OBLIQUE IMPACTS ON BICYCLE HELMETS

NJ Mills & A Gilchrist

Metallurgy and Materials, University of Birmingham, UK

ABSTRACT

Oblique impact tests on bicycle helmets were used to validate Finite Element Analysis

(FEA). Analysis of the forces acting on the ‘road’ surface showed the helmet to have an

effective friction coefficient on the road ≅ 0.2. The peak head rotational acceleration, < 6 krad

s-2, was largely determined by the offset of the line of action of the normal impact force from

the head centre of gravity. Oblique tests on frontal impact sites should be included in

Standards.

Keywords: HELMETS, MODELS, OBLIQUE IMPACTS, FINITE ELEMENT METHOD

BICYCLE HELMET OBLIQUE IMPACTS were performed by Mills and Gilchrist

(2003), without measuring the forces on the simulation road surface. Such force

measurements are now made, to reveal the frictional conditions at the interface. The aim is to

test current helmets in representative oblique impacts, measuring the linear and rotational

acceleration of the headform, and to develop FEA so that design changes can be considered.

To date, very little FEA of bicycle helmet impact has been published, and that only for direct

impacts on the crown of the helmet (Filardi et al, 2001).

OBLIQUE IMPACT TEST RIG A free-falling instrumented headform plus helmet (Figure 1) impacts the upper of two

aluminium plates separated by two triaxial quartz load cells (Kistler 9348B). The assembly, of

total mass 7.5 kg, is moved horizontally at up to 5 m s-1 by a pneumatic cylinder of 1 m

stroke. An Ogle headform, with a plasticized PVC skin and an acrylic wig, is fitted with two

Kistler rotational accelerometers, and a triaxial linear accelerometer.

Fig. 1 – Headform instrumentation and axes, and moving road surface

IRCOBI Conference - Madrid (Spain) - September 2006 425

HEADFORM ACCELERATIONS Figure 2 shows that the headform acceleration traces have a single peak, with some

superimposed oscillation. The peak values of the rotational acceleration components are less

than 6 krad s-2, while the peak linear accelerations (Table 1) are well below the 250 g allowed

in EN 1078 (1997). A plot of FT against FN, for many tests, has a trend line slope of

approximately 0.2 (Fig. 2b and Table 1). During the impact, there is slip then rolling on the

road surface, and slip at the wig/liner interface.

Table 1. Oblique impacts with VH = 3.6 m s-1

VV = 4.5 m s-1

on left 70° site

Helmet Max.

Accel.

G

Max.

FN

kN

Max.

FT

kN

Max. z axis

rot. acc.

krad s-2

Max. y axis

rot. acc.

krad s-2

FT

FN

Aventicum 114 5.2 0.5 3.9 3.6 0.21

Avanti 121 5.6 0.7 3.7 3.0 0.25

Indicator 115 5.5 0.7 3.9 2.8 0.23

Specialized 129 5.9 0.7 4.7 5.6 0.18

-2

-1

0

1

2

3

4

5

-50

0

50

100

150

0 2 4 6 8 10 12

rota

tio

na

l a

cc

ele

rati

on

kra

d s

-2

time ms

lin

ea

r a

cc

ele

rati

on

g

y axis

a

z axis

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 1 2 3 4 5 6

tan

ge

nti

al

forc

e

FT

kN

normal force FN kN

Fig. 2 a) resultant-linear and rotational accelerations vs. time for Aventicum helmet, b)

tangential vs. normal force for oblique impact of Arc helmet on the frontal 90° site

For direct (vertical, with road surface not moving) impacts on some test sites, the headform

rotated on rebound, so the headform undergoes rotational acceleration during the impact. The

peak values were in the range 3.5 to 5.5 krad s-2 for impacts in which the vertical velocity VV=

4.5 ms-1.

FINITE ELEMENT ANALYSIS OF OBLIQUE IMPACTS Dynamic (Explicit) FEA was performed using version 6.5 of ABAQUS. Dr Brüwiler of

EMPA, St Gallen, Switzerland, kindly provided the scanned shapes of a Specialised S1

bicycle helmet (Fig. 1) and a headform. The helmet .stl file was converted, and the number of

triangular facets reduced to 2500, and their aspect ratio restricted to < 2:1, to allow reasonably

fast FEA. There is a 3 or 4 mm gap between the headform and the liner interior surface at the

start of the simulations. The headform was modelled as a rigid shell, with a 4.26 kg point

mass at the centre of gravity. The headform axes are shown in figure 1. The moments of

inertia at the centre of mass are Ixx = 192 kg cm2, Iyy = 240 kg cm2, Izz = 163 kg cm2.

To accurately predict helmet rotation on the headform, the chin strap must closely follow

the headform surface. Separate FEA was performed on 15 mm wide straps, modelled as a

membrane, pulled upward to contact the headform. The deformed strap shape was used as an

orphan mesh in the main FEA. Frictional contact at the interfaces between the head/helmet

interior and between the helmet exterior/road had the values shown in Table 2. The helmet

liner was simulated as polystyrene bead foam (EPS) of density 83 kg m-3, protected on the

exterior by a 0.4 mm micro-shell of polycarbonate; details of the materials models are given

by Mills & Gilchrist (2006a).

The predicted maximum headform accelerations are given in Table 3. The horizontal

component of the impact velocity does not cause any increase in the predicted peak linear

426 IRCOBI Conference - Madrid (Spain) - September 2006

Table 2. Interface friction conditions- tangential to the contact surfaces

Interface friction

coefficient

shear stress limit

Pa

elastic slip stiffness

Pa

shell/road 1.2 5 x 106 5 x 107

liner/head and strap/head 0.6 5 x 105 5 x 106

Table 3. Oblique impacts of S1 helmet with velocity component VV = 5.4 m s-1

Impact site

and direction

VH

m s-1

FN

max

kN

FT

max

kN

Head

amax

g

Head max

1θ&&

krad s-2

Head max

2θ&&

krad s-2

Head max

3θ&&

krad s-2

0 5.51 1.00 132 1.5 0.0 0.2

3.6 6.75 1.10 158 1.6 0.0 0.5

Front 70°

down

10* >5.9 >0.9 >138 <-1.9 ±0.2 0.6

0* 7.22 0.0 165 -4.7 0.0 6.4

3.6* >6.7 -.26 >158 <-6.7 0.0 >6.0

Side 70°

back

10 6.88 1.13 163 -8.0 -1.3 6.6

0 6.12 -.30 145 0.6 0.5 3.3

3.6 5.77 0.57 136 -1.3 -0.7 3.45

Crown

side

10 6.21 1.46 161 -3.0 -1.6 4.1

If > signs are used, the simulation stopped due to element instability

Fig. 3 Predictions for S1 helmet in an oblique test with VH= 3.6, VV= 5.4 ms-1

on the

frontal 70° site at: a) 1 ms, b) 6.8 ms (peak force)

Fig. 4 Upper: predictions for S1 helmet impacted on left 70° site with VN = 5.4 and VT

=10.0 m s-1

, at times: a) 1.8, b) 4.0, c) 6.0 ms. Lower : video frames of Aventicum helmet

impacted on the right 70° site with VN = 4.5 and VT =3.6 m s-1

.

IRCOBI Conference - Madrid (Spain) - September 2006 427

acceleration. The peak rotational accelerations are low for the frontal impact site, as the road

friction acts in opposition to the headform centre of gravity being offset from the impact

point. They are low at the crown site, due to the normal impact force passing close to the head

centre of gravity, and largest at the side 70° site.

The time sequence of events is shown either by high speed video of oblique impacts (lower

part of fig. 4) or by FEA (upper part) to be:-

a) from 0 to 1 ms, the internal pads of comfort foam compress, so the headform moves

into contact with the inside of the helmet liner. The shell slides on the road surface.

b) from 1 to 5 ms, the liner foam compresses (crushes), and a ‘flat’ develops on its outer

surface. The helmet shell ceases to slip, and begins to roll on the road. For lateral

impact sites, the helmet appears to act as a deformable ‘roller’ of near constant

thickness, sliding at the head/helmet interface while rolling on the road. Figure 2a is

at 1.8 ms just after the shell starts to roll. By 4 ms (figure 2b) the liner has crushed

considerable. The ‘flat’ on the outside of the helmet hinders rotation, but slip occurs

at the head/ liner interface where the wearer’s hair acts as a low-friction layer.

c) from 5 to about 10 ms the liner foam partly recovers as the headform moves away

from the road. There is more rotation of the helmet on the head. By 7.8 ms (figure 2d)

the helmet has rotated by about 15° relative to the head’s neck to crown axis.

DISCUSSION

The maximum rotational accelerations were smaller than the 10 krad s-2 at which diffuse

axonal injury is likely (Gennarelli and Thibault L, 1989), but the horizontal velocity

components were only moderate. Bicycle helmets, which only cover the upper regions of the

head, can rotate relatively easily on the head in an oblique impact; the hair and scalp keep the

shear stress at the head/helmet interface to a low value, while the chin strap system only

prevents the helmet lifting from the head. The maximum rotation of the helmet is determined

by interactions such as the brow of the helmet on the nose for forward rotation. Comparison

with experimental oblique impacts suggests that the effective overall friction coefficient with

the road, based on the helmet being fixed to the head, is only about 0.2.

The direct impacts in EN 1078 at VV = 5.4 m s-1 do not include the frontal site, in spite of this

being common in crashes (Depreitere et al., 2004). This is due to the poor performance at this

site and/or the risk of damaging instrumentation. It is possible to test this site in an oblique

impact test. EN 1078 should contain oblique impact tests at the front and sides of helmets.

Velocities of VH = 5 m s-1 VV = 5.4 m s-1 are proposed, with the same 250 g linear acceleration

limit as at present, and the angular acceleration limited to 5 krad s-2 . This would lead to

helmets that are more effective in reducing head injuries. Mills & Gilchrist ( 2006b) used

FEA to consider design changes in bicycle helmets, and showed that protection levels can be

improved.

References

Depreitere B. et al. Bicycle-related head injury: a study of 86 cases, Accid. Anal. & Prevent.

36, (2004) 561.

EN 1078:1997 Helmets for pedal cyclists and for users of skateboards and roller skates, BSI,

London.

Filardi V. et al., Optimisation of the impact performance of a bicycle helmet (in Italian)

www.pcm.unifi.it/Lavorisalerno/ ART_126.pdf.

Gennarelli T.I. and Thibault L.E., Clinical rationale for a head injury angular acceleration

criterion, pp 5-8 in Head Injury Mechanisms, Washington 1989, AAAM.

Mills N.J. & Gilchrist A., Reassessing bicycle helmet impact protection, IRCOBI (2003) 15.

Mills N.J. & Gilchrist A., Dynamic FEA of bicycle helmet oblique impacts, Int. J. Impact

Engng. (2006a) submitted.

Mills N.J. & Gilchrist A., Bicycle helmet design, Proc. I. Mech E, Part L, J. Materials :

Design and Applications (2006b) submitted.

428 IRCOBI Conference - Madrid (Spain) - September 2006