Design of Motorcycle Helmets - TRIPP

Design of Motorcycle Helmets
Puneet Mahajan
Indian Institute of Technology Delhi
INDIA.
Helmet Components and Principle
Some facts about helmets (from a US study)
More than 80 percent of all motorcycle crashes result in injury or death to the
motorcyclist.
Per mile driven, a motorcyclist is 16 times more likely to die in a crash than an
automobile driver.
Wearing a motorcycle helmet reduces that risk by almost one-third (29
percent).
Wearing a helmet is the single most critical factor in preventing or reducing
head injuries among motorcycle drivers and passengers.
Some facts about helmets Hurt Report, (1981)
First, about half of all serious motorcycle accidents happen when a car pulls
in front of a bike in traffic. These accidents typically happen at very low
speeds, with a typical impact velocity, after all the braking and skidding,
below 40 kmph.
Actual crash speeds are slow, but the damage isn't. These are serious,
often fatal crashes. Most of these crashes happen very close to home.
Some facts about helmets Hurt Report, (1981)
The next-biggest group of typical accidents happens at night, often on a
weekend, at higher speeds.
They are much more likely to involve alcohol, and often take place when a
rider goes off the road alone.
These two groups of accidents account for almost 75 percent of all serious
crashes.
So the accident we are most afraid of, and the one we tend to buy our
helmets for—crashing at high speeds, out sport riding—is relatively rare.
Some facts about helmets
(Hurt Report and a similar study in Thailand )
A vast majority of head impacts occur when the rider falls off his bike and
simply hits his head on the flat road surface.
The energy is proportional to the height from which the rider falls—not his
forward speed at the time.
Going faster when you fall off does not typically result in your helmet taking a
harder hit.
90 % of the head impacts were equal to or less than the force involved in a 2.2
m drop (23 km/hr). 99% of the impacts were at or below the energy of a 3.3 m
drop (29 km/hr).
A high-speed crash may involve a lot of sliding along the ground, but all modern
full-face helmets do an excellent job of protecting you from abrasion.
Some facts about helmets
Helmets hit a flat asphalt surface (75-85%)
Helmets do hit curbs a small percentage of the time, but usually after sliding
along on the road first.
Some other facts about helmets
Helmets don't obscure vision.
All helmets provide a field of vision of more than the 140 degree standard that
state driver licensing agencies use to identify vision problems.
Helmets don't impair hearing.
for someone without a helmet, the wind and sound of the engine are very
loud, and any other important sounds must be even louder to be heard over
all that noise. With a helmet on, surrounding sounds are quieter, but in equal
proportions.
Technically speaking, the signal to noise ratio stays the same.
Visibility.
Although black helmets are popular among motorcyclists, they offer the least
visibility to motorists. A rider wearing a plain white helmet rather than a black
one reduces his or her chance of collision by 24% because it is so much more
visible — day or night. Nevertheless, black helmets outsell white ones by
20:1.
Sagittal view of Human head and Meninges (Ref. Nucleus Medical art, 2001)
Brain Injury
Brain basically floats inside the skull, within a bath of cervical-spinal fluid
(CSF)and a protective cocoon called the dura.
When the skull stops suddenly—as it does when it hits something hard—the
brain keeps going, and has its own collision with the inside of the skull.
If that collision is too severe, brain injuries such as shearing of the brain tissue
bleeding in the brain, or between the brain and the dura, or between the dura
and the skull can occur.
When the brain is injured internally, bleeding and inflammation make it swell
and it presses harder against the inside of the skull and tries to squeeze
through any opening, bulging out of eye sockets and oozing down the base of
the skull.
How does it work ?
When the helmet hits the road or a curb, the outer shell stops instantly.
Inside, the head keeps going until it collides with the liner. When this
happens, the liner brings the head to a gentle stop.
Δv
F =m
Δt
more Δt
Kinetic energy of head
less impact force on the head
Strain energy of liner
Deformations
F
F
FOAM
STEEL
(Δx)foam
(Δx)steel
Force on the head
F
F
Δt
Δt
(Fav)foam (ΔX)
(Fav)steel (ΔX)
2
1
= mv
steel 2
2
1
= mv
foam 2
ΔX
foam
(F av )
foam
> ΔX steel
<
(F av )
steel
Extended Poly-styrene (EPS) Liner
The great thing about EPS is that as it crushes, it absorbs lots of energy at a
predictable rate. It doesn't store energy and rebound like a spring, which would
be a bad thing because the head would bounce back up, shaking the brain not
just once, but twice.
EPS actually absorbs the kinetic energy of the moving head, creating a very
small amount of heat as the foam collapses.
Outer Shell
• Prevents penetration of EPS by sharp objects -almost
never happens
• Shell protects against abrasion when sliding on the road
•Absorbs energy
• as it flexes in a polycarbonate helmet,
•or flexes, crushes and delaminates in a fiberglass composite
helmet.
•The EPS liner inside the shell is better at absorbing energy
than the shell. It absorbs energy by crushing.
Helmet Performance and Tests
Impact attenuation test - dropping a helmet in a guided fall onto a steel test
anvil and measure Acceleration time history from an accelerometer at the
headform center of mass.
Drop Test Impact sites
Four impact sites B (front), P (crown), R (rear), and X (side)
IS requires that the peak acceleration of head should be less than 275g and Head
Injury Criterion (HIC) should be less than 2400 when the head is dropped with an
impact velocity of 7.5m/s. ECE 22:05 has increased the impact velocity to 8.5 m/s
Helmet Performance and Tests
roll-off test recommends that a helmet may be shifted but must not roll off the
head form.
dynamic retention test for testing the strap during an abrupt guided fall. The
retention system fails if it cannot support the mechanical loads or if the
maximum instantaneous deflection of the retention system exceeds 30 mm.
chin bar test, a weight is dropped through a guided fall to strike the central
portion of the chin bar. The maximum deflection of the chin bar must not
exceed a stated distance.
shell penetration test, a sharply pointed striker is dropped in a guided fall
onto the helmet from a prescribed height. The test striker must not penetrate
the helmet or even achieve momentary contact with the head form.
Work at IIT Delhi
• Ventilation and Computational Fluid Dynamics
•
Improve air velocities inside the helmet to evaporate
sweat without degrading impact behavior
• Impact analysis – Explicit Finite Element
• Alternate shell materials •Composite shell and Metal foam
Ventilation in Helmets
• Ventilation in Helmets was studied with wind tunnel experiments
and Computational Fluid Dynamics (Fluent)
• The Head and Helmet models used were of actual geometry
• Methods tried to improve the ventilation
•
- Providing the grooves and slot in liner foam
•
- Lifting the helmet by providing foam blocks at few locations
Computational Fluid Dynamics
Assumptions in CFD analysis
•
Fluid flow assumed as steady.
•
The walls were stationary with no slip.
•
k-ε turbulence model was used.
•
Symmetry boundary conditions were used at mid plane
•
Inlet boundary condition was 15.7 m/s velocity and the
outlet condition was Outflow.
Helmet without grooves & slot
Velocity contours in the central plane
‰ With
normal helmet the gap is ‘zero’ at few locations
and there is no space for air to flow
Ventilation models
Groove
Slot
Various dimensions of the groove were tried and the slot
dimension was fixed at 42mm x 7mm
Velocity contours in Helmet - Head
With slot at 30 deg.
With tangential slot
Velocity contours in 42mm x 7mm model
Comparison of air velocities in various helmet ventilation models
Alternate Designs
Four foam blocks were placed by lifting the helmet by 2mm
Velocity contours in helmet-head gap with foam blocks (60mm x 20mm)
Velocity contours in helmet-head gap
Results from CFD study
• In the helmet-head gap, air velocities were higher in helmet with slot
as compared to the helmet without slot.
• With tangential slot, velocities were higher through out the gap
compared to the slot at 30 deg.
• Grooves of 42mm x 7mm & 14mm x 14mm gave comparable
improvement in air velocities
Impact Dynamics
Helmet testing & standards
Impact Test Rig Type Twin-Wire or Monorail
Anvil type
Flat/ Kerb
Impact velocity
7.5 m/s
Peak Acceleration
275 g
HIC
2400
All the standards measure the shock transmitted through
the helmet into the headform by means of an accelerometer
mounted at the headform’s center of gravity
• Impact analysis done with various Helmet models (LS-DYNA)
• For Deformable head model in impact studies, forces,
intracranial pressures, and stresses were studied
• For rigid head model HIC was calculated
Finite Element model of helmet-head in front, side,
and oblique impacts
Material modeling
Outer shell
• Prevents the penetration of sharp objects
• ABS material was considered for outer shell
• Young’s modulus of ABS = 1.7 GPa
• Poisson’s ratio
= 0.3
• Yield stress
= 34.3 MPa
• *MAT_PLASTIC_KINEMATIC material model had been
•
used for ABS shell in LS-DYNATM
Liner foam
• Inner crushable foam is of EPS 30 mm thick
[Ref. Yettram, 1994]
Stress/Strain relationship of EPS under quasi-static loading
*MAT_CRUSHABLE _FOAM material model had been used for EPS foam
in LS-DYNATM
Young’s modulus of EPS foam
= 18 MPa
Poisson’s ratio
= 0.05
Yield stress
= 0.7 MPa
Yield surface and its evolution for EPS are defined by,
Yield surface description:
f = f ( I 1 , J 2)
Hardening formulation:
Y = Y 0 + H (e v )
Y t = Y t0
where,
Y is the yield stress, Y0 is initial compressive yield stress, Yt is tensile cut off stress
and H is strain hardening.
Human head model
The FE model of Head consists of
‰ Skin
‰ Skull
‰ CSF
‰ Brain
‰ Tentorium
‰ Falx
[Ref. Remy Willinger, 2000]
Parts in FE model of Head
Skin
Face
Skull
CSF
Brain
Falx
Tentorium
Measure of Injury
If head is assumed rigid - Head Injury criterion
Injury Criterion for Deformable Head
Applied brain pressure tolerance (ABPT)
Intracranial pressure
170 kPa moderate injury
> 230 kPa fatal injury.
„ Brain von-Mises shear stress (VMSS)
27 kPa for moderate
39 kPa for severe DAI.
„
Front impact
Force on the head with and without helmet
Intracranial pressures at coup & contrecoup with & without helmet
Deformed shapes of polystyrene foam (7.5m/s velocity)
Front impact
Oblique impact
Force on the head without helmet
ventilation at different velocities
Force on the head with ventilated helmet
at 7.5 m.s-1 velocity
Coup & contra-coup pressures
at 7.5 m.s-1 velocity
von Mises stress in the brain
• The maximum is observed on the right side of the brain
• Variation in von Mises stresses was observed in different Helmet models
• It is lower with 14x14 groove & 48x7 slot helmet and is 18.4 kPa
Various biomechanical parameters at 7.5 m/s velocity
Intracranial Pressure (N.m-2 )
Force on
the helmet
(N)
Force on
the head
(N)
No ventilation
7441
14mmx 7mm groove
Helmet type
Von-Mises
stress in the
brain (kPa)
HIC
Peak accel.
( in g)
47.4
867
170
774
158
Coup
Contra-coup
7230
2.1 x105
-1.19x105
6846
6574
1.87 x105
-1.1 x105
28mm x7mm groove
7363
7058
2.1 x105
-1.2 x105
50.9
868
168
42mm x7mm groove
7739
7364
2.36 x105
-1.28x105
55.6
1051
183
14mmx14mm
groove
7057
6862
1.94 x105
-1.15x105
46.1
744
155
14mm x7mm - 3
grooves
7137
6850
1.95 x105
-1.14x105
46.9
691
160
45.7
Side impact
Force on the head without helmet
ventilation at different velocities
Force on the head with ventilated helmet
at 7 m.s-1 velocity
Various biomechanical parameters at 7.5 m/s velocity
Intracranial Pressure (N.m-2 )
Force on
the helmet
(N)
Force on
the head
(N)
No ventilation
10202
14mm x7mm groove
Helmet type
Von-Mises
stress in the
brain (kPa)
HIC
Peak accel.
(in g)
Coup
Contra-coup
9513
2.2 x105
-1.43 x105
63.5
1680
229
10132
9504
2.2 x105
-1.43 x105
62.3
1693
231
28mm x7mm groove
10207
9552
2.2 x105
-1.43 x105
62.1
1701
235
42mm x7mm groove
10074
9384
2.2 x105
-1.43 x105
63.4
1690
234
14mm x14mm groove
10187
9531
2.2 x105
-1.43 x105
62.2
1730
223
14mm x7mm - 3
grooves
10126
9512
2.19 x105
-1.43 x105
63.2
1682
230
Results of Ventilation and Impact studies
• The helmet with ventilation was bottomed out at 9m/s
velocity where as without ventilation bottomed out at
10m/s
• Based on the ventilation and dynamics study, groove
with 14mm x 14mm is preferable in helmets.
Composite shell
• Composite materials apart from ABS/Polycarbonate are used in Motorcycle
helmets for outer shell because of high specific strength and stiffness.
• Helmets with Composite laminated shells are considered better in
minimizing the peak acceleration of the rider.
•Polyester resin, which is used as matrix material, reinforced by either
Carbon or Kevlar or Glass fibres.
• During impact, composites undergo damage (matrix cracking, debonding, fiber
breakage) and delamination between plies.
• The damage mechanisms provide another mechanism for absorprion of energy.
• Investigated the delamination and in-plane damage in Carbon fibre / Epoxy
matrix Helmet with 0 /90 plies
• Delamination in composite shell was studied by using Cohesive Zone Model
• Interface layer of ‘zero thickness’ was modeled with cohesive elements
between the composite plies of the outer shell in motorcycle helmet
Cohesive Zone Model
• Cohesive zone model (CZM) is a fracture mechanics approach to study the
interfacial effects in a material
• An idealized model with cohesive elements (5,6) between 4-noded bilinear
continuum elements (1,2,3,4) was introduced to model the interface between
two materials.
Variation of traction at the interface
• The mechanical response of cohesive interface can be described through a
constitutive law relating ‘traction’ and ‘separation’
• The area under the traction – separation curve is the energy absorbed in
separation
• Traction across the interface reaches maximum at point A then decreases to
point B and vanishes when complete decohesion occurs
Traction
A
tc
B
O
δc
Separation
Damage initiation
Damage initiates when the quadratic interaction function involving the
nominal stress ratios reaches a value of ‘1’
2
2
2
⎛ t n ⎞ ⎛ t s ⎞ ⎛ tt ⎞
⎜⎜ 0 ⎟⎟ + ⎜⎜ 0 ⎟⎟ + ⎜⎜ 0 ⎟⎟
⎝ t n ⎠ ⎝ t s ⎠ ⎝ tt ⎠
=1
Damage initiation stresses in Mode-I , II, and III are 57 MPa, 100 MPa, and
100 MPa respectively [Ref. Iannucci , 2006]
t
0
n = Damage initiation stress in Mode - I
Damage evolution
Mixed mode fracture criterion was used for damage evolution
G I + G II + G III =
G IC G IIC G IIIC
f
Where,
GI
G IC
= Strain energy in Mode - I
= Critical Strain energy in Mode - I
Critical energies used for cohesive layer in Mode – I, II, and III are 281 N/m,
900 N/m, and 900 N/m respectively [Ref. Iannucci , 2006]
Hashin’s in-plane damage criteria
2
⎛ σ 11 ⎞
⎜
⎟
⎝ XT ⎠
σ
11C
X
2
⎛ σ 12 ⎞
⎟⎟
⎜
⎝ S 12 ⎠
+⎜
≥1
≥1
Fiber tension failure
Fiber compression failure
C
2
⎛ σ 22 ⎞
⎜
⎟
⎝ YT ⎠
2
⎛ σ 12 ⎞
+⎜
⎟⎟
⎜
⎝ S 12 ⎠
≥1
Matrix tension failure
2
2
2
⎡
⎤
σ 22 ⎢⎛⎜ Y C ⎞⎟ − 1⎥ + ⎛⎜ σ 22 ⎞⎟ + ⎛⎜ σ 12 ⎞⎟ ≥ 1
⎢
⎥
Y C ⎢⎜⎝ 2 S 12 ⎟⎠ ⎥ ⎜⎝ 2 S 12 ⎟⎠ ⎜⎝ S 12 ⎟⎠
⎣
⎦
Matrix compression failure
Impact velocity = 9m/s
Delamination zone in the cohesive layer of composite shell
Separation of plies
Matrix tensile damage
Matrix compression damage
Energy during impact
IE in helmet
EPS foam
= 67.7 J
Composite lamina = 34.4 J
Cohesive layer
= 0.56 J
Force on the head with composite shell
Force on the head (with half-model) = 10900N without damage and delamination
=
9780N
with damage and delamination
Due to damage and delamination in the composite shell, force on the head is
reduce but is higher compared to ABS shell
Force on the head (with full model) = 17750N with ABS shell
= 19560N with composite shell
Metal foam
•
Lately, metal foams are being used in crash applications
because of its light weight, high strength and energy
absorption capabilities.
• Metal foams are expensive but much lighter than the
Polycarbonate, which commonly used as material for shell.
• One application of the Metal foam can be Helmet
A constitutive model proposed by Deshpande and Fleck used
for Metal foam
The yield function Φ is defined by
∧
φ =σ −Y ≤ 0
2
⎛ ⎞
⎜σ ⎟
⎝ ⎠
∧
∧
σ
=
1
⎡
⎢1 + ⎛ α
⎢ ⎜
⎢⎣ ⎝ 3
⎞⎥
⎟⎥
⎠ ⎥⎦
= equivalent stress
Y = Yield stress
∧
R
(ε ) = strain hardening
∧
ε
2
[σ
⎤
= equivalent strain
+α
e
2
2
σ
2
m
]
Deformation in top impact at 7.5 m/s velocity
Top impact at 15 m/s velocity
Experiments Vs prediction from FEA
Impact velocity = 7.5 m/s
Top impact
Front impact
Studies with Full-face helmets
Front impact , 7.5 m.s-1 velocity , Rigid headform
Headform acceleration traces : ABS vs Metal foam shell
Helmet impact characteristics
With Rigid headform
Outer shell
Mass (kg)
Peak Acceleration (in g)
HIC
ABS
0.938
208
1510
MF-1
0.829
210
1773
MF-2
0.497
202
1706
MF-3
0.248
160
1229
Front impact , 7.5 m.s-1 velocity , Deformable head
Deformation in helmet with 150 kg/m3 density Metal foam
Al foam EPS foam
Head
Resultant force : ABS vs Metal foam shell
Variation of kinetic energy (KE) and internal energy (IE) in the outer shell
ABS shell
Metal foam shell
Energy absorbed by ABS is almost same as its initial kinetic energy (26 J).
The low-density Al foam absorbed more energy than its own kinetic energy
and reduced the forces on the head
The initial kinetic energy of Metal foam (density = 150 kg/m3) shell is 7J but it
absorbed 16J
The energy absorption in EPS foam is same in all the helmets
von Mises stresses in the brain
Among all helmets, the von Mises stress in the brain is lower in 150 kg/m3
density Metal foam helmet and is 17.7 kPa
Front impact , 10 m.s-1 velocity , Deformable head
Resultant force : ABS vs Metal foam shell
Top impact
Resultant force : ABS vs Metal foam shell
Force on the head
= 13913 N with ABS helmet
= 13900 N with MF (500 kg/m3)
= 11500 N with MF (150 kg/m3)
CONCLUSIONS
Ventilation studies
9 Various ventilation models in helmets were investigated
9 The helmet with ventilation was crushed at 9m/s velocity where as
without ventilation it crushed at 10m/s
9 Pressure and stresses in the brain were investigated and found not to
change significantly due to the presence of grooves in the helmet
Composite shell studies
9 Delamination between the plies was not observed at low impact velocities
9
Matrix tensile and compressive damage were observed at 7.5 m/s and 9.0 m/s
velocities
9 2% energy was absorbed by the cohesive layer at 9m/s velocity
9 Composite shell didn’t absorb much energy compared to ABS shell
Metal foam shell studies
9 Experiments were performed on open-face helmets
9 Impact analysis with Metal foam shell was carried out and found
the lower contact forces on the head compared to ABS shell.
9 The weight of the shell is reduced by 73% with low-density metal
foam compared to ABS
9 von Mises stress in the brain is lower with metal foam.
Thank You.
X-velocity contours within the gap of 10mm
Max. velocity = 8.0m/s
Injury Scale and head injury
Abbreviated Injury Scale, or AIS, to describe how severely a patient is hurt
when they come into a trauma facility.
AIS 1 = Minor
AIS 2 = Moderate
AIS 3 = Serious
AIS 4 = Severe
AIS 5 = Critical
AIS 6 = Unsurvivable
Acceleration of
200 g - 250 g generally corresponds to a head injury of AIS 4
250 g - 300 g corresponds to AIS 5
> 300g corresponds to AIS 6.
Wayne State Tolerance Curve
exposing a human head to a force over 200 Gs for more than 2
milliseconds is what medical experts refer to as "bad.“
Its not going to kill you but can cause Traumatic Brain Injury leading to
disabilities.
At high speed tests show that the EPS had cracked and compressed at
the impact sites without bottoming.
Composite shell
• Composite materials are used in Motorcycle helmets for outer shell because
of high specific strength and stiffness.
• Helmets with Composite laminated shells are considered better in
minimizing the peak acceleration of the rider.
•Polyester resin, which is used as matrix material, reinforced by either
Carbon or Kevlar or Glass fibres.
• Composite materials exhibit a significant number of failure modes in impact.
Various biomechanical parameters in oblique impact
at 7m.s-1 velocity
Force on the
helmet (N)
Force on
the head
(N)
7248
Intracranial Pressure (N.m-2 )
Coup
Contra-coup
Von-Mises
stress in the
brain (kPa)
6339
1.76 x105
-1.57 x105
33.9
0.361
1524
198
7663
6657
1.81 x105
-1.56 x105
33.5
0.367
1195
184
28x7 groove
7596
6474
1.82x105
-1.56 x105
33.3
0.366
1264
177
42x7 groove
7483
6506
1.78 x105
-1.55 x105
33.6
0.366
1314
201
14x14 groove
7467
6502
1.85 x105
-1.57 x105
33.5
0.366
1550
199
14x7 - 3 grooves
8043
6691
2.17 x105
-1.43 x105
35.4
0.363
-
-
Helmet type
No ventilation
14x7 groove
Max.
strain in
brain
HIC
Peak
accel. (in
g)