Motorcycle helmets are vital to protect from recurrent road accidents as they prove crucial in reducing brain trauma. This research piece presents a new and plausible bio-inspired design affined to the foam liner material and structure in helmets. The proposed liner design is inspired from animal horn micro-structure and tubule arrangement. An innovative drop-testing apparatus is presented with a spring-ratchet mechanism for experimental testing. The aim is to validate the new design by meeting the ECE 22.05 standard for motorbike helmets using peak linear acceleration and HIC criteria. Experimental results are partly verified against FEA simulations for two proposed samples. Further samples call for more complex simulations at a later stage to best describe material properties and structures.

1. Department of Mechanical Engineering American University of Beirut
Towards a Safer Design of Helmets:
Finite Element and Experimental Assessment
The 2016 ASME
International Mechanical Engineering
Congress & Exposition
Phoenix Convention Center, Phoenix, AZ
November 16, 2016

2. Department of Mechanical Engineering American University of Beirut
Sarah Siblini
Structural Engineer
MEA – Air Liban
Authors & Current
Affiliations
Mutasem Shehadeh, PhD
Associate Professor
American University of Beirut (AUB)
Bilal Wehbi
Graduate Engineer
Petrofac Emirates LLC
Omar Abro
Post-graduate Studies
Imperial College London
Sari Kassar
Rackham Graduate School
University of Michigan

3. Motivation
3
Did you know? Motorcyclists …
➢ More vulnerable to injury than car passengers
➢ Best preventive method of head injury to date  motorcycle helmets
➢ Helmet design is effective but not ideal  reduces head injury by 72%
Objectives
❖ Improve energy absorption in motorcycle helmet foam liner
❖ Decrease peak linear acceleration and head injury criterion values
❖ Allow helmet to sustain multiple impacts and ensure safety

4. Project Deliverables
① Literature Review
• Biomechanics of Head Injury and Tolerance Limits
• Helmet Anatomy
• Bio-inspired Design
② Experimental Assessment
• Materials Used for Liner
• ISO J Head-Form
• Drop Testing Apparatus
• Tested Samples and Multiple-Impact Results
• Proposed Bio-inspired Sample and Results
③ Finite Element Analysis
• Geometry
• UTM Compression of Cork and EPS
• Material Properties
• Boundary Conditions 4

5. The helmet will be tested based on the ECE 22.05 European standard with PLA
< 275g and HIC < 2400 at impact speed 7.5m/s
Biomechanics of Head Injury and
Tolerance Limits
5
Head Injury
Accounts to higher % among other body parts in fatalities
1. Brain Injury  primary “failure” mode
2. Skull Fracture
3. Neck Injury
4. Scalp damage
Head Tolerance Limits and Criteria
1- Peak Linear Acceleration (PLA):
Maximum value of acceleration measured at COG of head-
form i.e. Brain; in units of “g” (gravitational
acceleration=9.81m/s2)
2- Head Injury Criteria (HIC):
Based on Wayne state tolerance curve; takes into account
impact duration Wayne State Tolerance Curve
(Kelvin,2002)

6. Helmet Anatomy
6
The Basic Safety Components of a
Motorcycle Helmet:
1. Outer Shell
2. Liner Foam
3. Visor
4. Fastening Strap
5. Interior Padding
The Expanded Polystyrene (EPS) foam liner:
• Thickness: 30 - 40 mm
• Absorbs the most impact energy of 30-50% (Pinnoji et
al. and Kostopoulos et al.)
• Closed-cell low weight foam, cheap, and undergoes
permanent crushing

7. Bio-Inspired Design
7
Cattle horns, rhinoceros horns, and equine hooves offer great shock
absorbance for linear and rotational impact application
Horn Structure
•Arranged in a lamellar structure of keratin
sheets.
•Lamellar structures are in the radial direction
with tubules (d=40-500 µm) longitudinally.
(8-12% porosity outside and 0% inside)
•Compression in the radial direction has the
lowest elastic modulus and yield strength
allowing for more energy absorbance
Horn Anatomy
Oriented Keratin Fiber Structure
Loaded in Radial Direction
(redrawn from McKittrick et al.)

8. Materials Used for Liner in Testing
8
Micro Agglomerate Cork (MAC)
Acts as a spring, has high elasticity, viscoelastic in
nature but much denser
Expanded Polystyrene (EPS)
Light material, can be modeled as isotropic, one-time
use
Sandwiched Configuration:
A combined structure of Cork-EPS-Cork with MAC
(20-40%) in a linearly packed structure would show
best properties in shock absorption (Coehlo et al.)

9. ISO-J Head form
9
• Made according to ECE 22.05 ISO-J Headform Standard
• High strength aluminum “ALUMEC” of density 2830 kg/m3 and tensile
strength of 575 MPa
• Weighs 4.038kg with accelerometer (SlamStick-X)
• Drawn with CAD software “Rhinocerous”
• CNC manufactured at AUB
Extracted from: http://www.cadexinc.com/en960_full_urethane_headform.php
Plastic SlamStick
Accelerometer by MIDE
Can take 500g and 20KHZ
sampling

10. ISO-J Head form Video
10

11. Drop Testing Apparatus
11
• Spring loaded mechanism
(of stiffness k instead of drop tower)
to reach speed of 7.5m/s
• Heavy bottom base (100kg)
to simulate flat impact and anvil
D=14cm
• The pulling mechanism consists
of a manual ratchet pulley
mechanism that can withstand
2.5 tons
In-House Built Impact Apparatus and
Components

12. Tested Samples
12
Figure: Preliminary Foam Configurations Tested
Sample 1: CEC: MAC Layer- EPS Layer - MAC Layer
Sample 2: CECE: MAC Layer- EPS Layer - MAC Layer -EPS Layer
Sample 3: EPS Only
Sample 4: CE: MAC Layer(top) - EPS Layer
Sample 5: Preliminary Bio-inspired Sample
Sample 6: Enhanced Bio-inspired Sample (covered later)

13. Drop Testing Apparatus Video
13

14. Multi-Impact Results:
CEC and CECE
14
cec impact1
cec impact3
cec impact2
time (ms)
acceleration(g)
Head Impact Curves for Multiple Impact on CEC Configuration
time (ms)
acceleration(g)
Head Acceleration Curves for Multiple Impact on CECE Configuration
cece impact1
cece impact3
cece impact2
260 278Average

15. Multi-Impact Results:
Bio-inspired Design 1
15
time (ms)
acceleration(g)
Head Impact Curves for Multiple Impact on Preliminary Bio-inspired Design
Configuration
biodesign
biodesign

16. MAC
EPS
Updated Bio-inspired Design
16
Exploded View of Configuration
using Creo Parametric
Impact this side up

17. Multi-Impact Results
17
The black helmet (no bio-inspired design insert) failed after
2 impacts (duration: 4min)
The helmet with bio-inspired insert failed after 3 impacts
(mean duration of 3:42:5 min)
Black Helmet PLA(g) HIC
Trial 1 85.086 70.25
Trial 2 461.27 943.21
Trial 3 N/A N/A
Bio PLA(g) HIC
Trial 1 112.48 144.34
Trial 2 133.76 112.27
Trial 3 210.93 663.32

18. Finite Element Analysis
18

19. Finite Element Analysis
19
EPS Foam Compression Curve Explained (Same for MAC)
Elastomeric forms typically show the following
behavior composed of 3 stages in the figure
(extracted from Di Landro et al.)
Figure : Typical Stress-Strain Diagram of
rigid foam
elastic
Plastic plateau
Densification
(Vaitkus et al.)
EPS microstructure
EPS macrostructure
density 23kg/m3
Stress Strain Curves EPS representative densities
(L Cui. Et al)

20. Finite Element Analysis
- Materials
20
Modeling Cork in ABAQUS
• Quasi-Static compression (strain rate 10mm/s)
• Eight-node brick element with reduced
integration (C3D8R)
• Permanent energy dissipation using the
“Mullins Effect” along with the Hyper-foam
model. (Fernandes et al.)
• Result: 2.4mm deformation (verified)
Modeling EPS in ABAQUS
• Same conditions as above
• “Crushable Foam” in ABAQUS library with
volumetric hardening along with linear
elastic model
• Result: 7.5mm deformation (verified)
Side Note on Modeling ABS:
Literature values of E = 2 GPA v=0.37 , and
yield stress of 60 MPa; elastic and perfectly
plastic(L.T Chang et al.)

21. Finite Element Analysis
Geometry and BC
21
• Geometry of the head-form is imported to ABAQUS for meshing and
introducing material properties (modeled as rigid along with anvil)
• The outer foam liner and shell are drawn with PTC Creo
• Boundary Conditions:
Anvil is fixed, helmet outer liner and foam liner and Head are in contact, impact
velocity of 7.5m/s
PLA can be extracted as soon as meshing and BC are finalized

22. References
22
• F. M. Shuaeib, A. M. S. Hamouda, R. S. Radin Umar, M. M. Hamdan, and M. S. J. Hashmi, “Motorcycle helmet - Part I.
Biomechanics and computational issues,” J. Mater. Process. Technol., vol. 123, no. 3, pp. 406–421, 2002.
• B. Liu, R. Ivers, R. Norton, S. Boufous, S. Blows, S.K. Lo, Helmets for preventing injury in motorcycle riders, 2008
• M. C. Tsai and D. Hemenway, “Effect of the mandatory helmet law in Taiwan,” Inj Prev, vol. 5, no. 4, pp. 290–291, 1999.
• Ouellet, J.V, Thom,D. R.,Smith, T. and Hurt Jr., H. H., 1986.,"Helmets and Neck Injuries in Fatal Crashes”
• E. A. Luce, T. D. Tubb, and A. Moore, “Review of 1,000 major facial fractures and associated injuries.,” Plast Reconstr
Surg, vol. 63, no. 1, pp. 26–30, 1979.
• P. K. Pinnoji, P. Mahajan, N. Bourdet, C. Deck, and R. Willinger, “Impact dynamics of metal foam shells for motorcycle
helmets: Experiments & numerical modeling,” Int. J. Impact Eng., vol. 37, no. 3, pp. 274–284, 2010.
• V. Kostopoulos, Y. . Markopoulos, G. Giannopoulos, and D. . Vlachos, “Finite element analysis of impact damage
response of composite motorcycle safety helmets,” Compos. Part B Eng., vol. 33, no. 2, pp. 99–107, 2002.
• R. M. Coelho, R. J. Alves de Sousa, F. A. O. Fernandes, and F. Teixeira-Dias, “New composite liners for energy absorption
purposes,” Mater. Des., vol. 43, pp. 384–392, 2013.
• Snell Foundation, 'Philosophy and Concept of Helmet Testing’, 2015. Available: http://www.smf.org/testing
• Vaitkus,S. Laukaitis,A. Gnipas, I. Kersulis, V. and Vejelis, S. “Experimental Structure and Deformation Mechanisms of
Expanded Polystyrene (EPS) Slabs” ,Materials Science, 2006
• Cui, L., Forero Rueda, M. A., & Gilrist, M. D. (2009). Optimisation of energy absorbing liner for equestrian helmets. Part
II: Functionally graded foam liner. Materials and Design, 30(9), 3414–3419.
http://doi.org/10.1016/j.matdes.2009.03.044

23. Acknowledgements
23
We would like to thank:
•Mr. Bahaa Aboulkhoudoud for his assistance in drawing the head
form according to standards
•The AUB Engineering Workshop staff particularly Joseph Zoulikian
for helping manufacture the head form by CNC Machining
•The Department of Mechanical Engineering at AUB

24. Thank You
Questions?