Improving Energy Dissipation to Lower Concussion
Risk in Football Helmets
by
Christine Elizabeth Labaza
MASC HUSETTS 1NS rKf9
OFTECHNOLOGY
JUN 0 42014
LIBRARIES
Submitted to the
Department of Materials Science and Engineering
in Partial Fulfillment of the Requirements for the Degree of
Bachelor of Science in Materials Science and Engineering
at the
Massachusetts Institute of Technology
June 2014
@ 2014 Massachusetts Institute of Technology. All rights reserved.
Signature redacted
Signature of Author:
Department of MateLals Science and Engineering
2 May 2014
Certified by:
Signature redacted
Lorna J. Gibson
Professor of Materials Science and Engineering
Thesis Supervisor
Accepted by:
Signature redacted
V /Yf
Jeffrey C.Grossman
Chairman of the DMSE Undergraduate Committee
1
2
Improving Energy Dissipation to Lower Concussion
Risk in Football Helmets
by
Christine Elizabeth Labaza
Submitted to the
Department of Materials Science and Engineering
in Partial Fulfillment of the Requirements for the Degree of
Bachelor of Science in Materials Science and Engineering
Abstract
American football is notorious for being a high impact sport. There
exists an especially high amount of danger to each player's brain, created in
part by gameplay, but also from the helmets worn by the athletes. Football
helmet pads were comparatively investigated, in order to find a better
alternative that can lower the amount of acceleration on the brain. A new
pad system was introduced that allows for the force to be dissipated
horizontally, through use of a dashpot-like center, also employing a foam
shell to assist in the vertical energy dissipation. The pad currently used,
along with the new dashpot system were further tested inside helmet shells
on a head form drop test, and compared to the national standards that
regulate athletic equipment.
Thesis Supervisor: Lorna Gibson
Professor of Materials Science and Engineering
3
4
Acknowledgements
I would like to thank Christopher di Perna, Mike Tarkanian, Geetha
Berera, and Daniel Newman for their constant help with anything that I
could ask for. I would also like to thank Scott Stephens, Emily McDonald,
and James Balchunas for starting this project and laying the foundation for
the ideas within. Furthermore, Joseph Crisco and Ryan Rich provided
excellent help and feedback in the testing of the full helmets at Brown
University. Lastly, I would like to thank Professor Lorna Gibson for her
advice and support.
5
6
Table of Contents
1.Introduction .............................
2. Background
11
.........................
.... 14
2.1 Traumatic Brain Injury in Football ...............
14
2.2 Medical impacts of Traumatic Brain Injuries.....15
2.3 Previous Research.................................................
16
2.4 Overview of Foams and Energy Absorption..... 20
3. Materials and Methods.................................. 21
3 .1 Pad M aterials .......................................................
21
3.1.1 Vacuum Grease Sandwich Structure..... 21
3.1.2 Asics Shoe and Gel Sandwich Structure 22
3 .2 D rop Tow er............................................................
23
3.2.1 ISN Drop Tow er.........................................26
3 .3 H ead Form .............................................................
29
3.4 Instron M achine...................................................
32
3.5 D SC M achine........................................................
33
.........
4. Results................................. .. MEE.
me..m.m
35
4.1 ISN Drop Tower Results.....................................
35
4.2 MIT Drop Tower Results.....................................
37
4 .3 Instron Testing......................................................
39
4 .4 D S C D ata...............................................................
40
7
4.5 Brown University Head Form.............................. 41
5. Discussion.............................
43
6. Conclusion..............................
50
8
Figure 1: Riddell Revolution Speed helmet with vinyl nitrile pads.................................12
Figure 2: Linear acceleration of im pacts.................................................................................... 16
17
Figure 3: Linear acceleration of vinyl nitrile. ..............................................................................
Figure 4: Linear impacts of sandwich structures. .....................................................................
Figure 5: V iscous fluids choices..................................................................................................
Figure 6: A final comparison of the results for the vinyl nitrile control pad and the vacuum
18
grease filled sandw ich structure pad.........................................................................................
19
Figure 7: The four 'pad' choices - Asics shoe heel, vinyl nitrile, gel sandwich structure, and
vacuum grease sandw ich structure ...........................................................................................
19
22
23
Figure 8: Unwrapped vacuum grease sandwich structure........................................................
Figure 9: MIT drop tower used for linear impact tests to compare pad choices..................... 25
Figure 10 : ISN D ro p Tow er ............................................................................................................
27
Figure 11: Drop Tower pushes grease horizontally outward ....................................................
Figure 12: Positions of strikes for Head Form testing................................................................
28
Figure 13: Head Form in action..................................................................................................
Figure 14: Vacuum grease pad helm et. .....................................................................................
31
Figure 15: M axim um Load At Im pact.........................................................................................
Figure 16: M axim um Failure At Im pact ....................................................................................
35
Figure 17: Deflection vs. Energy - Vinyl Nitrile ..........................................................................
37
Figure 18: Deflectin vs. Energy - Vacuum Grease ......................................................................
Figure 19: Force at Im pact - Vinyl Nitrile .................................................................................
37
Figure 20: Force at Im pact - Asics Shoe ....................................................................................
Figure 21: Stress Strain Curve - Asics Foam ...............................................................................
39
Figure 22: Stress Strain Curve - Vinyl Nitrile .............................................................................
40
Figure 23: Severity Index - Low Drop .........................................................................................
Figure 24: Severity Index - High Drop ........................................................................................
42
Figure 25: Energy Dissipation directions. ..................................................................................
Figure 26: Polydim ethylsiloxane structure ...............................................................................
44
Figure 27: Dim ethyl Siloxane structure ......................................................................................
46
9
30
32
36
38
40
42
45
10
1.
Introduction
Concussions and brain trauma are an extremely dangerous reality of
playing football, from the professional level to young children's leagues.
Recently, the danger of concussions in players has swept the country, from
media reports to lawsuits. Take, for example, Mike Webster, a retired
football player. He was found dead, at the early age of 50, and the autopsy
showed a brain that had signs of repeated hits to the head. Webster had a
history of depression, amnesia, and depression in the years before his
death.8 Medical studies have linked concussive injuries to both short and
long term health effects. Short term concussive effects are well known, and
those in the athletic training field who work with athletes, monitor those
cases closely, and do what they can to help the athlete return to play after
all symptoms have subsided. In fact, the football helmet manufacturer
Riddell has included impact sensors inside some of the helmets used, so that
dangerous circumstances can be watched closely, and if necessary, a player
removed from them. However, that doesn't do enough for the long-term
consequences that can be found after even low intensity impacts, if they are
11
repeated often enough. "Seven years after concussion, participants
displayed disrupted higher-order neurocognition in the form of chronically
impaired attention, working memory, inhibition, and interference control". 4
As it is not much of an option to remove helmets altogether, simply because
of the way football is played, the next step is to look at the helmets
themselves and find solutions for bettering their components. A Riddell
helmet is the most commonly used helmet, especially in the NFL. These
helmets have a set of vinyl nitrile foam pads.
Figure 1: Riddell Revolution Speed helmet with vinyl nitrile pads
12
The purpose of this thesis work is to build off the research done by Scott
Stephens, Emily McDonald, James Balchunas, and Christine Labaza while
working at MIT. Testing more thoroughly a dashpot system pad, and looking
at other possible solutions in gel-heeled shoes will show if a difference can
be made to improve energy dissipation. Building off the ideas of the previous
research, the pad choices, as well as the control were tested on two droptowers to measure the linear impact force and deceleration, and compare
between the choices as well as the control vinyl nitrile pad. Furthermore, the
best pad choice and the original control pad were tested on a head form at
Brown University. All football helmets must pass testing standards put in
place by NOCSAE, or the National Organizing Committee for Standards of
Athletic Equipment. Therefore, the final test is to pass those standards at a
better level than the original helmet. Many helmet types are tested at the
head form at Brown University. It is a certified laboratory, which allows for
valid testing of the newly designed and control helmets.
13
2. Background
2.1 Traumatic Brain Injury in Football
Traumatic brain injury is of major concern to those who are in
constant danger of receiving multiple high force impacts. These impacts can
cause concussions which lead to more severe brain diseases, mental health
problems, and sometimes death. A concussion happens when an impact to
the head causes the brain to shift inside the skull. Although high force
impacts are a larger risk, especially in playing football, concussions can be
caused even from very low force impacts. In fact, very recently, Russell
Allen of the Jacksonville Jaguars had a minor hit, and suffered a stroke. As
reported, "If you look at the replay of the game, there's nothing about that
play that stands out as unusual. A center and a linebacker meeting on a run
play."'
0
Impacts can cause shifting and stretching of brain fibers. After a
concussion, an athlete is much more susceptible to brain injuries, even at
lower impacts.5 Because of this common knowledge, in some games,
sensors are placed inside a player's football helmet to record the number of
impacts and the acceleration of the impact. Beyond being hit by another
14
player, football players have to be protected from concussion from hitting
the ground. There has been outcry to ban football helmets altogether, as a
harder helmet creates harder hits. However, the proponents of that possible
alternative forget about tackles. A player can be hit by another player,
causing a possible concussion; however, those two players will almost
certainly hit the ground. To try to get rid of a helmet, would be much like
saying motorcyclists or bikers shouldn't wear helmets. There is danger from
not only someone else, but the gameplay and its relationship to the
environment of the field as well. Even in the history of football, before the
use of modern helmets, there were deaths from skull fractures and cranial
hemorrhaging.9
2.2 Medical impacts of Traumatic Brain Injuries
There are many medical problems from receiving multiple concussions,
including diseases, changes in behavior, and even suicide. One of these
diseases is called Chronic Traumatic Encephalopathy, or CTE. CTE happens
when these multiple impacts cause the neurofibers to become entangled
with each other. 5 Furthermore, it isn't just heavy impacts that can cause this
entanglement, but also, repeated smaller hits contribute to this damage.
Over a season, a college player can sustain somewhere between 420 and
2400 hits. Although, the incidence of concussions for high school and college
players is close to only 6% per year, as there are close to 70,000 players, so
15
that becomes 4000 concussions. And this does not include the professional
players, who can receive impacts up to 7000 Newtons of force.7
The following chart shows the number of impacts received during a season,
and the maximum deceleration distribution.'
47918
Impact
Frequency
Fz-T-JT-
12342
6189
1340
0-10
10 1-20
20.1-30
30 140
40,1-50
3272
1745
924
549
272
537
50 1-60
60.1-70
70 1-80
80.1-90
9011-100
>100
Linear Acceleration of Impact (g)
Figure 2: The above plot shows the linear acceleration of impacts in one season. Concussions occurred between 74.09g and
146.09g.'
2.3 Previous Research
The current project seeks to further substantiate previous research
7
into these football helmet pads that has been conducted in the past year.
Many iterations of different pad combinations were tested by using the drop
16
tower for a large number of drops, from 6 feet, repeated one immediately
after another. In the prior project, both different foam types as well as
energy absorbent inserts, including d30, a material used in motorcycle
equipment, shear-thickening fluids, such as corn starch and water, and
various fluids of varying viscosity were tested. These tests were based on
the work of Goel, who researched sandwich structures filled with either
glycerin or water, for use in ski helmets. 3
The results of the different pad choices showed that the best pad was the
vacuum grease filled pad, as seen in the following charts.
Vinyl Nitrile Control Pad - Peak Acceleration vs.
Experimental Trial
900
4
800
700
600
=0 500
-
*Control Vinyl Nitrile Pad
400
300-200
100
--
-
-
-
----
-_~
---~
-- - -
0
1
2
3
4
5
6
7
8
9
10
Trial Number
Figure 3: The graph shows the linear accleration of the vinyl nitrile pad over a series of 10 drops.
17
Figure 4: The graph shows the effects of a filled sandwich structure during linear impacts.
Although no vacuum grease is shown to be tested in this above data, the
chart shows the system's success at achieving a lower maximum force. An
alternative had to be found, since corn starch and water doesn't survive for
more than a few days. Corn syrup was a high viscosity fluid, but a higher,
and better fluid was later found in vacuum grease.
18
Average Max Acceleration for Sample Range
600
" Control (Plain Vinyl
Nitrile)
500
" Corn Syrup
~400
" Machine Lubricant
0
1.300
* Glycerin
200
* Petroleum Jelly
100
0
* Vacuum Grease
Specimen
Figure 5: A final result of multiple viscous fluids, showing that the vacuum grease lowered the maximum acceleration by the
7
most.
Average
5.2
KLA
sr.
i=n
Minimum
4-2
Maximum
12.2
19.1
117
is
11-2
18.6
16.4
19.7
12.6856
18794
15472
0-3581
Coef. ol
Variation
8.7708
Std. Dev
0.4574 ,
Figure 6: A final comparison of the results for the vinyl nitrile control pad and the vacuum grease filled sandwich structure
pad.
This thesis seeks to confirm these previous results and test additional energy
dissipating systems. As the pads were only compared one to one, by testing
the linear impact force, more testing must be done to see how each pad
works in conjunction with a helmet shell. Furthermore, these full helmets
19
must be compared to national standards to further confirm their usefulness
in lowering concussion risk.
2.4 Overview of Foams and Energy Absorption
Foams make good energy absorption materials because of their
internal structure. While under some amount of force, they allow for a low
peak force from, which occurs because of the collapse of cells within the
foam itself. This collapse of cells is shown in stress-strain curves as a long
plateau before reaching the yield stress. This cellular design of foam allows
the material to be lighter than a solid material, as well as reduce a force
applied, no matter the incoming direction. This has led manufacturers of
various helmet types to incorporate foams into their designs. It can reduce
the danger to the wearer, as well as be cost effective for production.'
20
3. Materials and Methods
3.1 Pad Materials
After narrowing down different pad choices, the main tests were
conducted on the vinyl nitrile pad, the vinyl nitrile sandwich structure filled
with vacuum grease, and on a gel padded shoe heel. The original pad is
standard in all Riddell Revolution helmets. These pads make up most of the
helmet with the exception of the crown pad. The crown pad is an unknown
material, and not the focus of this thesis.
3.1.1 Vacuum Grease Sandwich Structure
The pad system that was tested in both the drop tower and the head
form is a sandwich structure made from a Riddell vinyl nitrile pad, filled with
a package of vacuum grease, and wrapped in a latex band with a high elastic
constant. This system employs a dashpot like resistance to the incoming
vertical force. The intent of the design is to distribute the force outward, and
away from the football player's head.
21
3.1.2 Asics Shoe and Gel Sandwich Structure
Shoes, and especially the heel, must be able to undergo extreme
numbers of impact. For running shoes, the impacts are more frequent and a
harder force is applied. An Asics running shoe uses a gel technology,
combined with surrounding foam to produce the same type of system as in
the vacuum grease pad. The gel moves outward horizontally when impacted
vertically, and testing this force distribution provides more insight into the
limits of the system.
Figure 7: The four 'pad' choices - Asics shoe heel, vinyl nitrile, gel sandwich structure, and vacuum grease sandwich structure.
22
Figure 8: Unwrapped vacuum grease sandwich structure.
3.2 Drop Tower
In order to achieve a comparison between the different options of
testing, a drop tower system was developed. This allows the maximum force
from a linear drop, which has a force comparable to a hit from a football
player to be effectively measured. A drop tower was built out of an
aluminum frame provided by the company 80/20, with a shuttle attached to
the side. This tower was carefully built so that the linear force was perfectly
perpendicular to the ground. Screwed into the shuttle, a section of 80/20 is
necessary for hitting the samples at the bottom with a flat surface. The
samples sit on an immovable aluminum block that has a force sensor sunk
into its side. This force sensor is a DLC101 with a maximum impact of up to
23
20,000 N. A small sphere sits on top of the force sensor, in order to
distribute the force of the impact of the shuttle evenly to the sensor. On top
of the sphere is a metal plate, which holds the sample that is attached
securely, in order to minimize noise. To collect the data, the force sensor is
attached to an OMEGA accelerometer power supply, which is connected to
an oscilloscope and then connected to a computer. Data is collected with a
program in LabView, which allows the full impact, its maximum force, to be
found. Testing procedures were to include multiple hits on each pad.
However, there were problems late in the course of the thesis that will not
be resolved. There was both a problem of tampering with the calibration, as
well as a broken wire. While there is some data, further research should
include some drops using different orientations of viscous liquid movement,
as well as many more iterations per pad.
24
Figure 9: MIT drop tower used for linear impact tests to compare pad choices.
25
3.2.1 ISN Drop Tower
The drop tower used at the Institute for Soldier Nanotechnologies
(ISN) works much the same way as the tower built in the Laboratory for
Engineering Materials (LEM) at MIT. A shuttle is lifted in the air, and
released, much like a guillotine. This drop tower has a 45kN load cell along
with a photo diode that is capable of measuring the speed immediately
before impact. Data acquired from this drop tower can show time of impact,
maximum force transmitted, and deflection of the sample. The samples
tested on the ISN drop tower included the vinyl nitrile control, the vacuum
grease filled sample, an Asics shoe heel, and a gel filled sample. Each
sample was tested with an impact at energy of 20 Joules.
26
Figure 10: ISN Drop Tower
27
Figure 11: It is shown that the vacuum grease gets pushed outwards, horizontally, while force is applied.
28
3.3 Head Form
Head forms are used to test various helmets, many times in order to
pass national standards. It consists of a model of a head that can be
attached to a drop shuttle, which is then raised to certain heights and used
to find impact data. The head can rotate to different orientations, so that
testing can be done on all parts of the helmet. Two different standards
organizations use different heads, which includes the NOCSAE form of a
silicone rubber, urethane structure, with a glycerin filled brain cavity, while
the ASTM head is solid magnesium. For this thesis, the NOCSAE head form
at Brown University, and standards for testing and certification were used.
The information from a head form test is used to calculate the Severity
Index (SI) of each hit, and it can also find the maximum acceleration of the
hit. The severity index is calculated with the formula
T
SI =
A2.sdt
Where A is the acceleration of the head and T is the duration of the impact.
Since the main calculation uses the acceleration of the head, the severity
index that is lower, means that there is a lower risk of concussion for a
29
player. NOCSAE standards state that a helmet pass the test with a severity
index of under 1200 for each orientation of drop.
Calibration of the machine involved dropping the head without a helmet on.
NOCSAE defines different drop heights for different head sizes, for three
axes of calibration. These axes test the side, the front, and the top
measurements, to make sure that the Severity Index is being calculated
correctly. Inside the head, an accelerator is secured, in order to measure a
simulation of what happens to the brain during a drop. Actual testing of the
helmet then consists of seven different head orientations, including front,
top, side, rear, rear boss, front boss, and random. These locations are
shown in the schematic below.
ac
OsIO
.. FFer
FRgure 2
Figure 12: Positions of strikes for Head Form testing, per NOMSE Document .001-06 12
30
Figure 13: Head Form in action.
31
Figure 14: Vacuum grease pad helmet, fastened on the head form, ready to be dropped.
3.4 Instron Machine
An Instron Machine (Istron Model 1361) can apply compressive and
tensile forces on materials, in this case to find the stress-strain curves of the
gel, the shoe foams, and the vinyl nitrile foam. Stress and strain can give
very important information about the material properties as pertaining to
energy absorption and dissipation. Stress and strain are related through
Young's Modulus, E, through the equation
E =-
cE
32
Where stress, a equals the compressive force over the sample area
F
A
And strain,
E equals
the change in deformation, over the original length, I
E
Al
=
3.5 DSC Machine
The foam and the gel were put into a differential scanning calorimeter,
or DSC. This machine heats and cools the material being tested over a
period of time, in order to find such properties as the specific heat capacity,
melting point, boiling point, and the glass transition temperature.1 5 The TA
brand DSC machine used can heat a sample up to 400 degrees C, which is
usually more than enough to show changes in a material, while it is being
tested. In order to find proper data during testing, the DSC machine is
calibrated with a sapphire sample. The information on the sapphire is known,
and can be repeated several times in order to make sure the information is
correct. Furthermore, a baseline test must be run, with an empty sample.
With this, any noise can be subtracted from the testing, leaving a much
cleaner result. However, the gel sample showed no changes up to 250 C. In
fact, as there was a baseline test, after subtraction, there was no change to
33
the gel. This means that there was not even softening of the material from
such high heat. This makes sense, as the gel has to withstand high friction
gains from constant impacts to the ground while running or walking. This
leads to the hypothesis of a cross-linked elastomer that resists thermal
4
changes.1 ' 15
34
4. Results
4.1 ISN Drop Tower Results
While there were only five trial runs with this drop tower, a general
trend is shown. The first graph shows the maximum load in kiloNewtons for
each pad sample. It is easy to see which samples are stable, and dissipate
the most energy through the pad. There is also evidence of hardening of the
vinyl nitrile control pad. Around 15-20 seconds passed between each trial
run, similar to plays in a football game, especially in rushed circumstances.
Maximum Load at Impact
5.5
5
A
-A
-i
-
4.5
4
AL
3.5
--
3
0
--
U-
*
Vinyl Nitrile
*
Vacuum Grease
A Gel Pad
2.5
*
2
0
1
2
4
3
Drop Number
Figure 15: Maximum Load At Impact
35
5
6
Asics Heel
It is evident from this chart that the vacuum grease pad sample performs
the best. However, for more robustness, more trials should be performed.
In the next chart, a comparison between the best pad, the vacuum grease
sample, and the vinyl nitrile control pad is shown of the maximum load to
failure, again in kN. Again, the vacuum grease pad performs better and
more consistently than the control pad.
Maximum Failure at Impact
0.9
-
0.8
z
-
--
0.7
--
-
0.6
-
----
--
_
+E Vinyl Nitrile
-
Vacuum Grease
0.0.4
0
1
2
3
4
5
6
Drop Number
Figure 16: Maximum Failure At Impact
The last comparison done with the ISN drop tower, shows the relationship
between deflection and energy. Each test was run with a standard energy of
20 Joules. In the following graph, the vacuum grease pad reaches the
maximum energy transmitted, but with less deflection than the control pad.
36
Deflection vs Energy - Vinyl Nitrile
E
0
- -------
--
00
-5
5
10
15
20
Energy (J)
Figure 17: Deflection vs. Energy - Vinyl Nitrile
Deflection vs Energy - Vacuum Grease
2-5
_
_
_
-
-
-
E
.0
-5
5
10
15
20
Energy (J)
Figure 18: Deflectin vs. Energy - Vacuum Grease
4.2 MIT Drop Tower Results
While there were not enough results before the force pad broke to
draw any conclusions, there was an interesting comparison between the
control pad, and the Asics shoe heel with the gel inside of the foam. The
37
control pad is extremely noisy, which looks like it reaches a high amount of
force several times, whereas the shoe heel has a very clean force with a
singular peak. Unfortunately, during the vinyl nitrile drop, the oscilloscope
program was not calibrated to record force in Newtons. There was not
enough time to keep testing this phenomenon, but it would be an interesting
discussion in the future.
Force at Impact - Vinyl Nitrile
4-50E-&1-
---
-
-----
o 3-OGE-O-
L2
2-G
E-O-1-
__
-
-
_
_
_
-
-
_
_
-
_
0
-1.0
j-)0
E+00-M0E
2->.
Time (s)
Figure 19: Force at Impact - Vinyl Nitrile
38
E-02
Force at Impact - Asics Shoe
±50E+G&
±O-.OE+03U
0
U-.
&5-OGE+G-2-
-1.00E-02
0.0 0:+00
1.OOE-02
2.OOE-02
3.OOE-02
4.OOE-02
5.OOE-02
Time (s)
Figure 20: Force at Impact - Asics Shoe
4.3 Instron Testing
Instron testing was done in order to get the stress stain curves of both
the vinyl nitrile foam and the shoe foam. While the vinyl nitrile foam was
used in the vacuum grease pad, it is also one of the variables that can be
researched in order to find the best combination. Through these curves, the
differences in the energy absorption of the foams can be shown, and the
best foam of the two, chosen.
39
Stress- Strain Curve - Asics Foam
5000000(U
0~
U)
U)
GJ
4.'
-1
-0.9
-0.8
-0.7
-0.6
-0.4
-0.3
-0.2
-0.1
0
Figure 21: Stress Strain Curve - Asics Foam
Stress Strain Curve - Vinyl Nitrile Foam
--350000--
-250000-
(U
0.
--200G0--
U)
U)
aJ
-150000-
4.'
S10000
---5000G-1
-0.8
-0.6
-C.4
Strain
Figure 22: Stress Strain Curve - Vinyl Nitrile
4.4 DSC Results
The results of the gel tests with the differential scanning calorimetry
machine were mostly unremarkable. However there was one interesting
feature of the data. There was a slight curve around 250 degrees Celsius,
40
but it was small enough that it might have shown some interference from
the machine itself. Though, a paper about DSC changes of elastomers shows
that there might have been similarities between the Asics gel and the
elastomers tested in the paper. Those segmented elastomers, with soft and
hard alternating regions, also showed that curved region around 200-250
degrees C.' 6 There is not enough information to draw further conclusions, as
there was always the fear of damaging the machine at a higher heat. But the
Asics gel must be able to resist thermal activity from friction caused by
running, and a cross linked elastomer would make a logical choice.
4.5 Brown University Head Form
The results changed as the drop height and velocities changed. At a
low drop with 3.46 m/s drop velocity, the vacuum grease helmet performed
very consistently, and had a lower severity index than the vinyl nitrile
helmet. However, at the high drop, with a velocity of 5.34 m/s, the results
reversed. The average peak acceleration at the low drop was 65.226 g for
the vacuum grease, and 65.654 g for the vinyl nitrile. At the high drop, the
average acceleration was 127.7 g for the vacuum grease, and 114.2 g for
the vinyl nitrile.
41
Severity Index - Low Drop
250
U
200
U
x
-
-
_
~
47 150
A Vacuum Grease
100
N Vinyl Nitrile
50
0
0
1
2
3
4
5
6
Drop Number
Figure 23: Severity Index - Low Drop
Severity Index - High Drop
700
600
500
U~ ~k
-o
..
0
400
300
*
200
M Vinyl Nitrile
100
0
0
1
2
3
4
Drop Number
Figure 24: Severity index - High Drop
42
5
6
Vacuum Grease
5. Discussion
5.1 Linear Impact Testing
Foam is a good material for energy absorption. When looking at a
compression stress-strain curve of the foam, it has a wide plateau in
between the elastic region and the densification region. The longer the
plateau, the better the absorption. The reason for this plateau has to do with
the cells inside the foam. During a stress-strain compression test, the
individual cell walls are collapsing. This stops the entire material from
collapsing as in other, non-cellular materials. Once the walls have fully
collapsed, then the entire sample can start compressing. This method allows
the energy going through a foam to be lessened as the time to full
compression is much longer. This reason is why foams are used as the
materials inside football helmets currently. However, this is not enough to
stop the massive impacts sustained on football players. A problem with using
foam pads inside each helmet, is that after repeated impacts, the cell walls
do not fully return to their original size and height. It takes a very long
amount of time for it to fully return to the original position, and in football,
43
an elastic response just is not a reasonable assumption. The new pad design
incorporates the nitrile foam already used, with an internal damping system
to supplement the foam and its flaws. As mentioned previously, the
proposed replacement pad employs a pad made into a sandwich structure
with a sack of vacuum grease in the middle. Around the outside, there is a
latex band to keep the vacuum grease from staying squeezed outside of the
pad, after impacts. This setup acts like a dashpot system. The extremely
high viscosity vacuum grease slows the impact considerably. Furthermore,
because the vacuum grease is not confined to the space inside the pad, it
can move out the open sides to an extent. At impact, the force is translated
downwards, towards the bottom of the pad. Before it can reach that point,
however, it reaches the sack of grease. This grease translates the force
horizontally, away from the bottom of the pad.
Figure 25: Vacuum Grease pad dissipates the impact energy outwards, whereas the vinyl nitrile control pad allows
too much energy to be transmitted to the player's head.
44
The original foam structure is also helping to decrease the force. This means
that the force that reaches the bottom, which means the head, inside the
helmet is considerably less than what it was from the impact. In addition to
this system, there is a comfort pad underneath the modified pad. In the
original helmet design, it is simply a soft pad, designed to provide minimal
comfort. The newly designed system incorporates a gel comfort pad. As the
one tested was a Dr. Scholl's foot insert, it also acts as a damping against
the impact forces. Not only does it bring comfort, but an added safety factor,
due to the shock absorbent material included inside each foot insert.
Vacuum grease was as the most effective damping solution because it has
such a high viscosity. According to a report from Dow-Corning, the brand of
vacuum grease used is composed of amorphous silica, polydimethylsiloxane,
and dimethyl siloxane.
H3C
H3C
H3 CQ-.
\
H3C
Si
Si
Si
/
,H
CH 3
3
C0H
3
cK
-e
2-
n
Figure 26: Polydimethylsiloxane structure
45
CH3
CH 3
O-Si
I
OH
3
n
Figure 27: Dimethyl Siloxane structure
As shown in the schematics above, the aforementioned materials are large
and long chained. Because of this, a higher viscosity is produced, as the
polymer chains have trouble sliding past each other. This relates to the
dashpot system in that oil is generally used, instead of much lower viscosity
fluids. At impact, the energy is slowed considerably because of the sluggishly
moving fluid."
When comparing the stress strain curves of the Asics heel foam and
the vinyl nitrile, it looks like the heel foam has a longer plateau for energy
absorption. It also is shown that it can reach a higher maximum stress. This
shoe heel should be further tested inside the helmet, for future work, to
determine if these are the properties required for better pads.
Shoe inserts were chosen to be similar to gel baskets used in helmets.
They are not only comfortable, but also energy absorbent. Much like the
system used in the heel of the shoes, the gel inserts must undergo constant
hard impacts between the foot and the ground. Needed between the helmet
pads and the players head is a soft object for good fit and comfort. This
46
insert can conform to the head to make it very tight and immovable, as well
as provide an extra layer of protection.
When Riddell improved its helmets recently, the company added a
better distribution of pads around the temple, ear, and neck area. This
reduced concussions by 10- 2 0%. However, there has been no mention of a
change in pad design. If that design for pad placement was coupled with a
better performing, and more comfortable pad, concussion risk could be
lowered even further. It is interesting, that the brain protects itself with a
viscous fluid between it and the skull. This natural solution is very similar to
the winning pad in linear energy dissipation.' 3
5.2 Head Form Testing
The results of the NOCSAE head form testing at Brown University gave
interesting conclusions and insight into possible future work. It was
mentioned while there that the when researchers test single helmet pads
against one another with only a linear force they can find which pad seems
to be the best. However when the researchers put the pads inside the
helmets and tested the full system, the results would reverse, and the pads
that seemed to work so well, performed terribly as compared to the current
helmet designs. Based on the results gathered about the vacuum grease
pad, this same dilemma did not happen. At a 3.46 m/s drop velocity, the
vacuum grease outperformed the control vinyl nitrile pad by BLANK percent.
47
However, a shift in results did happen at the highest velocity tested, which
was 5.34 m/s (0.12 m/s below NOCSAE standards).' 2 The vinyl nitrile pad
performed better than the vacuum grease, by BLANK percent. Although
there could be many variables as to why this happened, the main
hypotheses include both a significantly higher mass in the vacuum grease
pad and human fabrication error. As higher mass can be a problem in a
direct comparison, since it can cause a higher force to occur on impact. In
order to combat this, and to find the impact a higher mass has on the
acceleration of the head inside the helmet, it is common practice to retrieve
results from a variety of velocities and calculate the effects of mass from
those different data points. There was not enough time during testing to get
all of that data, so if this system were to be tested again, that would a
telling experiment. It could possibly be expected that there is a mass
problem, as the vacuum grease did perform better at a lower velocity.
Furthermore, another explanation for the change in results is human error in
the creation of the final helmet pads. At one point, after a drop, it was
evident that at least one of the pads was leaking vacuum grease,
counteracting the entire system.
For one of the highest drop tests, though, the vacuum grease
performed better, since it was dropped from a different location. The
previous tests were done on the side of the helmet, and the second tests
were performed on the top of the helmet. The fabrication was marginally
48
better there, as it had been done first. Results showed that the vacuum
grease performed better in the first couple hits, but then started to lose out
to the vinyl nitrile helmet. Unlike the hardening of the foam in the vinyl
nitrile pads during linear drop testing, there was very little increase of
maximum acceleration values or Severity Index. When the pads are put into
the helmet, the entire system worked together much better than expected.
The shells of the helmet are a large factor in the success of the entire
helmet. Because of this, the fact that the vacuum grease pads use the vinyl
nitrile foam as their outer container may have worked in their favor. If there
had been more time, it would have been an interesting experiment to test
some of the Asics shoe heels. During the linear drop tests, they performed
very close to each other. As the shoe heel system is very similar to the
vacuum grease system, though with a different mass, it would be useful to
test it in further research.
49
6. Conclusion
Preliminary testing showed that the vacuum grease pad continues to
perform better than the vinyl nitrile pad that is currently used in Riddell
helmets. With vacuum grease and a sandwich structure, possibility of
concussions can be reduced significantly.
Testing at Brown University on a head form showed that there was
promising results in the vacuum grease pad. At lower velocities, the helmet
performed better than the current Riddell helmet. Most impacts happen
below 70 g of acceleration, which can have a big impact over a football
player's career. However, in order to really prevent concussions, some
redesigns must be done on the vacuum grease helmet in order to decrease
the weight for higher drop velocities and harder impacts.
50
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