Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BLAST/IMPACT FREQUENCY TUNING AND MITIGATION
[0001] Continue to [0002].
[0002] Continue to [0003].
FIELD
[0003] The present disclosure relates to a novel concept for the
design of structures
to protect against blast and impact.
BACKGROUND AND SUMMARY
[0004] This section provides background information related to the
present disclosure
which is not necessarily prior art. This section provides a general summary of
the disclosure,
and is not a comprehensive disclosure of its full scope or all of its
features.
[0005] A design strategy for a composite material, and an exemplary embodiment
of
that design, is presented that optimally and repeatedly dissipates energy
transmitted through
a composite as a result of an impact event. The design strategy, according to
the principles
of the present teachings, uses one or more elastic layers to modulate the
frequency content
of the stress wave traveling through the composite, and a viscoelastic layer
to dissipate
energy at that frequency. Our current experimental and computational results
demonstrate
that this design efficiently mitigates the pressure and dissipates the energy
transmitted
through the composite.
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[0006]
In some embodiments of the present teachings, a composite structure
consisting of lightweight elastic and viscoelastic components chosen and
configured to
optimally reduce the impulse, while simultaneously mitigating the force
(pressure)
transmitted through the composite material from an impact load, is provided
and is
generally referred to as the MITIGATIUMTm design. The innovation of the
approach
that led to the development of this MITIGATIUMTm design rubric is that it
recognizes
that a highly dissipative material alone is generally not going to be useful
in impact
loadings. Rather, optimal, repeated dissipation can be obtained only by means
of a
layered composite in which the dissipative component is matched to the other
components based on specific relationships among their respective mechanical
properties.
[0007]
According to the principles of the present teachings, the properties of the
elastic and viscoelastic components, and their placement within the layered
system, are
optimally chosen to achieve three outcomes: 1) attenuate the pressure
transmitted
through the composite; 2) modulate the frequency content of the stress waves
within
the composite layers so that 3) the energy imparted by the impulse is
efficiently
dissipated as it is transmitted through the composite. The synergistic nature
of
MITIGATIUMTm arises because it couples the dissipative component to other
component(s) specifically chosen to tune the stress wave traveling through the
elastic
materials to a frequency at which it can most efficiently be dissipated by the
viscous
response of the dissipative layer. Thus the innovation has little to do with
the actual
materials chosen for this demonstration of MITIGATIUMTm, but instead lies with
the
concept of tuning and with the method to choose the specific combination of
material
properties required for a given application. In theory there is no limit to
the number of
combinations of elastic and viscoelastic materials that can satisfy the
MITIGATIUMTm
design rubric. However, the design would need to be tailored to different
applications.
[0008]
Further areas of applicability will become apparent from the description
provided herein. The description and specific examples in this summary are
intended
for purposes of illustration only and are not intended to limit the scope of
the present
disclosure.
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DRAWINGS
[0009] The drawings described herein are for illustrative purposes
only of
selected embodiments and not all possible implementations, and are not
intended to
limit the scope of the present disclosure.
[0010] FIG. 1 illustrates a multi-layer tuning and mitigation system
according to
the principles of the present teachings having a single layer tuning layer
assembly and
a single layer dissipative layer assembly configuration;
[0011] FIG. 2 is a graph illustrating the kinetic energy (KE)
dissipation results of
the multi-layer tuning and mitigation system of FIG. 1 for various
viscoelastic materials;
[0012] FIG. 3 illustrates a multi-layer tuning and mitigation system
according to
the principles of the present teachings having a single layer tuning layer
assembly and
a multi-layer dissipative layer assembly configuration;
[0013] FIG. 4 is a graph illustrating the kinetic energy (KE)
dissipation results of
the multi-layer tuning and mitigation system of FIG. 3 for various
viscoelastic materials;
[0014] FIG. 5 illustrates the model geometry for indenter impact
simulations;
[0015] FIG. 6A illustrates the model geometry of a convention helmet
design;
[0016] FIG. 6B illustrates the model geometry of a MITIGATIUMTm helmet
design
according to the present teachings;
[0017] FIG. 6C is a graph illustrating pressure vs. time history of
oblique impact
loading;
[0018] FIGS. 7A-7C are graphs illustrating the peak pressure,
translational
acceleration, and rotational acceleration histories inside the brain in
conventional and
MITIGATIUMTm helmet designs.
[0019] Corresponding reference numerals indicate corresponding parts
throughout the several views of the drawings.
DETAILED DESCRIPTION
[0020] Example embodiments will now be described more fully with
reference to
the accompanying drawings.
[0021] Example embodiments are provided so that this disclosure will
be
thorough, and will fully convey the scope to those who are skilled in the art.
Numerous
specific details are set forth such as examples of specific components,
devices, and
methods, to provide a thorough understanding of embodiments of the present
disclosure. It will be apparent to those skilled in the art that specific
details need not be
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employed, that example embodiments may be embodied in many different forms and
that neither should be construed to limit the scope of the disclosure. In some
example
embodiments, well-known processes, well-known device structures, and well-
known
technologies are not described in detail.
[0022] The
terminology used herein is for the purpose of describing particular
example embodiments only and is not intended to be limiting. As used herein,
the
singular forms "a," "an," and "the" may be intended to include the plural
forms as well,
unless the context clearly indicates otherwise. The terms "comprises,"
"comprising,"
"including," and "having," are inclusive and therefore specify the presence of
stated
features, integers, steps, operations, elements, and/or components, but do not
preclude
the presence or addition of one or more other features, integers, steps,
operations,
elements, components, and/or groups thereof. The method steps, processes, and
operations described herein are not to be construed as necessarily requiring
their
performance in the particular order discussed or illustrated, unless
specifically identified
as an order of performance. It is also to be understood that additional or
alternative
steps may be employed.
[0023]
When an element or layer is referred to as being "on," "engaged to,"
"connected to," or "coupled to" another element or layer, it may be directly
on, engaged,
connected or coupled to the other element or layer, or intervening elements or
layers
may be present. In contrast, when an element is referred to as being "directly
on,"
"directly engaged to," "directly connected to," or "directly coupled to"
another element or
layer, there may be no intervening elements or layers present. Other words
used to
describe the relationship between elements should be interpreted in a like
fashion (e.g.,
"between" versus "directly between," "adjacent" versus "directly adjacent,"
etc.). As
used herein, the term "and/or" includes any and all combinations of one or
more of the
associated listed items.
[0024]
Although the terms first, second, third, etc. may be used herein to
describe various elements, components, regions, layers and/or sections, these
elements, components, regions, layers and/or sections should not be limited by
these
terms. These terms may be only used to distinguish one element, component,
region,
layer or section from another region, layer or section. Terms such as "first,"
"second,"
and other numerical terms when used herein do not imply a sequence or order
unless
clearly indicated by the context. Thus, a first element, component, region,
layer or
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section discussed below could be termed a second element, component, region,
layer
or section without departing from the teachings of the example embodiments.
[0025]
Spatially relative terms, such as "inner," "outer," "beneath," "below,"
"lower," "above," "upper," and the like, may be used herein for ease of
description to
describe one element or feature's relationship to another element(s) or
feature(s) as
illustrated in the figures. Spatially relative terms may be intended to
encompass
different orientations of the device in use or operation in addition to the
orientation
depicted in the figures. For example, if the device in the figures is turned
over, elements
described as "below" or "beneath" other elements or features would then be
oriented
"above" the other elements or features. Thus, the example term "below" can
encompass both an orientation of above and below. The device may be otherwise
oriented (rotated 90 degrees or at other orientations) and the spatially
relative
descriptors used herein interpreted accordingly.
[0026] INTRODUCTION
[0027] At the
outset, it is anticipated that the present invention will find utility in a
wide range of applications, including, but not limited to, vehicle armor,
personal armor,
blast protection, impact protection, vests, helmets, body guards (including
chest
protection, shin protection, hip protection, rib protection, elbow protection,
knee
protection, running shoes), firing range protection, building protection,
packaging of
appliances and devices, and the like. It should be appreciated that the
present
teachings are applicable to any blast and/or impact situation.
[0028]
According to the principles of the present teachings, as illustrated in the
figures, a multi-layer tuning and mitigation system 10 is provided for blast
and/or impact
mitigation. In some embodiments, the multi-layer tuning and mitigation system
10
comprises a tuning layer assembly 12 and a dissipative layer assembly 14. In
some
embodiments, the tuning layer assembly 12 can comprise one or more individual
elastic
layers having an acoustic impedance. Similarly, dissipative layer assembly 14
can
comprise one or more individual viscoelastic layers. As a result of an impact,
a stress
wave is produced whose frequencies entering the dissipative layer assembly 14
are
determined by the mechanical and physical properties (e.g. acoustic impedance)
of the
tuning layer assembly 12 and the geometry and nature of the impact event
itself.
[0029]
The dissipative layer assembly 14 is chosen to be complementary to the
tuning layer assembly 12 to tune the frequencies of the stress waves into a
range that
is damped by the dissipative layer assembly 14. The damping frequencies
required for
5
the dissipative layer assembly 14. The damping frequencies required for the
dissipative
layer assembly 14 are application specific; that is, they depend upon the
impact event itself
as well as on the shape and size of the impact mitigating structure itself.
[0030]
With particular reference to FIGS. 1 and 2, in some embodiments, multi-
layer
tuning and mitigation system 10 can comprise a single-layer tuning layer
assembly 12 and
a single-layer dissipative layer assembly 14. In this way, single-layer tuning
layer assembly
12 is an elastic material that is sufficient to work with single-layer
dissipative layer assembly
14 to tune the frequencies of the stress waves of the impact. Single-layer
dissipative layer
assembly 14 is a viscoelastic material selected to mitigate the resulting
tuned frequencies
of the stress wave to dissipate the kinetic energy. As illustrated in FIG. 2
and described
herein, the viscoelastic material is selected based on the particular tuned
frequencies,
wherein, for example, viscoelastic material V1 is sufficient to dissipate
approximately 77%
of the kinetic energy (KE) of the tuned frequencies, V2 is sufficient to
dissipate approximately
95% of the kinetic energy (KE) of the tuned frequencies, and V3 is sufficient
to dissipate
approximately 96% of the kinetic energy (KE) of the tuned frequencies. FIG. 2
was
generated in response to an indenter impacting the structure of FIG. 1 with a
kinetic energy
of approximately 10 J. In this embodiment, the tuning layer assembly 12 is a
thin elastic
material and dissipative layer assembly 14 is a thicker viscoelastic material.
The dominant
frequencies that enter the second layer, namely the dissipative layer assembly
14, in this
example are in the range of 0.01 ¨100 Hz (approximately).
[0031]
With particular reference to FIGS. 3 and 4, in some embodiments, multi-
layer
tuning and mitigation system 10 can comprise a single-layer tuning layer
assembly 12 and
a multi-layer dissipative layer assembly 14. In this way, single-layer tuning
layer assembly
12 is an elastic material that is sufficient to work with multi-layer
dissipative layer assembly
14 to tune the frequencies of the stress waves of the impact. Multi-layer
dissipative layer
assembly 14 can comprise two or more viscoelastic materials selected to each
mitigate a
portion of the resulting tuned frequencies of the stress wave to dissipate the
kinetic energy.
In some embodiments, several layers of multi-layer dissipative layer assembly
14 can be
used to dissipate the same frequencies, different frequencies, and/or
overlapping
frequencies. For example, the single-layer tuning layer assembly 12 can work
to tune the
stress waves to a range of frequencies, and one layer of dissipative layer
assembly 14 can
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dissipate a second subrange of the frequencies. The first and second subranges
can
be different, overlapping, or the same. As illustrated in FIG. 4 and described
herein, the
viscoelastic materials of multi-layer dissipative layer assembly 14 are
selected based
on the particular tuned frequencies, wherein, for example, viscoelastic
material
composite V1 is sufficient to dissipate approximately 80% of the kinetic
energy (KE) of
the tuned frequencies, viscoelastic material composite V2 is sufficient to
dissipate
approximately 94% of the kinetic energy (KE) of the tuned frequencies, and
viscoelastic
material composite V3 is sufficient to dissipate approximately 95% of the
kinetic energy
(KE) of the tuned frequencies.
[0032] It should
also be appreciated that, in some embodiments, multi-layer
tuning and mitigation system 10 can comprise a multi-layer tuning layer
assembly 12
and a single-layer dissipative layer assembly 14, or a multi-layer tuning
layer assembly
12 and a multi-layer dissipative layer assembly 14.
[0033]
In some embodiments, tuning layer assembly 12 can be modified, thereby
varying its performance and acoustic impedance, by selecting the material,
thickness,
and, in the case of a multi-layer configuration, how and if the layers are
bonded.
Likewise, dissipative layer assembly 14 can be modified, thereby varying its
dissipative
performance, by selecting the material, thickness, and, in the case of a multi-
layer
configuration, how and if the layers are bonded. By way of non-limiting
example, in
some embodiments, tuning layer assembly 12 can be made of an elastic material,
such
as thermoplastics (e.g., polycarbonate, polyethylene), metals, ceramics,
polymers
(elastic type), composites, and biological solids (e.g. bone, ligament).
Furthermore,
dissipative layer assembly 14 can be made of viscoelastic material, such as
polymers.
It should be understood, however, that polymers may be elastic and/or
viscoelastic.
Whether they are elastic or viscoelastic in a given application depends upon
the
application temperature and the frequencies under consideration. In other
words, a
given polymer at a given temperature responds elastically to some frequencies
and
viscoelastically to other frequencies.
[0034]
The tuning layer assembly 12 is typically chosen based on other
functional requirements of the application, such as chip resistance of a
layered paint,
ballistic penetration resistance in a military armor, and protecting the skull
against
facture in a sport helmet. The acoustic impedance of the tuning layer assembly
12 is
therefore set once this choice is made (however there may be several materials
that fit
the bill). The thickness of the tuning layer assembly 12 may also be set by
these
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existing functional requirements. The mechanical and physical attributes of
the tuning
layer assembly 12 determine one of the frequencies that will be passed to the
dissipative layer assembly 14 in a tuned design. They also provide the mass of
the
tuning layer assembly 12, which together with the dissipative layer assembly
14, will
determine an additional frequency that is passed to the dissipative layer
assembly 14 in
a dynamic system (mass-spring in which the tuning layer assembly 12 is the
mass and
the dissipative layer assembly 14 is the spring). The dissipative layer
assembly 14 is
chosen to have a lower acoustic impedance than the tuning layer assembly 12,
to
provide the tuning and to mitigate the force transmitted. The elastic
properties of the
dissipative layer assembly 14 determine this impedance; optimal tuning
requires a
significant impedance reduction in layer 2 from that of layer 1. The
dissipative layer
assembly 14 may include portions that are elastic, in which it acts as the
spring in a
mass-spring dynamic system that has a characteristic frequency, or it may
include
portions that are viscoelastic to additionally damp either the tuned frequency
or the
mass-spring frequency, or both. If the dissipative layer assembly 14 is
elastic in part,
additional viscoelastic layers are required to dissipate the impulse. A
viscoelastic
dissipative layer assembly 14 is both elastic and viscous, so that it
satisfies all of the
previously described functions of the dissipative layer assembly 14 to tune
with the
tuning layer assembly 12 and vibrate with the tuning layer assembly 12 as a
mass-
spring system. In addition it is chosen to damp one or more of the
frequencies. If the
dissipative layer assembly 14 is elastic, an additional layer is chosen to
damp the
transmitted frequencies.
[0035]
For purposes of illustration, the present invention will be discussed in
connection with design of a football helmet. However, as set forth herein, the
following
should not be regarded as limiting the present invention to only the
illustrated
embodiments.
[0036] TECHNICAL APPROACH
[0037]
Strategy for head health ¨ When the head is subjected to an impulsive
force such as an impact or blast wave, there are two attributes to the event
that can
lead to damage in the brain. The first is the directly transmitted force
(corresponding
directly to the acceleration of the head). The second is the transmitted
impulse
(corresponding to the absolute change, not the rate of change, of the velocity
of the
head). It has been known, but not generally recognized, for more than 70
years, that
the damage in long duration impulses depends on the peak force, while the
damage in
8
=
short duration impulses depends on the magnitude of the impulse. To limit the
force in the
design of a helmet, one can utilize elastic impedance mismatch to reduce the
force, and
energy dissipation to reduce the impulse. Our design strategy is unique in
that it specifically
targets both in a deliberate, rather than incidental, fashion.
[0038]
Description of the material ¨ The technical approach is a strategy to design a
composite material for the optimal mitigation of an impulse using elastic and
viscoelastic solids. Additional reference should be made to PCT Application
Serial
No. PCT/US2014/065658 entitled: "Blast/Impact Frequency Tuning and
Mitigation".
[0039]
A sport's (football) helmet is chosen as a design example. Current
helmet
designs have other functions, such as preventing skull fracture; therefore we
chose materials
for the present demonstration that are similar to those currently used. The
outer shell of a
football helmet is often a thermoplastic, such as polycarbonate (PC),
therefore we limited
our choice of outer shell layer to similar polymers. These materials do not
plastically deform
under the impact loadings seen in sports. Therefore, they respond as linear
elastic solids.
Mitigating the force transmitted through elastic materials is easily
accomplished by an
impedance mismatch approach. Current helmets utilize this strategy effectively
by coupling
the first, high elastic impedance layer to a second, low-elastic-impedance
layer. We chose
an elastic material for the second layer having elastic impedance much lower
than that of
the first layer to preserve the force mitigating properties of existing
helmets, and to provide
the tuning that is at the heart of our design. A vinyl foam serves this
purpose in our design.
Elastic materials do not dissipate any of the energy associated with an
impact; therefore, a
strategy that focuses on reducing the force of an impact through elastic
impedance
mismatch does nothing to mitigate the impulse. Stated another way, this
strategy does not
dissipate the energy of the impact. A third or dissipative, viscoelastic
layer, can dissipate
energy; the optimal choice for the dissipative properties of the third layer
depends on the
properties of the first two layers.
[0040]
We limited the selection of the dissipative third material layer to
viscoelastic
materials because the design must be capable of dissipating the same amount of
energy
every time the helmet is impacted. Plastically deforming materials and
materials that
fracture, delaminate, craze, and/or crack upon an initial impact will not be
effective in
dissipating energy upon subsequent impacts of equal intensity. A linear
viscoelastic material
can dissipate energy repeatedly. However, it is most
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effective at dissipating energy at one specific frequency: this critical
frequency (fcR/T) is
a function of its unrelaxed and relaxed moduli and its characteristic
relaxation time. In
an impact, the stress wave transmitted to a solid material contains a broad
spectrum of
energy, therefore, this same viscoelastic material acting alone will not be
effective in
dissipating impact energy.
[0041]
Our novel solution to optimizing viscoelastic dissipation is to tune the
stress wave that enters the viscoelastic material to a frequency that matches
f GRIT and
effectively damp that frequency. The first one or two layers of the composite
in
MITIGATIUMTm modulate the stress wave to a frequency that is dependent upon
their
elastic, physical, and geometric properties in addition to mitigating the
magnitude of the
stress wave. Thus both the force (or stress) magnitude and the impulse
transmitted are
reduced using the MITIGATIUMTm approach. A fourth layer of comfort foam is
optionally
used in the design because it serves important functions in current helmet
designs. In
addition to providing comfort to the wearer, it enables an adjustable fit.
[0042] Data
supporting energy dissipation ¨ Impact experiments have been
conducted on MITIGATIUMTm and on an existing helmet design and determined that
MITIGATIUMTm results in a significantly lower peak acceleration than the
existing
helmet does. We have compared these experimental results to computational
analyses
to validate our computational models of impact loading and stress wave
propagation.
We also conducted one- and two-dimensional computational analyses of a
MITIGATIUMTm helmet design and an existing helmet design on a skull/brain
system to
demonstrate the energy dissipating capabilities of MITIGATIUMTm.
Our results
demonstrate that the MITIGATIUMTm helmet reduces the pressure and impulse
transmitted to the skull and hence, the brain, and MITIGATIUMTm also reduces
translational and rotational accelerations within the brain compared to those
of an
existing helmet design.
[0043]
Impact measurements ¨ A MITIGATIUMTm prototype specimen was built
as follows: layer 1, 2.4 mm thick PE (McMaster Carr); layer 2, 12.7 mm thick
vinyl nitrile
(Grainger); layer 3, 14.3 mm thick polyurethane (PU, McMaster Carr, actually
three 4.1
mm layers of PU stacked together); layer 4, 12.7 mm thick soft "comfort" foam
(McMaster Carr). The overall dimensions of the MITIGATIUMTm specimen were 305
mm X 305 mm X 42 mm ["MITIGATIUMTm unbounded]. A test specimen based on an
existing helmet design was also built. It consisted of PC (3.2 mm thick,
McMaster Carr),
vinyl (25.4 mm thick, Grainger), and soft foam (12.7 mm thick, McMaster Carr)
layers
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such that its overall size was 305 mm X 305 mm X 41 mm ['Current unbounded].
Duplicate sets of each specimen type were built and these layers were bonded
together
using a spray-on adhesive (3M Super 77) ['MITIGATIUMTm bonded" and "Current
bonded]. A cylindrical steel indenter (2.8 kg, 7.5 cm diameter, 7.5 cm length,
McMater
Carr) was used to impact each specimen. The indenter was dropped from a height
of
72 cm (20 J) using a quick release and the position vs. time of the indenter
was filmed
via a high-speed digital video camera (Optotrak Certus) at a rate of 400
images/s.
Each sample type was indented five times.
[0044]
The derivative of the position vs. time data was computed using a 5-point
centered finite differencing method to obtain velocity vs. time data. The
derivative of the
velocity vs. time data was similarly computed to obtain acceleration vs. time
data. The
peak acceleration of the indenter was determined for each sample type and the
results
appear in Table 1. The peak accelerations of the indenter during impact of the
bonded
specimens exceeded those of the unbonded specimens for both MITIGATIUMTm and
Current samples. The peak accelerations of the indenter during impact of the
two
"Current" samples exceeded those of the MITIGATIUMTm samples for both bonded
and
unbonded cases. Therefore, the lowest peak indenter acceleration was that
impacting
the unbonded MITIGATIUMTm sample. As described herein, the acceleration of the
head in an impact is directly proportional to the peak force transmitted
through a helmet
to the skull. The impact experiments performed here are not a direct
indication of the
force transmitted through the samples, but the acceleration of the indenter
serves as a
proxy for the skull and provides an indication of the force mitigating
response of the
samples. Therefore, these results indicate the MITIGATIUMTm sample is a better
attenuator of force than the current helmet design is, and unbounded layers
attenuate
force better than bonded layers.
Specimen Peak Acceleration SD
[m/s2]
MITIGATIUM'm unbonded 519 22
Current unbonded 689 43
MITIGATIUM'm bonded 599 15
Current bonded 696 27 [0045]
Indenter
Table 1: Peak acceleration experimental results. 30
Impact simulations ¨ The
experimental indenter impact procedure was replicated computationally using
the same
geometries for the specimens and indenter as in the experiments, and the
mechanical
and material properties for the layers in Table 2. Simulations assumed all
layers in the
samples were bonded (to avoid prescribing frictional contact properties) but
no bonded
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layers existed; nodes from layer one were tied to nodes of layer two, et
cetera. Thus the
effect of the mechanical properties of the adhesive layers in the experiments
is not
examined in these computational simulations. The commercial finite-element
package
ABAQUS Explicit was used for the simulations. The computational model geometry
appears in FIG. 5. The indenter was given an initial velocity of 3.7 m/s
corresponding to
the velocity of a 2.8 kg indenter dropped from a height of 72 cm, in
accordance with the
experiments. A body force of 79,000 kg/m2s2 (density * gravity) was also
applied to the
indenter to account for the gravitational force. The maximum indenter
accelerations
determined from these analyses are: MIGATIUMTm bonded, 550 m/s2; Current
bonded,
700 m/s2. The computational results are within 10% of the mean experimental
values
for the peak accelerations given in Table 1. These results replicate what was
determined experimentally, namely, MITIGATIUMTm is a better force attenuator
than the
Current helmet design. These computational results provide reasonable
confidence that
we can explore the impact response of various helmet designs in transmission
to
predict the force and impulse mitigation properties, and therefore injury
preventative
responses, of the current MITIGATIUMTm embodiment, or of an optimal
embodiment,
vs. current helmet
designs.
Young's Unrelaxed Relaxed
Poisson's Density
Characteristic
Modulus Modulus Modulus
[MPa]
Ratio [kg/m3 [MPa] [MPa]
] Time
[seconds]
PE 755 0.35 950
Vinyl 0.16 0.1 130
PU 0.4 1200 100 0.2 3.5E-8
Foam 80 1.0 0.052 1.9E-8
PC 2200 0.35 1175
Table 2: Mechanical and physical properties of layers used in computational
analysis of impact.
[0046]
One-dimensional analysis of transmission through elastic and viscoelastic
layers ¨ The mechanics of impact wave transmission through layers of elastic
and
viscoelastic materials, such as those found in existing football helmets, were
analyzed
and the MITIGATIUMTm design was developed for a new sports helmet comprised of
layers that can optimally dissipate impact energy. Our results demonstrate
that an
existing helmet design may reduce the over-pressure transmitted to the skull
on the
interior of the helmet by an order of magnitude over that delivered by the
impact to the
external surface of the helmet, but it has no effect on the impulse
transmitted.
[0047]
The new MITIGATIUMTm design paradigm cannot only further reduce the
over-pressure by an additional order of magnitude over existing approaches, it
can also
reduce the impulse delivered to the brain by an order of magnitude.
12
[0048]
This is accomplished by a viscoelastic layer chosen to match the tuning
induced by the other one or two layers. Linear viscoelastic materials
dissipate energy at
specific frequencies and do so repeatedly. It should again be emphasized that
an arbitrary
impact to a helmet will not result in a stress wave with an optimal frequency
distribution to
be dissipated, whether these be designs with monolithic materials or fluid-
filled or air-gap
designs. All of these designs, like the viscoelastic design, will dissipate
energy optimally at
specific frequencies. Therefore, the optimally dissipative design concept
needs to contain
the frequency tuning aspect.
[0049]
A single- or multi-layer design allows for tuning of an arbitrary impact
into a
specific frequency that can be optimally dissipated by the viscoelastic layer.
The viscoelastic
layer, acting alone, is not effective. Our one-dimensional analysis shows that
the use of a
viscoelastic material alone, without tuning components, transmits 90% of the
impulse of an
impact event. However, when a viscoelastic material is optimally coupled to
elastic materials
that tune the stress wave to the critical damping frequency of the
viscoelastic material, less
.. than 30% of the impulse is transmitted.
[0050]
In some embodiments, this optimal MITIGATIUMTm design can comprise a
tuned frequency that is high, so the thickness of the third dissipative layer
is reduced
because of the higher tuning frequency. Therefore, this optimal MITIGATIUMTm
would be
thinner and lighter weight than current football helmets. The required
properties of the
viscoelastic material are well within any expected range of polyurethanes.
[0051] Two-dimensional analysis of impact response of helmets ¨ A MITIGATIUMTm
helmet design was compared to an existing sport helmet using two-dimensional
finite
element analyses of impact loading. The commercial finite-element package
ABAQUS
Explicit was again used for the simulations. The geometries used in the finite-
element
models are shown in FIGS. 6A and 6B. In these simulations, the head was
modeled as a
two-component system consisting of an outer rim with a material having
properties that
approximated a skull 60, and an inner region of material having properties
approximating
the brain 61. The model corresponding to an existing football helmet design
has a 4 mm
outer layer of ABS plastic 62, a 23 mm second layer of a hard foam 64, and a 9
mm inner
layer of "comfort" foam 66, as shown in FIG. 6A. The MITIGATIUMTIvi helmet in
FIG. 6B was
chosen to have the same mass and volume as the existing helmet. The 4 mm outer
shell
layer is polyethylene 68, the 20.5 mm second layer is a styrene-based elastic
foam 70, and
the 2.5 mm third layer is a viscoelastic urethane-based material 72. The
fourth layer on this
helmet is not necessary; it is included
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to match the size and weight of the existing helmet, and because the comfort
foam is
important to helmet wearers. In fact, the MITIGATIUMTm helmet design can be
made
significantly thinner and lighter than the existing helmet. Choosing equal
mass designs
normalizes the response, as the effectiveness of armor in reducing momentum
transfer
depends on mass. The helmet models were subjected to an oblique impact
pressure
load of shape and duration shown in FIG. 6C. Peak pressure and impulse
transmitted
to the skull were determined. Linear and rotational accelerations were
examined
throughout the region of the brain and peak values recorded for comparison.
The
results are shown in Table 3 and in FIGS. 7A-7C. As the table shows, the
choice of
outer layer affects the pressure, impulse, and duration of the impact imparted
to the
helmet from a given impact load. The last two columns compare the pressure and
impulse transmitted to the skull by the two helmet designs, these are
normalized by the
values transmitted by the existing helmet design. The MITIGATIUMTm helmet
transmits
less than 1% of the pressure and 31% of the impulse that the existing helmet
transmits.
It is important to appreciate that it is only in this type of geometry¨where
there is
interaction between the head and the helmet¨that the full effects of impulse
transmission be considered. Ultimately, the validation needs to be conducted
with this
type of geometry, rather than considering impulses transmitted to a massive
rigid plate.
[0052]
FIGS. 7A-7C show the peak pressure, translational acceleration, and
rotational acceleration histories inside the brain in both helmet designs. The
peak
values occur at different nodes for the various quantities recorded, and for
different
nodes in each helmet, but in every case, the highest magnitude was searched
within
the entire brain region and that is what is recorded for comparison. The
significant
reductions in the peak pressure and accelerations for the MITIGATIUMTm helmet
are
clearly seen in the figure. It is also evident from FIGS. 7A-7C that in the
existing
undamped helmet, a single-impact loading-event results in multiple peak-
acceleration
events.
Po [MPa] I t0 [ms] Ptrans 'trans
[Pa-sec] 'existing existing
Existing helmet 1.3 5400 8 1 1
Optimal 0.54 3400 12 0.0012 0.31
MITIGATIUMTm
helmet
Table 3: Pressure transmitted, impulse transmitted, and duration of
transmission
for an existing helmet design vs. the MITIGATIUMTm design.
[0053]
The foregoing description of the embodiments has been provided for
purposes of illustration and description. It is not intended to be exhaustive
or to limit the
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WO 2016/205380 PCT/1JS2016/037645
disclosure. Individual elements or features of a particular embodiment are
generally not
limited to that particular embodiment, but, where applicable, are
interchangeable and
can be used in a selected embodiment, even if not specifically shown or
described. The
same may also be varied in many ways. Such variations are not to be regarded
as a
departure from the disclosure, and all such modifications are intended to be
included
within the scope of the disclosure.
