Improving Energy Dissipation to Lower Risk in

MASC HUSETTS 1NS rKf9 by OFTECHNOLOGY JUN 0 42014 Christine Elizabeth Labaza 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 Signature redacted Certified by: Lorna J. Gibson Professor of Materials Science and Engineering Thesis Supervisor Signature redacted Accepted by: 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 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 ...... 11 2. Background ...... 14 2.1 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 ...... 2 1 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...... 4 3

6. Conclusion...... 5 0

8 Figure 1: Riddell Revolution Speed helmet with vinyl nitrile pads...... 12 Figure 2: Linear acceleration of im pacts...... 16 Figure 3: Linear acceleration of vinyl nitrile...... 17 Figure 4: Linear impacts of sandwich structures...... 18 Figure 5: V iscous fluids choices...... 19 Figure 6: A final comparison of the results for the vinyl nitrile control pad and the vacuum 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 ...... 22 Figure 8: Unwrapped vacuum grease sandwich structure...... 23 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 ...... 28 Figure 12: Positions of strikes for Head Form testing...... 30 Figure 13: Head Form in action...... 31 Figure 14: Vacuum grease pad helm et...... 32 Figure 15: M axim um Load At Im pact...... 35 Figure 16: M axim um Failure At Im pact ...... 36 Figure 17: Deflection vs. Energy - Vinyl Nitrile ...... 37 Figure 18: Deflectin vs. Energy - Vacuum Grease ...... 37 Figure 19: Force at Im pact - Vinyl Nitrile ...... 38 Figure 20: Force at Im pact - Asics Shoe ...... 39 Figure 21: Stress Strain Curve - Asics Foam ...... 40 Figure 22: Stress Strain Curve - Vinyl Nitrile ...... 40 Figure 23: Severity Index - Low Drop ...... 42 Figure 24: Severity Index - High Drop ...... 42 Figure 25: Energy Dissipation directions...... 44 Figure 26: Polydim ethylsiloxane structure ...... 45 Figure 27: Dim ethyl Siloxane structure ...... 46

9 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 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 drop- towers 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 3272 1745 924 549 272 537 0-10 10 1-20 20.1-30 30 140 40,1-50 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

900 800 4 700 600 -

=0 500 -

400 *Control Vinyl Nitrile Pad 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 * Vacuum Grease 0 Specimen

Figure 5: A final result of multiple viscous fluids, showing that the vacuum grease lowered the maximum acceleration by the most.7

Average 5.2 12.2 19.1

KLA i =n sr. 117 is Minimum 4-2 11-2 18.6

Maximum 16.4 19.7 Coef. ol Variation 8.7708 12.6856 18794 Std. Dev 0.4574 , 15472 0-3581

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 . These pads make up most of the helmet with the exception of the 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

c- E =- E

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

Al E =

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 changes.14' 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 4.5 4 * Vinyl Nitrile AL - 3.5 -i * Vacuum Grease -- 0 3 U- -- A Gel Pad 2.5 * Asics Heel 2 0 1 2 3 4 5 6 Drop Number

Figure 15: Maximum Load At Impact

35 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

0.- Vacuum Grease

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->. E-02 Time (s)

Figure 19: Force at Impact - Vinyl Nitrile

38 Force at Impact - Asics Shoe

±50E+G&

±O-.OE+03-

U 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. U) --200G0-- U) aJ 4.' -150000- 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 U _ 200 x - - ~ 47150

100 A Vacuum Grease 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 * Vacuum Grease 200 M Vinyl Nitrile 100 0 0 1 2 3 4 5 6 Drop Number

Figure 24: Severity index - High Drop

42 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 CH 3 ,H 3

H3CQ-. \ C0H3 Si Si Si / cK H3C CH3 -e 2- n

Figure 26: Polydimethylsiloxane structure

45 CH3 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.

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