Note: Descriptions are shown in the official language in which they were submitted.
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PROTECTIVE HELMETS HAVING
ENERGY ABSORBING LINERS
Cross-Reference to Related Application
This application claims priority to co-pending U.S. Provisional Application
Serial Number 62/078,007, filed November 11, 2014, which is hereby
incorporated
by reference herein in its entirety.
Background
Sports concussion and traumatic brain injury have become important issues in
both the athletic and medical communities. As an example, in recent years
there has
been much attention focused on the mild traumatic brain injuries (concussions)
sustained by professional and amateur football players, as well as the long-
term
effects of such injuries. It is currently believed that repeated brain
injuries such as
concussions may lead to diseases later in life, such as depression, chronic
traumatic
encephalophathy (CTE), and amyotrophic lateral sclerosis (ALS).
Protective headgear, such as helmets, is used in many sports to reduce the
likelihood of brain injury. Current helmet certification standards are based
on testing
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parameters that were developed in the 1960s, which focus on the attenuation of
linear impact and prevention of skull fracture. An example of a linear impact
is a
football player taking a direct hit to his helmet from a direction normal to
the center of
his helmet or head. Although the focus of headgear design has always been on
attenuating such linear impacts, multiple lines of research in both animal
models and
biomechanics suggest that both linear impact and rotational acceleration play
important roles in the pathophysiology of brain injury. Although nearly every
head
impact has both a linear component and a rotational component, rotational
acceleration is greatest when a tangential blow is sustained. In some cases,
the
rotational acceleration from such blows can be substantial. For instance, a
football
player's facemask can act like a lever arm when impacted from the side, and
can
therefore apply large torsional forces to the head, which can easily result in
brain
trauma.
Although the conventional wisdom is that the components of modern
protective headgear that are designed to attenuate linear impact inherently
attenuate
rotational acceleration, the reality is that such components are not designed
for that
purpose and therefore do a relatively poor job of attenuating rotational
acceleration.
It therefore can be appreciated that it would be desirable to have means for
attenuating not only linear impacts to but also rotational accelerations of
the head, so
as to reduce the likelihood of brain injury.
Brief Description of the Drawings
The present disclosure may be better understood with reference to the
following figures. Matching reference numerals designate corresponding parts
throughout the figures, which are not necessarily drawn to scale.
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Fig. 1 is a cross-sectional side view of an embodiment of a protective helmet.
Fig. 2A is a front view of an embodiment of an energy absorber that can be
used in the helmet of Fig. 1.
Fig. 2B is a side view of the energy absorber of Fig. 2A.
Fig. 3 is a partial detail view of an energy absorbing column of the energy
absorber of Fig. 2.
Fig. 4 is a side view of a further embodiment of an energy absorber that can
be used in the helmet of Fig. 1.
Fig. 5 is a side view of a compressed energy absorber illustrating bending and
buckling of its energy absorbing columns.
Fig. 6A is a bottom view of a protective helmet of the type shown in Fig. 1
immediately prior to impact from another helmet.
Fig. 6B is a bottom view of the protective helmet of Fig. 6A during an impact
from another helmet.
Fig. 7 is a side view of a further embodiment of an energy absorber that can
be used in the helmet of Fig. 1.
Fig. 8A is a cross-sectional side view of a protective helmet incorporating an
energy absorbing outer shell immediately prior to an impact.
Fig. 8B is a cross-sectional side view of the protective helmet of Fig. 8B
during the impact.
Fig. 9 is a rear perspective view of a first embodiment of a passive helmet
tether system.
Fig. 10 is a rear perspective view of a second embodiment of a passive
helmet tether system.
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Fig. 11 is a rear perspective view of a third embodiment of a passive helmet
tether system.
Fig. 12 is a rear perspective view of a first embodiment of an active helmet
tether system.
Fig. 13 is a rear perspective view of a second embodiment of an active helmet
tether system.
Detailed Description
As described above, current protective headgear is primarily designed to
attenuate linear impact. However, it has been determined that both linear
impact and
rotational acceleration from torsional forces contribute to brain injury, such
as
concussion. Disclosed herein are energy absorbing systems that comprise means
for absorbing energy from impacts to a protective helmet that minimize both
translational and rotational accelerations experienced by the head of the
helmet
wearer. In some embodiments, these means comprise an inner liner that includes
energy absorbing columns that are designed bend and buckle to attenuate both
translational and rotational accelerations. In some embodiments, the means
comprise energy absorbing outer shell that locally deforms upon hard impacts
to
absorb energy. In some embodiments, the means comprise an energy absorbing
tether system that limits linear movement and rotation of the helmet upon hard
impact. These various means can be used independently of each other or in
conjunction with each other to protect the helmet wearer.
In the following disclosure, various specific embodiments are described. It is
to be understood that those embodiments are example implementations of the
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disclosed inventions and that alternative embodiments are possible. All such
embodiments are intended to fall within the scope of this disclosure.
Described below are energy absorbing systems that can be incorporated into
protective helmets that not only address linear forces but also tangential
forces that
cause the highest shear strains on the brain and the brain stem. By optimizing
protection from both linear impacts and rotational acceleration, the
transmission of
shear force to the brain from head impacts can be reduced and so can the
incidence
of brain injury, such as concussion.
In some embodiments, a protective helmet can be provided with an energy
absorbing inner liner that utilizes energy absorbing columns having various
lengths
and/or cross-sectional dimensions that are sandwiched between two elastomeric
layers. The use of columns of varying lengths and/or cross-section dimensions
enables protection against impacts over a range of energy levels. When columns
of
different lengths are used, low-energy impacts will activate only the tallest
columns,
which are connected to both layers, resulting in low translational
accelerations.
Higher energy impacts, however, will also activate shorter columns, which are
connected to only one layer to prevent bottoming out and unacceptably high
translational accelerations. The liner can be designed to provide optimal
stiffness by
tuning the distribution of columns to control the peak accelerations applied
to the
wearer's head during impact.
As disclosed herein, the inner liner uses controlled buckling and bending of
the columns to mitigate both linear and rotational accelerations experienced
by the
wearer's head. Traditional column buckling is a velocity-dependent process
that
produces high initial forces that drop very low as the column deforms. This
fundamental behavior must be overcome if columns are to become an efficient
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energy absorber for use in protective helmets and other protective equipment.
One
important advantage of precise column buckling that makes it attractive for
use as a
helmet liner is the directionality of its resistance forces during oblique
impacts that
apply rotational moments to the helmet. During this type of impact, the top of
the
column pushes the helmet in the direction of the applied moment while pushing
the
player's head in the opposite direction. An advance of the disclosed liners is
that
linear impact dissipation can be optimized without adversely affecting the
rotational
behavior of the columns.
A column buckles when the eccentricity, or misalignment, over its length
produces a bending moment in the center of the column that overcomes its
bending
stiffness. Hence, dynamic buckling utilizes forces from axial loading to push
the
middle of the column laterally. Unless the misalignment is very large, these
lateral
forces are small relative to the mass of the column. As a result, the column
produces
a large inertial impulse while dynamic buckling is initiated. This type of
impulse can
produce dangerous acceleration forces on the player's head. In some
embodiments,
the disclosed inner liner overcomes this problem using multiple features.
First, the
columns of the inner liner can be made of an elastomeric material that
provides
some level of axial compression during the period in which buckling is
initiated to
compensate for the magnitude of the inertial spike.
Second, the columns can be eccentric relative to the layers between which
they lie to reduce the load required to initiate column buckling. These
eccentricities
take the form of a misalignment of the column ends from the normal direction
of the
layers so that the columns will have a moment applied upon the onset of
loading.
This misalignment also results in additional stroke because it can cause the
column
halves to fold beside themselves as they collapse rather than stacking on top
of
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themselves. Furthermore, the curvature of the inner liner due to the curved
nature of
the helmet results in further eccentricity in the columns because it is likely
that only a
small portion of the activated columns will be normal to the impact direction,
thus any
inertial forces coming from these columns would be small in comparison to the
overall forces generated by the sum of activated columns.
Third, as mentioned above, the column lengths can be varied. Varying column
lengths accomplishes two goals. Firstly, it spreads out the inertial impulse
to
eliminate the high inertial spike during the early stages of impact. Secondly,
it
enables the liner stiffness to be increased with higher deflections.
Fig. 1 illustrates an example embodiment of a protective helmet 10 that is
designed to attenuate both linear impact and rotational accelerations. The
helmet 10
shown in Fig. 1 is generally configured as an American football helmet.
Although that
particular configuration is shown in the figure and other figures of this
disclosure, it is
to be understood that a football helmet is shown for purposes of example only
and is
merely representational of an example protective helmet. Therefore, the helmet
need
not be limited to use in football. Other sports applications include baseball
and
softball batting helmets, lacrosse helmets, hockey helmets, ski helmets,
bicycling
and motorcycle helmets, and racecar helmets. Furthermore, the helmet need not
even be used in sports. For example, the helmet could be designed as a
construction or military helmet. It is also noted that the principles
described herein
can be extended to protective equipment other than helmets. For example,
features
described below can be incorporated into protective pads or armor, such as
shoulder
pads, hip pads, thigh guards, shin guards, cleats, and other protective
equipment in
which energy absorption could be used to protect the wearer.
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With continued reference to Fig. 1, the helmet 10 generally includes an outer
shell 12 and an inner liner 14. In the illustrated embodiment, the shell 12 is
shaped
and configured to surround the wearer's head with the exception of the face.
Accordingly, the shell 12, when worn, extends from a point near the base of
the
wearer's skull to a point near the wearer's brow, and extends from a point
near the
rear of one side of the wearer's jaw to a point near the rear of the other
side of the
wearer's jaw. In some embodiments, the shell 12 is unitarily formed from a
generally
rigid material, such as a polymer or metal material. Example materials are
described
below in relation to Figs. 9A and 9B.
Irrespective of the material used to construct the shell 12, the shell
includes
an outer surface 16 and an inner surface 18. In some embodiments, the shell 12
can
further include one or more ear openings 20 that extend through the shell from
the
outer surface 16 to the inner surface 18. The ear openings 20 are provided on
each
side of the shell 12 in a position in which they align with the wearer's ears
when the
helmet 10 is donned. Notably, the shell 12 can include other openings that
serve one
or more purposes, such as providing airflow to the wearer's head.
As is further shown in Fig. 1, a facemask 22 can be secured to the front of
the
helmet 10 to protect the face of the wearer. In some embodiments, the facemask
22
can comprise one or more rod-like segments that together form a protective
lattice or
screen. When used, the facemask 22 can, for example, be attached to the helmet
10
at points that align with the forehead and jaw of the wearer when the helmet
is worn.
The facemask 22 can be attached to the helmet 10 using screws (not shown) that
thread into the shell 12 or into fastening elements (not shown) that are
attached to
the helmet. Although a particular facemask configuration is shown in the
figures,
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alternative configurations are possible. Moreover, the facemask 22 can be
replaced
with a face shield or other protective element, if desired.
The inner liner 14 generally comprises one or more pads that sit between the
shell 12 and the wearer's head when the helmet 10 is worn. In some
embodiments,
each of the pads is removable from the helmet. For instance, the pads can be
configured to releasably attach to the inside surface 18 of the helmet shell
16 with
snap, T-nut, or hook-and-loop fasteners. In the illustrated embodiment, the
pads
include a top pad 24, multiple lateral pads 26, 28, and 30, a front pad 32, a
rear pad
34, and jaw pads 36. The top pad 24 is adapted to protect the top of the
wearer's
head. In the illustrated embodiment, the top pad 24 is elongated in a
direction that
extends along the sagittal plane of the wearer so as to extend from a rear top
portion
of the head to a front top portion of the head. The top pad 24 is further
curved to
generally follow the curvature of the wearer's head. Accordingly, the top pad
24
forms a concave inner surface that is adapted to contact the wearer's head.
The lateral pads 26-30 are adapted to protect the sides of the wearer's head.
The lateral pads 26-30 extend from the edges of the wearer's face to points
behind
(and above) the user's ears. Like the top pad 24, the lateral pads 26-30 are
curved to
follow the curvature of the shell 12 and the wearer's head. Accordingly, the
lateral
pads 26-30 form concave inner surfaces that are adapted to contact the
wearer's
head.
The front pad 32 is positioned within the outer shell 12 so as to protect the
forehead of the wearer. Like the other pads, the front pad 32 is curved to
follow the
curvature of the wearer's head. The forward pads 30 therefore form concave
inner
surfaces that are adapted to contact the wearer.
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The rear pad 34 is adapted to protect the rear of the wearer's head. The rear
pad 28 is also curved to follow the curvature of the wearer's head and forms a
concave inner surface that is adapted to contact the wearer's head.
The jaw pads 36 are adapted to protect the jaw of the wearer. As with the
other pads, the jaw pad 36 can curved to follow the curvature of the wearer's
head
and forms a concave inner surface that is adapted to contact the wearer's
head.
Several or all of the above-described pads can be of similar construction. In
some embodiments, each of the pads comprise an outer energy absorber 40 that
is
adapted to absorb translational and rotational energy from helmet impacts and
an
inner cushion 42 that is adapted to provide comfort to the wearer's head. The
energy
absorbers 40 releasably attach to the inner surface 18 of the shell 12.
Details about
the construction of the energy absorbers 40 are provided below in relation to
Figs. 2-
8. It suffices to say at this point, however, that the energy absorbers 40
include
energy absorbing columns 44 that dissipate translational and rotational
accelerations.
The inner cushions 42 of the pads contact or are at least adjacent to the
wearer's head and/or face when the helmet 10 is donned. The cushions 42 can
have
any construction that is comfortable for the wearer. In some embodiments, the
cushions 42 are foam cushions. In other embodiments, the cushions 42 are air
bladder cushions.
Figs. 2A and 2B illustrate an example energy absorber 50 that can be used in
a pad that forms part of a helmet liner, such as the inner liner 14 shown in
Fig. 1. As
shown in Figs. 2A and 2B, the energy absorber 50 generally comprises a first
or
inner layer 52, an opposed second or outer layer 54, and a plurality of energy
absorbing columns 56 that are provided between the layers, which can bend and
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buckle to absorb energy. As illustrated in the figures, the inner and outer
layers 52,
54 comprise thin, generally planar members that are curved to conform to the
curvature of the human head and the outer shell 12. In some embodiments, the
layers 52, 54 have similar curvatures. The inner layer 52 comprises an inner
surface
58 that faces the outer layer 54 and an outer surface 60 that faces the
wearer's head
and provides a surface to which an inner cushion 42 can be attached. The outer
layer 52 comprises an inner surface 62 that faces the inner layer 52 and an
outer
layer 64 that can be attached to the inner surface 18 of the outer shell 12.
The energy absorbing columns 56 can comprise elongated cylindrical
members that are substantially perpendicular to the inner and outer layers 52,
54. As
is apparent in Figs. 2A and 2B, the columns 56 can have various lengths or
heights.
Relatively long columns 66 connect the inner and outer layers 52, 54. Such
columns
66 are attached at a proximal end (nearest the wearer's head) to the inner
layer 52
and are attached at a distal end (nearest the shell 12) to the outer layer 54.
Shorter
columns 68 are only attached to one of the layers 52, 54. In the illustrated
embodiment, the proximal ends of the shorter columns 68 are attached to the
inner
layer 52 while the distal ends of those columns are free ends. In addition to
the
lengths, the cross-sectional dimensions of the columns 56 can be varied.
In some embodiments, the energy absorber 50 can comprise columns of
several different lengths. For example, the energy absorber 50 could
incorporate
columns 56 of 2, 3, 4, 5, or more different lengths, in which case the energy
absorber
provides multiple stages of energy dissipation. In such cases, relative mild
impacts
may only affect the longest columns 56 (i.e., the first stage of the energy
absorber
50) while stronger impacts may affect columns of shorter lengths (i.e., other
stages
of the energy absorber). This multi-stage approach provides increased
stiffness as
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the deflection of the energy absorber 50 increases, as well as reduction in
the inertial
spike that comes prior to the onset of buckling in the columns 56.
An important measure of energy absorber efficiency is the achievable
absorber deflection divided by its original length. All multi-impact energy
absorbers
have a maximum useable deflection beyond which the stiffness becomes
excessive.
This difference is normally referred to as the stack-up distance. In some
embodiments, the columns 56 are arranged within the energy absorber 50 in a
manner that minimizes interaction between adjacent columns to minimize the
possibility of the columns stacking on top of one another as the energy
absorber
compresses.
The thicknesses of the inner and outer layers 52, 54, the lengths and cross-
sectional dimensions of the columns 56, and the ratio of columns attached to
both
layers versus attached to only one layer can be tailored to achieve a desired
load
capacity for the energy absorber 50 and the pad in which it will be used.
Thicker
layers 52, 54 will increase the load capacity of the columns 56 because of the
stiffened end conditions, thereby enabling the use of thinner columns.
However,
thicker layers 52, 54 will also increase the overall mass of the inner liner
14 because
the layers represent the highest volume of material in the system while also
reducing
the useable stroke. Thus, it is important to optimize the energy absorbers 50
to
provide the desired outcome at each location within the helmet 10, taking into
account factors such as available stroke, coverage area in the impact
location,
frequency of impact in the protected location, and overall liner mass. For
instance,
the front pad 32 (Fig. 1) may have larger diameter columns and a higher ratio
of
attached columns than other pads in the liner 14 to increase the pad stiffness
due to
the inherent weakness in the outer shell at that location and the increased
need for
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protection in the frontal region due to the increased likelihood of impacts in
that
location.
In some embodiments, the outer layer 54 has a thickness of approximately
0.5 to 3 mm and may contain holes for fasteners or ventilation. In some
embodiments, the inner layer 52 has a thickness of approximately 0.5 to 2.5
mm. In
some embodiments, the energy absorbing columns 56 that are attached to both
the
inner and outer layers 52, 54 have lengths of approximately 18 to 65 mm and
cross-
sectional dimensions (e.g., diameters) of approximately 3 to 7 mm, while the
columns that are attached to only one of the layers have lengths of
approximately 8
to 55 mm and cross-sectional dimensions (e.g., diameters) of approximately 2
to 6
mm. In some embodiments, the fraction of columns 56 that are connected to both
layers 52, 54 is approximately 15 to 40%, but can be increased to as much as
100%
if the pad will undergo consistent loading and does not need to provide
protection
against a variety of impact conditions. While the columns 56 are illustrated
in Figs.
2A and 2B as having constant cross-sectional dimensions along their lengths,
it is
noted that these dimensions can vary along the lengths of the columns. For
example, one or more columns 56 can have a larger cross-section at its base
than at
other points along its length.
In some embodiments, the columns 56 can be slightly eccentric to reduce the
magnitude of the inertial spike that occurs upon impact. This eccentricity can
come in
the form of an angling of the columns 56 from the direction normal to the
inner
surface of the inner and/or outer layers 52, 54. Fig. 3 illustrates an example
of this
form of eccentricity. As shown in this figure, a column 56 is offset from the
normal
direction of the inner surface 58 of the inner liner 52 by an angle 0, which,
for
example, can be an acute angle up to approximately 15 degrees. Other possible
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forms of eccentricities include a predefined curve or kink manufactured into
the
columns. Fig. 4 illustrates an example of this. In this figure, an energy
absorber 70
having an inner layer 72, and outer layer 74, and a plurality energy absorbing
columns 76. Some of the columns 78 comprise a medial kink 80 that facilitates
buckling.
Although the energy absorbing columns 56 have been described as
comprising cylindrical members, which typically comprise circular cross-
sections, it is
noted that other cross-sectional geometries are possible. For example, the
columns
56 can have an elliptical, polygonal, or other non-uniform cross-section. In
addition,
the columns 56 can have a twisted configuration in which the cross-section
changes
along the length of the columns. For example, if the column 56 had an
elliptical
cross-section, the orientation of the ellipse can rotate as the length of the
column is
traversed to form a twisted shape. Such a shape can force the columns 56 to
twist
while buckling, which both increases the energy dissipation rate in the later
stages of
collapse and forces the top half of the column to land beside the bottom half,
which
reduces the stack-up distance and maximizes available compression in the
energy
absorber.
Each of the inner liner 52, outer liner 54, and the energy absorbing columns
56 can be made of an elastomeric material. In some embodiments, these
components are made of a thermoplastic elastomer (TPE), such as thermoplastic
polyurethane (TPU). BASF Elastollan 1260D U is one commercial example of a
TPU. Other suitable TPEs include copolyamides (TPAs), copolyesters (TPCs),
polyolefin elastomers (TP05), and polystyrene thermoplastic elastomers (TPSs).
TPU may be preferable for construction of the energy absorbers for a variety
of reasons. This material can be made to be soft enough to provide consistent
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initiation of the buckling process, has a rapid relaxation time to assure high
rates of
energy dissipation, and has proven to be both durable and tolerant of large
temperature variations. Both the viscoelastic nature of TPU and the
sensitivity of
column buckling to impact speed enable the energy absorbing columns to absorb
greater amounts of energy as impact speed increases. This is important for
helmets
that must attenuate high speed impacts and simultaneously provide optimum
protection of helmet wearers who experience large numbers of low speed
impacts.
Furthermore, TPU is a low cost, versatile, and commercially available
material. It
offers a long list of performance characteristics that are desirable in an
environment
involving energy management, such as athletic equipment and military
applications.
For instance, all grades of unreinforced TPU have high elasticity with
elongation to
break values of 300 to 1000%, tensile strength to yield of 10 to 45 MPa,
hardness
values of 52 to 98 on the Shore A scale and 22 to 95 on the Shore D scale, and
material densities in the range of 1.05 to 1.53 g/cc.
TPU also has a low glass transition temperature of -69 to -17 C, meaning that
it will retain its elastic properties over the a broad range of temperatures,
such as
that in which sports are played. In addition, TPU provides excellent abrasion
resistance, impact strength, weather resistance, and antimicrobial properties.
Additionally, TPU can be modified to suit a particular application by adding
fillers,
colorants, or stabilizers. One desirable performance characteristic is that
TPU can be
optimized to achieve effective damping with optimal rebound speed (e.g. short
relaxation time). Finally, TPU provides fabrication flexibility, can be
injection molded,
and can be bonded or welded though a variety of processes.
One potential problem associated with varying column length is the possibility
that shorter columns will slide, resulting in a bending mode of failure rather
than
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buckling. The bending failure mode greatly reduces energy dissipation rates.
To
combat this issue, a texture can be added to the inner surfaces of the inner
and
outer layers as well as the outer surfaces of the columns. Such a texture is
schematically illustrated in Fig. 3 as texture 82 and can cause the columns to
lock in
place once contact is made with other columns and/or the inner and/or outer
layers.
In some embodiments, the texture 82 can comprise a rough surface that is
formed
on the layers/columns during energy absorber fabrication (e.g., injection
molding). In
other embodiments, this texture 82 can comprise a geometric (e.g., metal) mesh
that
is integrated into the surfaces during fabrication (e.g., injection molding).
In some embodiments, the energy absorbers can be manufactured in two
parts. The first part can comprise the outer layer and all the necessary
features for
attaching the energy absorber to the outer shell 12, while the second part can
comprise the inner layer and the columns that are connected thereto. The two
parts
can be produced through injection molding or another commercial manufacturing
process. Once formed, the two parts can be bonded together through use of
welding
or an adhesive. Alternatively, the layers and columns could each be
manufactured
as separate parts. In such a case, the columns can comprise notches at their
ends
that enable them to be snapped into place into pre-formed holes in the inner
and
outer layers. The columns could then be bonded to the layers through welding
or
adhesion.
As discussed above, the energy absorbers are designed to deform upon
impact to dissipate energy. Fig. 5 illustrates such deformation. As shown in
this
figure, an energy absorber 90 comprises an inner layer 92, an outer layer 94,
and a
plurality of energy absorbing columns 96. The outer layer 94 has been pushed
in
toward the inner layer 92 because of an external (downward) force and, as a
result,
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the columns 96 of the energy absorber 90 have bent and/or buckled under this
force,
thereby dissipating energy.
Figs. 6A and 6B illustrate operation of the energy absorbers when
incorporated into a protective helmet 100. As is apparent in this figures, the
helmet
100 includes an inner liner 102 comprising multiple pads 104 having energy
absorbing columns 106. Fig. 6A shows the helmet 100 prior to impact. In this
state,
the helmet 100 is centered on the wearer's head. Fig. 6B shows the helmet 100
upon receiving a tangential impact from another helmet 110. As can be
appreciated
from this figure, the energy absorbing columns 106 near the point of impact
have
deformed to absorb the energy of the impact. In addition, the helmet 100 has
rotated
relative to the wearer's head to dissipate the rotational force imparted by
the helmet
110 instead of delivering it directly to the wearer's head. In such a case,
the wearer's
head can remain relatively stationary, at least in terms of rotation, while
the helmet
100 rotates. Once the force is removed, however, the energy absorbing columns
106
can return the helmet 100 to its original orientation on the head.
It is noted that the energy absorbers can comprise other components besides
columns between their inner and outer layers. For example, Fig. 7 illustrates
an
energy absorber 120 comprising an inner layer 122, an outer layer 124, and a
plurality of energy absorbing columns 126. Provided between the inner and
outer
layers 122, 124 in a free space between the columns 126 is a supplemental
energy
absorbing member 128. The member 128 can comprise a foam element or an air
bladder element that provides increased energy dissipation where needed, such
as
near the front of a helmet. In the illustrated embodiment, the member 128 is
generally elliptical with its long axis extending along the normal directions
of the
inner and outer layers 122, 124.
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The basic premise of impact energy management is to optimize energy
absorption in each component of a system. So, while the inner liners described
above can be used to absorb energy, other components of the helmet can
likewise
be designed to absorb energy. Once such component is the outer shell of the
helmet.
The shell material in most commercial football helmets is made of
polycarbonate (PC) alloys or acrylonitrile butadiene styrene (ABS) plastic in
thicknesses ranging from 3 to 4 mm. While these materials have high impact
resistance, they exhibit a highly elastic response to impacts. Therefore, the
energy
absorbed by the shell material is minimal. Greater energy could be absorbed,
however, if the shell was made of a deformable, energy absorbing material.
With reference back to Fig. 1, the outer shell 12 of the protective helmet 10
can be made of such a material. In some embodiments, the shell 12 is made of a
polyethylene (PE) composition, such as high density polyethylene (HDPE). HDPE
is
a class of thermoplastic polymers that incorporate long chains of polyethylene
mers
with molecular weights in the range of approximately 100,000 to 3,000,000.
HDPE is
a suitable replacement for the elastic PC or ABS materials used in current
football
helmets, whether or not the helmets include an inner liner of the nature
described
above.
A protective helmet must meet the requirements under its working conditions.
Football helmets are required to absorb energy, resist gouging, fatigue, and
creep,
operate in extreme ambient temperatures (-12 C and 52 C), accept paint and
dyes,
and be readily manufacturable. HDPE is a low-cost, versatile, and commercially
available material. It offers a long list of performance characteristics that
are
desirable in an environment involving energy management, such as athletic
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equipment and military applications. Specific parameters of a suitable HDPE
composition include the following:
= Tensile Strength to Yield: ¨ 25-31 MPa
= Rockwell Hardness (Shore D): ¨ 55-75
= Elongation to Break: ¨ 900-1300%
= Flexural Modulus: ¨ 1000-1500 MPa
= Melt Flow Index: ¨ 5 to 8 g/10 minutes
Additionally, HDPE offers a lower density (0.95 g/cm3) when compared to
conventional PC (1.2 g/cm3) or ABS (1.05 g/cm3) formulations. A lower density
can
be advantageous by providing lower weight to the wearer or a thicker geometry
for
the same weight. In some embodiments, the shell has a thickness of
approximately
2.4 to 4 mm. HDPE also offers a low glass transition temperature of -70 C to -
80 C.
It is important to note that energy absorbing outer shells can be too soft. If
the
local deformation of the shell is too high, impacting helmets can become
entangled
or interlocked such that high forces can be generated parallel to the surface
of the
shell. This type of loading produces high rotational accelerations in the
helmet. High
rotational accelerations are also produced when hard accessories, such as
fasteners, gouge into the surface of the shell material. Typical HDPE's may
not offer
sufficient slip (low friction) or abrasion resistance to counter mechanical
interlock as
described above. The polyethylene of the HPDE can be compounded with one or
more additives to combat these issues. Such additives can include a processing
stabilizer that protects the polymer at high temperatures, a heat stabilizer
that inhibits
degradation of the end product, a slip agent that reduces friction between
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surfaces (i.e., increases slip), and an ultraviolet stabilizer that inhibits
environmental
degradation. ADDCOMP ADD-VANCE 148 and 796 are two example commercial
multi-functional additives that could be used. A range of approximately 1 to
8% by
weight of the additives can be compounded with the PE base in the composition.
Football helmet shells are often dyed in a similar color in which they will be
painted or coated after manufacturing. The HDPE composition described herein
can
readily accept up to 3.5% by weight, which is suitable for the range of colors
on the
market. Once a HDPE helmet has been manufactured, its surface energy can be
increased by 2-5 dynes/cm through corona treatment or other processes to
impart
wettability, which enables paint particles to adhere to the helmet.
Figs. 8A and 8B illustrate the effect of constructing the outer shell 12 of
the
protective helmet 10 out of an energy absorbing material, such as HDPE. Fig.
8A
shows the helmet 10 prior to impact. Fig. 8B shows the helmet 10 upon
receiving an
impact to the top of the shell 12. As can be appreciated from this figure, the
shell 12
has locally deformed at the point of impact and thereby dissipates some of the
impact force. As before, the energy absorbing columns 56 have also deformed
near
the point of impact.
Another way in which energy imparted to a protective helmet can be absorbed
is tethering of the helmet. Tethering a helmet involves attaching one or more
flexible
tethers between the wearer's helmet and an object securely anchored to the
wearer's body. Such tethers greatly increase the helmet's resistance to motion
by
firmly securing the helmet to the wearer's upper body. If properly designed,
tethers
can reduce peak accelerations by as much as 80 percent by raising the
effective
mass of the head and helmet from approximately 13 lbs. to over 70 lbs.
An effective helmet tether system can incorporate the following features: A)
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enables the head/neck complex to freely rotate and posterior flex when not
being
struck; B) provides resistance to acceleration when helmet is struck; C)
cannot apply
excessive force to helmet; D) cannot obstruct players vision; and E) easily
attaches
and detaches from the helmet.
A helmet tether system can be designed as a passive or an active system.
Passive tether systems are designed to resist extreme motions, such as
excessive
deflection or velocity. Active tether systems, however, incorporate sensors
that
sense when an impact has either begun or is about to occur and includes
actuation
mechanisms that actively respond to such sensed conditions.
Fig. 9 illustrates a first embodiment of a passive helmet tether system 140
that
links a protective helmet 142 to an article 144 (shoulder pads in this
example) worn
by the helmet wearer. The system 140 includes multiple tethers 146 that extend
between the helmet 142 and the shoulder pads 144. More particularly, a first
or
upper end of each tether is attached to the interior or exterior of the outer
shell 148
of the helmet 142, and a second or lower end of each tether is attached to the
outer
shells 150 of the shoulder pads 144. In the illustrated embodiment, the lower
ends of
the tethers 146 are attached to and wrapped around spools 152 that are fixedly
mounted to the shoulder pad outer shells 150. The spools 152 are free to
rotate to
enable lengthening of the tether 146 to enable turning of the head until the
maximum
length has been reached, at which point the tether limits further helmet
movement.
By limiting the degree to which the helmet 142 can be move relative to the
body, the
tether system 140 limits the forces that can be transmitted to the wearer's
head. In
some embodiments, the tethers 146 comprise high-strength, flexible, inelastic
cords.
Example inelastic cord materials include steel, nylon, polypropylene, and
polyethylene.
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In some embodiments, the spools 152 can comprise internal torsion springs
(not shown) that take up any slack that forms in the tethers 146. In other
embodiments, the spools 152 can further comprise internal locking mechanisms
(not
shown), such as centrifugal brakes, that automatically lock the angular
orientations
of the spools, and therefore halt lengthening of the tethers 146, upon a
threshold
angular acceleration being reached. The threshold angular acceleration can be
one
that is associated with movements of the helmet 142 that exceed the speed with
which the wearer can move his or own head, which are indicative of a helmet
impact.
Fig. 10 illustrates a second embodiment of a passive helmet tether system
160 that links a protective helmet 162 to an article 164 (shoulder pads) worn
by the
helmet wearer. The system 160 is similar to the system 140 in that a first or
upper
end of each tether 166 is attached to the outer shell 168 of the helmet 162,
and a
second or lower end of each tether is attached to the outer shells 170 of the
shoulder
pads 164. However, this embodiment comprises no spools. Instead, the tethers
146
comprise flexible, elastic cords that resist movement as they are stretched.
Example
elastic cord materials include elastomers such as synthetic rubber, and TPU,
and
fiber-reinforced elastomers.
Fig. 11 illustrates a third embodiment of a passive helmet tether system 180
that links a protective helmet 182 to an article 184 (shoulder pads) worn by
the
helmet wearer. This system 180 is also similar to the system 140 shown in Fig.
9.
Accordingly, the system 180 comprises multiple tethers 186 having a first or
upper
end attached to the outer shell 188 of the helmet 182, and a second or lower
end
attached to the shoulder pad outer shells 190. In this embodiment, however, an
extension mechanism 192 is provided along each tether 186. Lengths of the
tethers
186 are wound around an internal spool (not shown) within the extension
mechanism
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192, which also includes an internal torsion spring (not shown) that takes up
slack.
The extension mechanism 192 can further include a locking mechanism (not
shown)
that automatically locks the angular orientation of the internal spool, and
therefore
halts lengthening of the tether 186, upon a threshold angular acceleration
being
reached.
Fig. 12 illustrates a first embodiment of an active helmet tether system 200
that links a protective helmet 202 to an article 204 (shoulder pads) worn by
the
helmet wearer. The system 200 includes multiple tethers 206 that extend
between
the helmet 202 and the shoulder pads 204. More particularly, a first or upper
end of
each tether is attached to the interior or exterior of the outer shell 208 of
the helmet
202, and a second or lower end of each tether is attached to and wrapped
around
spools 210 that are releasably mounted to the shoulder pad outer shells 212.
The
system 200 further comprises pre-tensioned springs 214 that are attached at
one
end to a spool 210 and attached at the other end to the shoulder pad outer
shell 212.
In addition, the system 200 includes an impact sensor 216, such as an
accelerometer, that is mounted to the helmet 202 or the wearer's head. The
impact
sensor 216 is in communication with a central controller 218 that is adapted
to
actuate the spools 210.
During use of the system 200, the spools 210 are free to rotate to enable
lengthening of the tether 206 to enable turning of the head until an impact
that
exceeds a force threshold is sensed by the sensor 216. When such an impact
occurs, the central controller 218 activates actuation mechanisms (not shown)
associated with each spool 210 that halt further rotation of the spools and
decouple
the spools from the shoulder pads 204. When this occurs, the tethers 206 will
no
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longer lengthen and the springs 214 will pull down on the spools 210 to remove
slack
from the tethers.
Fig. 13 illustrates a second embodiment of an active helmet tether system 220
that links a protective helmet 222 to an article 224 (shoulder pads) worn by
the
helmet wearer. The system 220 includes multiple inelastic tethers 226 having a
first
or upper end attached to the outer shell 228 of the helmet 222, and a second
or
lower end attached to and wrapped around spools 230 that are fixedly mounted
to
the shoulder pad outer shells 232.
The system 220 further comprises multiple sensors 234, such as
accelerometers, that are mounted at multiple points on the helmet wearer's
body
(multiple locations of the shoulder pads 224 in the example of Fig. 13). The
data
collected by the sensors 234 can be provided to a central controller 236 that
executes a control algorithm that determines from wearer's body posture and
motion
that a helmet impact is likely to occur. In such a case, the central
controller 236 can
activate pre-tensioning mechanisms (not shown) associated with each spool 232
that
wind the tethers 226 onto the spools 230 to prepare the head for an impending
impact. In some embodiments, the control algorithm comprises a heuristic
algorithm
that adapts to the individual helmet wearer over time. In the case of a sports
helmet,
data from both practice and live game play can be used to refine the heuristic
algorithm. In some embodiments, the pre-tensioning mechanisms can comprise
electro-active materials used to form the tethers 226, such as dielectric
elastomers.
Signaling of such electro-active tethers could be used to induce changes in
stiffness
and strain.
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