Selection of Energy Absorbing Materials for Automotive ... · Selection of Energy Absorbing...

263Cellular Polymers, Vol. 23, No. 5, 2004

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 diffi cult 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 Traffi c 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 fl esh. The target location typically consists of an energy absorbing material and a

*Corresonding author, E-mail: [email protected]

Cell Poly 5 04.indb 263Cell Poly 5 04.indb 263 8/11/04 4:20:57 pm8/11/04 4:20:57 pm

264 Cellular Polymers, Vol. 23, No. 5, 2004

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 specifi c 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 diffi cult to fi nd 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 specifi c material category.

The work presented in this paper addresses the process to defi ne 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 F

max = 7 kN and the maximum loaded

area of the head at impact was defi ned as Amax

= 0.01 m2. Assuming a mass of the head of m

head = 4.54 kg(1) and engineering relations,

σ = =F

AF ma

(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




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 fi gures 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 defi ned as ρ*/ρ

s, 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 densifi cation and the strain at which it initiates is called the lock-up strain.

The properties of the solid material have strong infl uence 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




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, T

g, of the polymer. Thus, T

g is a signifi cant 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 specifi c 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 specifi c

Figure 1. Uniaxial quasi-static compression stress-strain response for Rohacell 31 IG foam




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 microfi brils 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, σ

pb, 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 ρ

s 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 specifi c 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 fl at 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 densifi cation




strain. Consequently, a material can have an almost ideal load-deformation response but still be of little use for a specifi c 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.

Figure 2. Stress-strain curves for a dense and a cellular material (redrawn from(8), not to scale)

Compression

Strain (%)

Stre

ss

0 10 20 30 40 50 60 70 80 90 100

Fully denseelastic solid

Foam

Energy indense solid

Energy in foam




Table 1. Description of investigated polymer foams and properties given by the supplier

No. Manufacturer Material/Designation

Cell structure

ρ∗

(kg/m3)E*

(MPa)σ

y*

(MPa)G*

(MPa)

1 ROEHM PMI/Rohacell 51 IG **

Closed 50.0(12) 70.0(14) 0.80(16) 21.0(17)

2 ROEHM PMI/Rohacell 31 IG **

Closed 30.0(12) 36.0(14) 0.40(16) 14.0(17)

3 DOW PP/Strandfoam EA1000 **

Option 40.0(13) 9.64(15) 0.34(15) -

4 Kaneka PE/EPE 25 * Closed 25.0 0.077 0.04 -

5 BASF PP/EPP 25 * Closed 25.0 - - -

6 BASF PP/EPP 55 * Closed 55.0 - - -

7 BASF PS/EPS 50 * Closed 50.0 - - -

8 BASF PS/EPS 100 * Closed 100.0 - - -

9 Caligen PU/XE70—H * Open 70.0 - - -

10 General Plastics PU/FR-6703 ** Closed 48.1(12) 12.2(16) 0.48(16) 4.20(17)




14 General Plastics PU/FP-8015 * Closed 240.3(12) - - -

15 Bayer AG PU/64if80 ** Closed 55.0 - 0.30 -

16 Bayer AG PU/67if80 ** Closed 110.0 - 1.50 -* Elastic ** Elastic-plastic

Table 2. Description of investigated balsa wood and properties given by the supplier


Cell structure

ρ∗

(kg/m3)E*

(MPa)σ

y*

(MPa)G*

(MPa)

17 Drünert Balsa/70 Closed ~70 - - -

Table 3. Description of investigated metal foams and properties given by the supplier


Cell structure

ρ∗

(kg/m3)E*

(MPa)σ

y*

(MPa)G*

(MPa)18 Cymat Al/3% ** Closed 81.0 1.50 0.06 -19 Hydro Al/6% ** Closed 170 190 3.80 -* Elastic ** Elastic-plastic




EXPERIMENTS

The experiments were divided into quasi-static uni-axial compression, quasi-static 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.

Table 4. Description of investigated honeycombs and properties given by the supplier

No. Manuf. Material/Designation

Cell structure

ρ∗

(kg/m3)E*

(MPa)σ

y*

(MPa)G*

(MPa)20 Tubus Bauer PEI/PEI3.5-70 ** Circular/

φ3.5 mm70.0 - 2.00 28.5(18)

21 Tubus Bauer PEI/PEI6-70 ** Circular/φ6 mm

70.0 - 1.80 30.0(18)

22 Tubus Bauer PC/PC6-70 ** Circular/φ6 mm

70.0 95 1.90 19.0(18)

23 Tubus Bauer PP/PP8-80 ** Circular/φ8 mm

80.0 97 2.00 12.0(18)

24 Plascore PP/PP30-5 ** Circular/φ7.6 mm

80.1 72.4 1.62 13.8

25 Plascore PP/PP40-4 ** Circular/φ10.2 mm

64.1 49.6 0.97 9.00

26 Euro-Composites

Aramid paper/ECA 4.8-32 **

Hexagonal/4.8 mm

32.0 - 1.15 31.0

27 Hexcel Al/1/4-5052.001N-2.3 **

Hexagonal/6.4 mm

36.8 310 0.83 221

28 Plascore Al/2.3-3/8-P-3003 **

Hexagonal/9.5 mm

36.8 - - -

* Elastic ** Elastic-plastic




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 confi gurations 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 t

1 and a reduced stiffness, E

1, with

respect to the intact material, see Figure 3. To examine the proposed origin of discrepancy, a simplifi ed model utilizing constant stress through the thickness was applied, see Figure 3, together with three different sample confi gurations. Confi guration 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. Confi gurations 2 and 3 were primed with epoxy resin on the loaded surfaces of the samples. Each confi guration 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 confi gurations 1 and 2 was 25 mm. The height in confi guration 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.




The reason for testing different confi gurations 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

tE E

t E t t E=

+ −( )1 2

1 2 1 12 2 (3)

where t represents the total height of the foam sample. Combination of the results from confi gurations 2 and 3 with knowledge about the Young’s modulus of epoxy allowed for determination of E

2 and t

1, 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, T

g, 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 specifi c 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).

Figure 3. Foam sample with presumed damaged cells at the free surfaces




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 ρ

0. 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.




Table 5. Mechanical properties of raw materials

Material Density, ρS

(kg/m3)Young’s modulus,

ES (GPa)Yield strength, σ

ys (MPa)

Tg

(°C)T

m

(°C)

PMI 1200 3.6 120 185 -PP 900 – 915 1.1 – 1.6 31 – 42 -20 – -5 165 – 175LD PE 910 – 940 0.15 – 0.24 6 – 10 -5 – 0 170 – 240HD PE 950 – 970 0.55 – 1.0 20 – 28 25 – 30 170 – 240PS 1050 1.4 – 3.0 30 – 35 100 180 – 250PU (rigid)

1200 1.6 130 130 130

PU (fl exible)

1200 0.045 25 – 50 20 – 30 130

AL 5052 2700 69 90 – 290 650 660AL 3003 2700 69 40 – 190 650 660PEI 1270 3.0 105 215 -PC 1200 ~2 ~60 ~140 310 – 350Balsa 1500 35 350 - -Nomex* 740* 3.0 – 5.0* 150 – 250* ~350* -* Derived from Nomex data-sheet

Figure 4. Young’s modulus, thermoplastics

ManufacturerTestTheory

You

ng's

mod

ulus

(M

Pa)

120

100

80

60

40

20

0

Rohacell 5

1

Rohacell 3

1

Strandfoam

KANEKA 25

BASF 25

BASF 55

BASF 50

BASF 100




Figure 5. Compression strength, thermoplastics

Figure 6. Young’s modulus, thermosets


Com

pres

sion

str

engt

h (M

Pa)

1.4

1.2

1

0.8

0.6

0.4

0.2

0

Rohacell 5

1


You

ng's

mod

ulus

(M

Pa)

120

100

80

60

40

20

0

GP 6703

Rohacell 3

1

Strandfoam

KANEKA 25

BASF 25

BASF 55

BASF 50

BASF 100

GP 67045

GP 6706

GP 6708

GP 8015

BAYER 64

BAYER 67




Figure 7. Compression strength, thermosets

Figure 8. Young’s modulus, balsa and honeycombs


3

2.5

2

1.5

1

0.5

0

GP 6703

GP 67045

GP 6706

GP 6708

GP 8015

BAYER 64

BAYER 67

Com

pres

sion

str

engt

h (M

Pa)


You

ng's

mod

ulus

(M

Pa)

1800

1600

1400

1200

1000

800

600

400

200

0

Balsa

TB PE135-70

TB PE16-70

TB PC6-70

TB PP8-80

P-CORE 30-5

P-CORE 40-4

Euro-Comp

Hexcel 2.3

P-CORE 2.3




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 densifi cation strain, the plateau stress varying strongly with strain, too high plateau stress, or poor specifi c properties (with respect to weight).

Figure 9. Compression strength, balsa and honeycombs


1800

1600

1400

1200

1000

800

600

400

200

0

Balsa

TB PE135-70

TB PE16-70

TB PC6-70

TB PP8-80

P-CORE 30-5

P-CORE 40-4

Euro-Comp

Hexcel 2.3

P-CORE 2.3

Com

pres

sion

str

engt

h (M

Pa)




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.

Figure 10. Shear modulus, foam

Figure 11. Shear strength, foam


25

20

15

10

5

0

Rohacell 3

1

Shea

r m

odul

us (

MPa

)

Strandfoam

GP 6703


Rohacell 3

1

Shea

r st

reng

th (

MPa

)

Strandfoam

GP 6703

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0




Figure 12. Shear modulus, honeycombs

Figure 13. Shear strength, honeycombs


TB PE16-70

TB PC6-70

P-CORE 30-5

P-CORE 40-4

HEXCEL 2.3

P-CORE 2.3

You

ng's

mod

ulus

(M

Pa)

400

350

300

250

200

150

100

50

0


TB PE16-70

TB PC6-70

P-CORE 30-5

P-CORE 40-4

HEXCEL 2.3

P-CORE 2.3

Shea

r st

reng

th (

MPa

)

2.5

2

1.5

1

0.5

0




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 (confi guration 1), the difference compared with previous compression results (Figure 4) is signifi cant. The tests were performed under identical conditions, which indicates that the difference is related to the foam samples being extracted from different batches. To confi rm 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 confi guration 1 in Table 6 are still 25% lower than the data given by the manufacturer (Table 1). Using the test results for confi gurations 2 and 3 and assuming the Young’s modulus for the epoxy primed surfaces E

1 = 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 specifi c stress versus strain for all the materials. Observe the different scales in the fi gures.

Dynamic Compression Experiments

The results from the experiments using the constant velocity impact rig(22) were initially infl uenced by noise. Due to this, some results were fi ltered and

Table 6. Results from uni-axial compression tests of Rohacell 31 IG

Confi guration no.

Primed with epoxy

Resultant Young’s modulus(MPa)

Yield stress(MPa)

1 No 26.6 (1.07) 0.450 (0.0004)2 Both ends 46.6 (3.14) 0.524 (0.02)3 Both ends 65.2 (4.95) 0.577 (0.017)




Figure 14. Stress vs strain normalized with density, materials 1-5





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)




Figure 21. Compression strength as function of temperature, 1.4 m/s (56 s-1)

Figure 22. Compression strength as function of temperature, 3 m/s (120 s-1)

Quasi-static 20 °C-20 °C20 °C60 °C

Rohacell 31 IG

Com

pres

sion

str

engt

h (M

Pa)

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0Strandfoam BASF EPP55

Quasi-static 20 °C-20 °C20 °C60 °C

Rohacell 31 IG

Com

pres

sion

str

engt

h (M

Pa)

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0Strandfoam BASF EPP55




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 t

1.

The analytical estimates are approximate and should be treated as such. Raw material properties are in general uncertain and diffi cult 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 benefi t 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 densifi cation 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 specifi c energy absorption than corresponding structures made of hyper-elastic materials. Unfortunately it seems like the specifi c energy absorption increases with increasing density, see for example Rohacell 51 IG and 31 IG in Figure 14, while for the current application the specifi ed 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.




As pointed out in the work by Gibson and Ashby(8) and Vural and Ravichandran(9,10,11) balsa possesses remarkable specifi c stiffness and strength in the longitudinal direction. This is confi rmed 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; fi rstly, the cutting of the specimens might have introduced deformed cells on the loaded surfaces infl uencing 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 disqualifi es 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 diffi cult to make defi nite conclusions about Rohacell 31 IG based on fl uctuations 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 infl uence 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.




� 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 diffi cult 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 stress-strain 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. fl exible 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 signifi cantly with temperature as Strandfoam did. As for Strandfoam, T

g of BASF epp55

is approximately -15 °C, which makes it sensitive to temperature changes in the studied temperature range.




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 specifi c 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 specifi c 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 diffi cult 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 specifi ed. The cost of Strandfoam ea1000 is higher than for BASF epp55, but the higher specifi c energy absorption at room temperature makes it a potential material candidate for countermeasure applications where space and/or weight is crucial.




Rohacell 31 IG is a quite expensive foam material. However, it might be useful for specifi c 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 fi nancial 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 acceleration G* shear modulus of the foam

Tg glass transition temperature G

s 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 pressure

ρ* density of the foam E* Young’s modulus of the foam

ρs density of the raw material E

s Young’s modulus of the raw material

ν poisson’s ratio ε strain rate

REFERENCES

1. National Highway Traffi c 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.

·




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 fl exible polyurethane foams”, Polymer engineering and science, Vol. 38, No. 1, pp 134-142, 1998.

6. Allsop, D.L. “Skull and Facial Bone Trauma: Experimental Aspects”, Accidental Injury, Biomechanics and Prevention, Nahum, A. M., Melvin, J. W., eds., Springer-Verlag, New York, pp 247 – 267, 1993.

7. 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.

11. Vural, M. and Ravichandran, G., “Failure mode transition and energy dissipation 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 Olefi n 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.




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), 6321-6341

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-Pacifi c conference on shock & impact loads on structures, 221-226, 1999.




APPENDIX 1. COMPRESSION AND SHEAR TEST RESULTS

Compression (standard deviation within parentheses)

Nr. Manufacturer Designation ρ*(kg/m3)

E*(MPa)

σy*

(MPa)

1 RÖHM Rohacell 51 IG 52.3 47.2 (5.64) 0.98 (0.004)

2 RÖHM Rohacell 31 IG 31.3 14.3 (1.43) 0.33 (0.004)

3 DOW Strandfoam 38.1 10.6 (0.58) 0.38 (0.007)

4 KANEKA EPE 25 25.4 0.88 (0.04) 0.04 (8.0∗10-4)

5 BASF EPP 25 31.5 3.98 (0.39) 0.12 (0.004)

6 BASF EPP 55 48.1 10.4 (0.54) 0.26 (0.005)

7 BASF EPS 50 36.9 12.0 (0.92) 0.30 (0.001)

8 BASF EPS 100 96.0 59.7 (2.85) 1.04 (0.03)

9 CALIGEN E7000H 71.7 0.33 (0.03) 0.009 (-)

10 GENERAL PLASTICS FR-6703 47.0 12.0 (0.25) 0.43 (0.007)



13 GENERAL PLASTICS FR-6708 128 72.6 (1.32) 1.88 (0.01)

14 GENERAL PLASTICS FP-8015 230 11.2 (0.34) 0.39 (0.01)

15 BAYER AG1 64if80 59.1 11.3 (0.75) 0.34 (0.01)

16 BAYER AG1 67if80 100 32.8 (0.86) 0.96 (0.01)

17 Drünert / Hobbyträ Balsa/ 70 72.0 353 (63.0) 4.30 (0.11)

18 CYMAT Al 3% 105 23.6 (6.05) 0.18 (0.01)

19 HYDRO Al 6% 128 40.2 (10.0) 0.44 (0.10)

20 TUBUS BAUER PEI3.5-70 94.7 176 (2.66) 2.35 (0.24)

21 TUBUS BAUER PEI6-70 72.2 180 (3.34) 2.73 (0.10)

22 TUBUS BAUER PC6-70 71.1 122 (0.66) 1.94 (0.02)

23 TUBUS BAUER2 PP8-80 90.0 123 (4.55) 1.88 (0.05)

24 PLASCORE2 PP30-5 131 83.9 (2.59) 1.44 (0.006)

25 PLASCORE2 PP40-4 110 76.1 (1.52) 1.24 (0.03)

26 EURO-COMPOSITES Aramid fi bre-paper/ECA 4.8-32

31.5 62.3 (1.32) 1.00 (0.02)

27 HEXCEL 1/4-5052-.001N-2.3

38.2 252 (7.44) 1.48 (0.01)

28 PLASCORE 2.3-3/8-P-3003 36.4 327 (10.2) 1.03 (0.02)1 Tested without any skin-layer on surface. 2 With polyester facings




Shear. Standard deviation in parenthesis.

Nr. Manufacturer Designation G*(MPa)

τy*

(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)28 PLASCORE1 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)

APPENDIX 2. EXTENDED UNI-AXIAL COMPRESSION TESTS OF ROHACELL 31 IG




APPENDIX 3. TEST RESULTS, CONSTANT VELOCITY IMPACT

ROEHM Rohacell 31 IG:

DOW Strandfoam ea1000:




BASF epp 55:


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