexpensive, and the shape
of the stress-strain curve is fairly consistent with changing strain rate and
temperature, which for instance could enable FE-modelling using only one
type of material model. The response of Strandfoam ea1000 varies too, but
in a different manner than BASF epp55. The difference is presumably related
to the inherent honeycomb-like structure of the foam. For FE-modelling it
would likely be necessary to make different material models of Strandfoam
for different temperatures.
Cost is most easily compared when the geometry of the desired part is specified.
The cost of Strandfoam ea1000 is higher than for BASF epp55, but the higher
specific energy absorption at room temperature makes it a potential material
candidate for countermeasure applications where space and/or weight is crucial.
Cellular Polymers, Vol. 23, No. 5, 2004
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Rickard Juntikka and Stefan Hallström
Rohacell 31 IG is a quite expensive foam material. However, it might be useful
for specific impact areas where a better overall performance is needed than
what BASF epp55 and Strandfoam ea1000 can provide.
Rohacell 31 IG is well suited for use as energy absorbing material. The behaviour
when compared to BASF epp55 and Strandfoam ea1000 is outstanding in terms
of consistency in mechanical properties and energy absorption over varying
temperatures and strain rates. For FE-modelling of head impact scenarios at
strain rates in the order of 0.5 × 102 to 1 × 102 s-1, the test results imply that the
strain rate effect would be necessary to include in a Rohacell 31 IG material
model while the temperature dependence could be ignored for the investigated
temperature span.
ACKNOWLEDGEMENTS
The financial support by Volvo CC is gratefully acknowledged and the assistance
provided by Björn Lundell at Volvo CC greatly enhanced the progress of the
work that formed the basis for this paper. The authors also wish to express their
gratitude to the material suppliers who provided material for the experiments
performed in this work.
NOMENCLATURE
a
Tg
acceleration
G* shear modulus of the foam
glass transition temperature
Gs shear modulus of the raw material
*
σ y yield strength of the foam
τ*y shear strength of the foam
σys yield strength of the raw material
p
*
ρ
density of the foam
pressure
*
E
Young’s modulus of the foam
ρs
density of the raw material
Es Young’s modulus of the raw material
ν
poisson’s ratio
ε·
strain rate
REFERENCES
1.
National Highway Traffic Safety Administration (NHTSA), (10-1-98 Edition),
Federal Motor Vehicle Safety Standard No. 201: Occupant Protection In Interior
Impact.
2.
Han, Z. and Gérard, G. “Crushing behaviour of aluminium honeycombs under
impact loading”, International Journal of Impact Engineering, Vol. 21, No. 10,
pp 827-836, 1998.
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3.
Fong, W., Nusholtz, G., Chaudhry, M., Williams, S. “Comparison of energy
management materials for head impact protection”, SAE-paper 970159, 1997.
4.
Everitt, L., Fialka, J., Kerman, M., Laabs, E. ”A comparative study of energy
absorbing foams for head impact energy management”, SAE-paper 980972,
1998.
5.
Sims, G.L.A. and Bennett, J.A. ”Cushioning performance of flexible polyurethane
foams”, Polymer engineering and science, Vol. 38, No. 1, pp 134-142, 1998.
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Allsop, D.L. “Skull and Facial Bone Trauma: Experimental Aspects”, Accidental
Injury, Biomechanics and Prevention, Nahum, A. M., Melvin, J. W., eds., SpringerVerlag, New York, pp 247 – 267, 1993.
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Yoganandan, N., Zhang, J., Pintar, F.A., Gennarelli, T.A., Kuppa, S. and Eppinger,
R.H. “Biomechanics of Lateral Skull Fracture”, IRCOBI Conference, Lisbon
(Portugal), pp 69-78, 2003.
8.
Gibson, L.J. and Ashby, M.F. “Cellular solids, Structure and properties”, second
edition, University press, Cambridge, UK, 1997.
9.
Vural, M. and Ravichandran, G., “Microstructural aspects and modeling of failure
in naturally occurring porous composites”, Mechanics of materials, Vol. 35, pp
523 – 536, 2003.
10. Vural, M. and Ravichandran, G. “Dynamic response and energy dissipation
characteristics of balsa wood: experiment and analysis”, International journal
of solids and structures, Vol. 40, pp 2147 – 2170, 2003.
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in naturally occurring composites”, Composites Part B: engineering, Vol. 35, pp
639 – 646, 2004.
12. ASTM D 1622, ”Standard Test Method for Apparent Density of Rigid Cellular
Plastics”, The American Society for Testing and Materials.
13. DIN EN ISO 845, “Cellular plastics and rubbers; determination of apparent (bulk)
density”, DIN Deutsches Institut für Normung.
14. ASTM D 638, “Standard Test Method for Tensile Properties of Plastics”, The
American Society for Testing and Materials.
15. ASTM D 3575, “Standard Test Methods for Flexible Cellular Materials Made
From Olefin Polymers”, The American Society for Testing and Materials.
16. ASTM D 1621-00, ”Standard Test Method for Compressive Properties Of Rigid
Cellular Plastics”, The American Society for Testing and Materials.
17. ASTM C 273, “Standard Test Method for Shear Properties of Sandwich Core
Materials”, The American Society for Testing and Materials.
Cellular Polymers, Vol. 23, No. 5, 2004
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Rickard Juntikka and Stefan Hallström
18. DIN 53294, “Testing of sandwiches; Shear test”, DIN Deutsches Institut für
Normung.
19. ASTM C 271-61, ”Standard Test Method for Density of Core Materials for
Structural Sandwich”, The American Society for Testing and Materials.
20. ASTM C365-57, ”Standard Test Methods for Flatwise Compressive Strength of
Sandwich Cores”, The American Society for Testing and Materials.
21. ASTM C263-00, “Standard Test Method for Shear Properties of Sandwich Core
Materials”, The American Society for Testing and Materials.
22. Juntikka, R. and Hallström, S., “Weight-balanced drop test method for
characterization of dynamic properties of cellular materials”, International
Journal of Impact Engineering, Vol. 30, Issue 5, pp. 541 - 554, 2004.
23. Åström, B.T. “Manufacturing of Polymer Composites”, Chapman & Hall, UK,
1997.
24. Budinski, K.G., “Engineering Materials, Properties and Selection”, Prentice Hall,
New Jersey, USA, 1992.
25. Li, Q.M, Mines, R.A.W and Birch, R.S., The crush behavior of Rohacell-51WF
structural foam, International Journal of Solids and Structures, 37 (2000), 63216341
26. Li, Q.M, Mines, R.A.W and Birch, R.S., Combined strain rate and temperature
effects on compressive strength of Rohacell-51WF structural foam, Proceedings
of the 3rd Asia-Pacific conference on shock & impact loads on structures, 221226, 1999.
292
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
APPENDIX 1. COMPRESSION AND SHEAR TEST RESULTS
Compression (standard deviation within parentheses)
Nr. Manufacturer
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
Designation
RÖHM
RÖHM
DOW
KANEKA
BASF
BASF
BASF
BASF
CALIGEN
GENERAL PLASTICS
GENERAL PLASTICS
GENERAL PLASTICS
GENERAL PLASTICS
GENERAL PLASTICS
BAYER AG1
BAYER AG1
Drünert / Hobbyträ
CYMAT
HYDRO
TUBUS BAUER
TUBUS BAUER
TUBUS BAUER
TUBUS BAUER2
PLASCORE2
PLASCORE2
EURO-COMPOSITES
ρ*
(kg/m3)
52.3
31.3
38.1
25.4
31.5
48.1
36.9
96.0
71.7
47.0
70.4
95.7
128
230
59.1
100
72.0
105
128
94.7
72.2
71.1
90.0
131
110
31.5
σy*
E*
(MPa)
(MPa)
47.2 (5.64) 0.98 (0.004)
14.3 (1.43) 0.33 (0.004)
10.6 (0.58) 0.38 (0.007)
0.88 (0.04) 0.04 (8.0∗10-4)
3.98 (0.39) 0.12 (0.004)
10.4 (0.54) 0.26 (0.005)
12.0 (0.92) 0.30 (0.001)
59.7 (2.85) 1.04 (0.03)
0.33 (0.03)
0.009 (-)
12.0 (0.25) 0.43 (0.007)
22.2 (1.76) 0.67 (0.04)
33.1 (2.63) 1.02 (0.05)
72.6 (1.32) 1.88 (0.01)
11.2 (0.34)
0.39 (0.01)
11.3 (0.75)
0.34 (0.01)
32.8 (0.86) 0.96 (0.01)
353 (63.0)
4.30 (0.11)
23.6 (6.05) 0.18 (0.01)
40.2 (10.0) 0.44 (0.10)
176 (2.66)
2.35 (0.24)
180 (3.34)
2.73 (0.10)
122 (0.66)
1.94 (0.02)
123 (4.55)
1.88 (0.05)
83.9 (2.59) 1.44 (0.006)
76.1 (1.52) 1.24 (0.03)
62.3 (1.32) 1.00 (0.02)
Rohacell 51 IG
Rohacell 31 IG
Strandfoam
EPE 25
EPP 25
EPP 55
EPS 50
EPS 100
E7000H
FR-6703
FR-67045
FR-6706
FR-6708
FP-8015
64if80
67if80
Balsa/ 70
Al 3%
Al 6%
PEI3.5-70
PEI6-70
PC6-70
PP8-80
PP30-5
PP40-4
Aramid fibrepaper/ECA
4.8-32
27 HEXCEL
1/438.2 252 (7.44)
5052-.001N-2.3
28 PLASCORE
2.3-3/8-P-3003
36.4 327 (10.2)
1
2
Tested without any skin-layer on surface.
With polyester facings
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1.48 (0.01)
1.03 (0.02)
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Rickard Juntikka and Stefan Hallström
Shear. Standard deviation in parenthesis.
τy*
G*
(MPa)
(MPa)
2
RÖHM
Rohacell 31 IG
20.2 (3.36)
0.46 (0.01)
3
DOW
Strandfoam
7.31 (0.34) 0.21 (0.003)
10 GENERAL PLASTICS
FR-6703
5.29 (1.33) 0.26 (0.003)
21 TUBUS BAUER
PEI6-70
37.3 (5.68)
1.06 (0.04)
22 TUBUS BAUER
PC6-70
24.5 (2.53)
0.72 (0.01)
24 PLASCORE
PP30-5
18.1 (1.43) 0.52 (0.004)
25 PLASCORE
PP40-4
14.4 (2.28) 0.38 (0.006)
27 HEXCEL1
1/4-5052-.001N-2.3 318 (49.6)
1.15 (0.03)
1
28 PLASCORE
2.3-3/8-P-3003
197 (19.4) 0.69 (0.02)§
1
Tested in the direction in which the cell walls has double thickness (the stiffest
direction)
Nr. Manufacturer
Designation
APPENDIX 2. EXTENDED UNI-AXIAL COMPRESSION TESTS OF
ROHACELL 31 IG
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
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APPENDIX 3. TEST RESULTS, CONSTANT VELOCITY IMPACT
ROEHM Rohacell 31 IG:
DOW Strandfoam ea1000:
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Selection of Energy Absorbing Materials for Automotive Head Impact Countermeasures
BASF epp 55:
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297
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