U.S. patent application number 15/549145 was filed with the patent office on 2018-02-01 for an impact absorbing structure and a helmet comprising such a structure.
The applicant listed for this patent is OXFORD UNIVERSITY INNOVATION LIMITED. Invention is credited to James COOK.
Application Number | 20180027914 15/549145 |
Document ID | / |
Family ID | 52705737 |
Filed Date | 2018-02-01 |
United States Patent
Application |
20180027914 |
Kind Code |
A1 |
COOK; James |
February 1, 2018 |
AN IMPACT ABSORBING STRUCTURE AND A HELMET COMPRISING SUCH A
STRUCTURE
Abstract
An impact absorbing structure comprises a unitary material
formed as a stretch-dominated hollow cell structure and a helmet
comprising such a structure as an inner impact resistant liner.
Inventors: |
COOK; James; (Oxford,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
OXFORD UNIVERSITY INNOVATION LIMITED |
Oxford |
|
GB |
|
|
Family ID: |
52705737 |
Appl. No.: |
15/549145 |
Filed: |
February 4, 2016 |
PCT Filed: |
February 4, 2016 |
PCT NO: |
PCT/IB2016/050587 |
371 Date: |
August 4, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A42B 3/12 20130101; A42B
3/124 20130101; A42B 3/283 20130101 |
International
Class: |
A42B 3/12 20060101
A42B003/12; A42B 3/28 20060101 A42B003/28 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 4, 2015 |
GB |
1501834.4 |
Claims
1. An impact absorbing structure, comprising a unitary material
formed as a stretch-dominated hollow cell structure wherein at
least a plurality of cells are configured to tessellate with a cell
axis normal to the surface or out-of-plane and the unitary material
has a relative density substantially between 0.05 and 0.15.
2. An impact absorbing structure as claimed in claim 1 wherein
substantially all the cells of the hollow cell structure are 2D
hollow-cells.
3. An impact absorbing structure as claimed in claim 2 wherein
substantially all the cells are aligned substantially out of
plane.
4. An impact absorbing structure as claimed in claim 1 wherein the
cells are formed as a micro-truss lattice.
5. An impact absorbing structure as claimed in claim 1 wherein the
cells are formed as a crystal lattice structure.
6. (canceled)
7. (canceled)
8. An impact absorbing structure as claimed in claim 1 wherein at
least a plurality of the cells are hexagonal.
9. An impact absorbing structure as claimed in claim 1 wherein at
least a plurality of the cells are triangular.
10. An impact absorbing structure as claimed in claim 1 wherein at
least a plurality of the cells are square.
11. An impact absorbing structure as claimed in claim 1 wherein at
least a plurality of the cells are a combination of octagons and
squares co-located in a tessellating pattern.
12. (canceled)
13. (canceled)
14. An impact absorbing structure as claimed in claim 1 wherein the
ratio of cell wall thickness to cell length is significantly
small.
15. An impact absorbing structure as claimed in claim 14 wherein
the wall has a maximum thickness of substantially 1 mm.
16. An impact absorbing structure as claimed in claim 1 wherein the
unitary material is a polymer material.
17. An impact absorbing structure as claimed in claim 16 wherein
the unitary material is an elastomer.
18. An impact absorbing structure as claimed in claim 16 wherein
the unitary material is elastic-plastic and elastic-brittle.
19. An impact absorbing structure as claimed in claim 16 wherein
the unitary material is selected from the group consisting of Nylon
11 and ST Elastomer.
20. (canceled)
21. (canceled)
22. A helmet, comprising an inner impact resistant liner at least
partly formed from an impact absorbing structure as claimed in
claim 1.
23. A helmet as claimed in claim 22 further comprising an outer
shell formed to substantially cover the inner impact resistant
liner.
24. A helmet as claimed in claim 22 further comprising an outer
shell formed to substantially cover the inner impact resistant
liner wherein the outer shell is at least partly formed from a
composite material.
25. A helmet as claimed in claim 22 further comprising an outer
shell formed to substantially cover the inner impact resistant
liner wherein the outer shell is at least partly formed from a
thermoplastic material.
26. A helmet as claimed in claim 22 further comprising an outer
shell formed to substantially cover the inner impact resistant
liner wherein at least one vent slot is formed in the outer
shell.
27-50. (canceled)
Description
FIELD
[0001] The present invention relates to an impact absorbing
structure. More particularly, the present invention relates to a
hollow-cell impact absorbing structure. Even more particularly, the
present invention relates to an impact absorbing structure formed
as a stretch-dominated hollow-cell structure. The present invention
also relates to impact absorbing structures where the impact
surface is curved, such as a sports helmet or aerospace nose
bumpers, at least part of the structure formed from a hollow-cell
impact absorbing structure, and even more particularly a
stretch-dominated hollow-cell impact absorbing structure.
BACKGROUND
[0002] Injury to a person or damage to an object can occur when the
person or object is subjected to an impact of sufficient force.
Considerable developmental effort has been expended in order to
produce materials and structures that provide protection from
potentially damaging or injurious impacts.
[0003] Impact protection is particularly important for preventing
head injury. A blow to the head can result in severe traumatic
brain injury (TBI). It is common for brain trauma to occur as a
consequence of either a focal impact upon the head, or by a sudden
acceleration/deceleration within the cranium, or from a combination
of both impact and movement. Traumatic brain injury can cause
long-term issues, and there are limited treatment options.
[0004] One of the most common causes of head injury is
participation in sports. For example, a fall from a bicycle when
riding may result in the head striking against a solid unyielding
object or surface such as a road surface or similar. In order to
help prevent injury, helmet usage is customary or mandatory in many
sports such as bicycle, motorcycle and horse riding, rock climbing,
American football and also winter or ice sports such as skating,
ice hockey, and skiing. Another common cause of head injury is an
impact caused by a falling object on a building or construction
site.
[0005] Sports helmets and safety helmets are individually designed
so as to be particularly suited to their particular use. However,
most or all of the helmets have common design elements such as a
hard outer shell (formed from a stiff thermoplastic or composite)
and a lining/liner, softer than the outer shell, but still stiff
enough to retain it's shape when unsupported. In combination, the
shell and liner act to absorb the force of an impact and to help
prevent this force being transmitted to the head and brain.
Virtually all helmets use expanded polystyrene as the energy
absorbing liner. The expanded polystyrene is formed as a unitary
structure (that is, without gaps) in the required shape.
[0006] U.S. Pat. No. 3,447,163 describes and shows a safety or
crash helmet intended for use by motorcyclists and/or racing
motorists. The helmet has an outer shell formed as a double-skinned
member, the two skins of the shell joined to one another around the
periphery of the shell by a gently curved peripheral portion that
has no sharp edges, and the space between the skins contains a
layer of a honeycomb type of material, the cells of the honeycomb
layer filled with an energy-absorbing foamed material.
[0007] U.S. Pat. No. 7,089,602 describes and shows an impact
absorbing, modular helmet having layers on the outer side of a hard
casing that increase the time of impact with the intention of
reducing the intensity of the impact forces. The layers are made up
of a uniformly consistent impact absorbing polymer material, a
polymer layer filled with air or a polymer structure. These
impact-absorbing layers can also be made and used as an
independent, detachable, external protective cover that can be
attached over a hard casing helmet.
[0008] U.S. Pat. No. 6,247,186 describes and shows a helmet having
a housing, an inner impact resistant layer shaped to the head of
rider, a protective covering spaced above and formed integrally
with the housing, and a chamber enclosed by the housing and
protective covering that is open in the front for ventilation. The
chamber has a net strap in the front side for preventing foreign
objects from entering and one or more inner channels in
communication with the inner space of helmet through a passageway.
In use, fresh air flows through the passageway and into the impact
resistant layer.
[0009] Sports helmets and safety helmets often have to be worn for
extended periods, and the weight of the helmet is an important
design consideration. When designing a helmet, there will generally
be a trade-off between the overall weight (and shape and size) of
the helmet, and the impact-absorbing properties. Increasing the
amount of impact-absorbing material will increase the overall
weight of the helmet, and may also result in an increase in the
external dimensions, which can in turn make wearing the helmet
relatively more unwieldy and uncomfortable to wear, especially
where aerodynamic considerations may also be important. Conversely,
impact protection can be compromised if the helmet has too little
impact-absorbing material.
[0010] Foams such as the foams used in helmets are typically
excellent energy absorbers because they are characterised by a long
plateau stress, and in most impacts the area is constant so the
stress can be directly converted to force, providing a long plateau
force. This means all the energy can be absorbed whilst maintaining
a low peak force and acceleration, optimal in reducing brain
damage. However, in an oval shape helmet, the area when crushing is
not constant.
[0011] In this specification where reference has been made to
patent specifications, other external documents, or other sources
of information, this is generally for the purpose of providing a
context for discussing the features of the invention. Unless
specifically stated otherwise, reference to such external documents
is not to be construed as an admission that such documents, or such
sources of information, in any jurisdiction, are prior art, or form
part of the common general knowledge in the art.
SUMMARY
[0012] It is an object of the present invention to provide a range
of optimised impact absorbing structures for improved impact
absorption, or which at least provide the public or industry with a
useful choice. It is a further object of the invention to provide a
range of optimised impact absorbing structures that can be used to
reduce traumatic brain injury by reducing peak acceleration and
force to the brain and directing energy away from vulnerable areas
of the brain, or which at least provide the public or industry with
a useful choice. It is a yet still further object of the invention
to provide a helmet at least partly formed from an optimised impact
absorbing structure that assists with reducing traumatic brain
injury by reducing peak acceleration and force to the brain and
directing energy away from vulnerable areas of the brain, or which
at least provides the public or industry with a useful choice. It
is a yet still further object of the present invention to provide a
method of optimising an impact absorbing structure for improved
impact absorption.
[0013] The term "comprising" as used in this specification and
indicative independent claims means "consisting at least in part
of", and is intended as an inclusive rather than exclusive term.
When interpreting each statement in this specification and
indicative independent claims that includes the term "comprising",
features other than that or those prefaced by the term may also be
present. Related terms such as "comprise" and "comprises" are to be
interpreted in the same manner.
[0014] As used herein the term "and/or" means "and" or "or", or
both.
[0015] As used herein "(s)" following a noun means the plural
and/or singular forms of the noun.
[0016] Accordingly, in a first aspect the present invention may
broadly be said to consist in an impact absorbing structure,
comprising a unitary material formed as a stretch-dominated hollow
cell structure.
[0017] In an embodiment, substantially all the cells of the hollow
cell structure are 2D hollow-cells.
[0018] In an embodiment, substantially all the cells are aligned
substantially out of plane.
[0019] In an embodiment, the cells are formed as a micro-truss
lattice.
[0020] In an embodiment, the cells are formed as a crystal lattice
structure.
[0021] In an embodiment, at least a plurality of the cells are
configured to tessellate.
[0022] In an embodiment, at least a plurality of the cells are
configured to tessellate with a cell axis normal to the surface or
out-of-plane.
[0023] In an embodiment, at least a plurality of the cells are
hexagonal.
[0024] In an embodiment, at least a plurality of the cells are
triangular.
[0025] In an embodiment, at least a plurality of the cells are
square.
[0026] In an embodiment, at least a plurality of the cells are a
combination of octagons and squares co-located in a tessellating
pattern.
[0027] In an embodiment, the unitary material is formed to have a
relative density substantially between 0.05 and 0.15.
[0028] In an embodiment, the cell shape, size, cell wall thickness,
cell width and cell length can be freely varied relative to one
another.
[0029] In an embodiment, the ratio of cell wall thickness to cell
length is significantly small.
[0030] In an embodiment, the wall has a maximum thickness of
substantially 1 mm.
[0031] In an embodiment, the unitary material is a polymer
material.
[0032] In an embodiment, the unitary material is an elastomer.
[0033] In an embodiment, the unitary material is elastic-plastic
and elastic-brittle.
[0034] In an embodiment, the unitary material is Nylon 11.
[0035] In an embodiment, the unitary material is ST Elastomer.
[0036] In an embodiment, the hollow cell structure is manufactured
by Laser Sintering.
[0037] In a second aspect, the present invention may broadly be
said to consist in a helmet, comprising an inner impact resistant
liner at least partly formed form an impact absorbing structure as
claimed in any one of the preceding statements.
[0038] In an embodiment, the helmet further comprises an outer
shell formed to substantially cover the inner impact resistant
liner.
[0039] In an embodiment, the outer shell is at least partly formed
from a composite material.
[0040] In an embodiment, the outer shell is at least partly formed
from a thermoplastic material.
[0041] In an embodiment, at least one vent slot is formed in the
outer shell.
[0042] In a third aspect, the invention may broadly be said to
consist in a method of optimising an impact absorbing structure for
improved impact absorption, comprising the steps of: [0043] (i)
choosing a material; [0044] (ii) forming the material into a
stretch-dominated hollow cell structure.
[0045] In an embodiment of the method, substantially all the cells
of the hollow cell structure are formed as 2D hollow-cells.
[0046] In an embodiment of the method, substantially all the cells
are formed so as to be aligned substantially out of plane.
[0047] In an embodiment of the method, the cells are formed as a
micro-truss lattice.
[0048] In an embodiment of the method, the cells are formed as a
crystal lattice structure.
[0049] In an embodiment of the method, at least a plurality of the
cells are formed so as to tessellate.
[0050] In an embodiment of the method, at least a plurality of the
cells are formed so as to tessellate with a cell axis normal to the
surface or out-of-plane.
[0051] In an embodiment of the method, the hollow cells are formed
to have a topology that propagates radially to a curved
surface.
[0052] In an embodiment of the method, at least a plurality of the
cells are formed as hexagons.
[0053] In an embodiment of the method, at least a plurality of the
cells are formed as triangles.
[0054] In an embodiment of the method, at least a plurality of the
cells are formed as squares.
[0055] In an embodiment of the method, at least a plurality of the
cells are formed as a combination of octagons and squares
co-located in a tessellating pattern.
[0056] In an embodiment of the method, the material is formed in
such a manner that the material has a relative density
substantially between 0.05 and 0.15.
[0057] In an embodiment of the method, the cells are formed so that
the cell shape, size, cell wall thickness, cell width and cell
length can be freely varied relative to one another.
[0058] In an embodiment of the method, the cells are formed so that
the ratio of cell wall thickness to cell length is significantly
small.
[0059] In an embodiment of the method, the cells are formed so that
the wall has a maximum thickness of substantially 1 mm.
[0060] In an embodiment of the method, the unitary material is a
polymer material.
[0061] In an embodiment of the method, the unitary material is an
elastomer.
[0062] In an embodiment of the method, the unitary material is
elastic-plastic and elastic-brittle.
[0063] In an embodiment of the method, the unitary material is
Nylon 11.
[0064] In an embodiment of the method, the unitary material is ST
Elastomer.
[0065] In an embodiment of the method, the hollow cell structure is
manufactured by Laser Sintering.
[0066] With respect to the above description then, it is to be
realised that the optimum dimensional relationships for the parts
of the invention, to include variations in size, materials, shape,
form, function and manner of operation, assembly and use, are
deemed readily apparent and obvious to one skilled in the art, and
all equivalent relationships to those illustrated in the drawings
and described in the specification are intended to be encompassed
by the present invention.
[0067] This invention may also be said broadly to consist in the
parts, elements and features referred to or indicated in the
specification of the application, individually or collectively, and
any or all combinations of any two or more said parts, elements or
features, and where specific integers are mentioned herein which
have known equivalents in the art to which this invention relates,
such known equivalents are deemed to be incorporated herein as if
individually set forth.
[0068] Therefore, the foregoing is considered as illustrative only
of the principles of the invention. Further, since numerous
modifications and changes will readily occur to those skilled in
the art, it is not desired to limit the invention to the exact
construction and operation shown and described, and accordingly,
all suitable modifications and equivalents may be resorted to,
falling within the scope of the invention.
BRIEF DESCRIPTION OF THE FIGURES
[0069] Further aspects of the invention will become apparent from
the following description which is given by way of example only and
with reference to the accompanying drawings which show an
embodiment of the device by way of example, and in which:
[0070] FIGS. 1a-c show schematic views of single cells that form
part of a cellular solid, showing the joints j and struts s which
the cell shares with adjoining cells, the struts s forming
surrounding faces that enclose the cells, FIG. 1a showing a
bending-dominated structure where the joints are locked and the
frame bends as the structure is loaded, stretch-dominated
structures shown in FIGS. 1b and 1c where the members carry tension
or compression when loaded, giving higher modulus and strength.
[0071] FIGS. 2a and 2b show plots summarising the difference
between stretch and bending-dominated structures in terms of
relative modulus E/E.sub.s and relative strength
.sigma./.sigma..sub.s against relative density p/p.sub.s.
[0072] FIG. 3 shows a perspective view from above, looking
downwards and sideways, of a honeycomb hollow cell structure
according to an embodiment of the present invention.
[0073] FIG. 4 shows a top view from directly above of the hollow
cell structure of FIG. 3.
[0074] FIG. 5 shows a section of a periodic lattice of hexagonal
cells, showing the positions of the joints j and struts s for this
stretch dominated structure.
[0075] FIG. 6 shows a perspective view from one side of an inner
impact resistant liner of a cycle helmet, the inner impact
resistant liner formed from a hollow cell structure similar to that
shown in FIGS. 3, 4, and 5, the liner shaped to follow and
substantially conform to the top portion of a user's head.
[0076] FIG. 7 shows a perspective view directly from the rear of
the inner impact resistant liner of FIG. 3, with an outer shell
covering the inner impact resistant liner, vent slots formed in the
outer shell to allow air to circulate within the inner impact
resistant liner.
[0077] FIG. 8 shows a perspective schematic view from the front and
to one side of a test rig used to test samples of a hollow cell
structure.
[0078] FIG. 9 is a graph showing the Head Injury Criterion (HIC)
and peak acceleration for a range of test samples.
[0079] FIGS. 10 to 12 show test samples of honeycomb hollow cell
structure according to embodiments of the present invention
post-testing, each sample having a different relative density, FIG.
10 showing brittle failure at a relative density of 0.111, FIG. 11
showing plastic work at a relative density of 0.143, and FIG. 12
showing linear elastic deformation at a relative density of
0.25.
[0080] FIG. 13 shows a graphical plot of energy per volume vs peak
acceleration for a range of test materials and conditions.
[0081] FIG. 14 shows graphical plots of acceleration vs time and
force vs displacement for test pieces formed from Nylon 11, Elasto
and EPS.
DETAILED DESCRIPTION
[0082] Embodiments of the present invention will now be described
with reference to the figures. As outlined above in the background
section certain structures are known to be suited for impact
absorption. However, it has not been fully understood how these
could be structured in order to optimise the impact absorption
properties. Outlined below are examples of optimised structures for
improved impact absorption. These can be used to form items
intended to reduce traumatic brain injury such as bicycle helmets.
A method of optimising the structure for improved impact absorption
is also described.
[0083] Previously, it has been assumed when assessing energy
dissipation in helmets or similar impact absorbing structures that
the liner foam is entirely responsible for dissipating the impact
energy. The reaction force is determined by the compressive
strength of the foam. A foam lattice is assumed to have a flat
plateau compressive strength over its densification strain.
However, the foam only provides an ideal force-displacement curve
if the compressed region is uniform in area. In a curved structure
such as a helmet with a substantially ovoid shape, the impact area
or crush area is not constant, or planar: the contact area
increases with displacement. This causes the reaction force to also
increase. Furthermore, if the curved helmet surface impacts another
curved surface, the force-displacement gradient will be further
reduced. The consequence of this is that a foam liner needs to be
thicker in order to provide adequate energy absorption by
maintaining the peak acceleration below the safety legislation. The
consistent plateau stress of foam limits it's effectiveness as an
energy absorbing structure when used as a curved structure (such as
for example in a helmet) due to the inherent curved contact
surface. The other assumption is that the liner is formed as a
unitary structure (that is, without gaps).
[0084] As outlined below, it is possible to create a structure that
has a stiffness and strength higher than would otherwise be the
case if creating the structure as a unitary structure formed from
e.g. foam, for a given relative density p/p.sub.s (where p is the
density of the foam and p.sub.s that of the bulk material), and
this allows more energy to be dissipated per volume. It is also
possible to create a structure that provides an initially high
strength when the contact area is very low and which has a gradual
post-yield softening proportional to the increase in contact
area.
[0085] This is achieved by forming the impact-absorbing structure
as a hollow-cell structure which is stretch-dominated, such as for
example a micro-truss lattice or out-of-plane honeycomb.
[0086] In this type of structure, the mechanism of deformation
involves `hard` modes such as compression and tension rather than
bending. For the same relative density as foam, stretch dominated
structures have a comparatively higher modulus and yield stress.
This is discussed below.
[0087] In stretch-dominated hollow-cell structures, yield stress
occurs due to localised plastic buckling and brittle collapse of
the struts. This is also known as the bifurcation point because the
structure becomes unstable and a post yield softening regime
ensues.
[0088] The stress rises steeply at the densification strain
(.epsilon..sub.d), which can be calculated from
d = 1 - ( .rho. .rho. s ) / ( .rho. crit .rho. s ) ##EQU00001##
where p is the density of the structure and p.sub.s that of the
bulk material, and where p.sub.crit/p.sub.s is the relative density
(or volume fraction solid) at which the structure locks up.
[0089] Apart from light weight and ventilation, there are
potentially two key benefits of using hollow-cell stretch-dominated
structures as an impact absorbing structure. Firstly, the
post-yield softening counteracts the area increase of a oval shaped
helmet dissipating energy at a more uniform plateau force.
Secondly, for a given yield stress, the relative density of a
stretch-dominated structure can be much lower providing a greater
densification strain and therefore increasing the potential energy
dissipated over the same displacement.
[0090] One particular form of stretch-dominated structure is a
cellular solid. A cellular solid is one made up of an
interconnected network of solid struts or plates that form the
edges and faces of cells. Typically the mechanical behaviour of
cellular solids can be distinguished by bending-(foam) and
stretch-(lattice) dominated mechanisms. The Maxwell stability
criterion is used to distinguish between bending- and
stretch-dominated structures. Cellular solids can be thought of as
joints j, joined by struts s, which surround faces that enclose
cells, as shown in FIG. 1.
[0091] The effect of material in the faces stiffens the structure
by a constant. In FIG. 1a, when the frame is compressed it has no
stiffness or strength in the loading direction. If the joints are
frozen (locked) the frame in FIG. 1a will bend as the structure is
loaded and can be called a bending-dominated structure. In the
stretch-dominated structures in FIG. 1b and 1c, the members carry
tension or compression when loaded, giving higher modulus and
strength. This is shown in FIGS. 2a and 2b, which summarise the
difference between stretch and bending-dominated structures in
terms of relative modulus E/E.sub.s and relative strength
.sigma./.sigma..sub.s against relative density p/p.sub.s. In the
structures in FIGS. 1a and 1b, the structure carries self-stress,
which means the struts carry stress even though the structure
carries no external load (this is prevalent in FIG. 1c). For
example if the vertical strut is shortened, it pulls the other
struts into compression.
[0092] The two key benefits of using stretch-dominated structures
as impact absorbing structures are as follows: firstly, the
post-yield softening counteracts the area increase of a oval shaped
helmet dissipating energy at a more uniform plateau force, and;
secondly, for a given yield stress, the relative density of a
stretch-dominated structure can be much lower providing a greater
densification strain and therefore increasing the potential energy
dissipated over the same displacement. This is discussed in detail
in Appendix E.
[0093] In the embodiments described below, the impact absorbing
structure is formed as a lattice--i.e. from interconnected hollow
cells. Also, in order to simplify manufacture, a periodic lattice
is described (i.e. the cells are regularly shaped and sized).
Hexagonal cells were used as this shape has the largest number of
side and which will still regularly tessellate--i.e. without
requiring a second shape to fill gaps (for example, if a regular
octagon lattice was chosen, a regular square shape would be
inherent). Hexagonal honeycomb cells have the highest number of
cell walls for each cell, and therefore the lowest connectivity.
which has been shown to be effective in high specific strength.
[0094] Other shapes (e.g. triangles and squares) will also
tessellate, but have fewer sides. However, the number of sides has
been shown to correlate positively with the dissipated energy per
unit mass of the structure (SAE).
[0095] The types of lattice structure described above can be
generally described as 2D hollow cell structures. Where these are
referred to in this specification, this indicates a
three-dimensional structure, with the cells of the structure formed
in such a way as to have depth, but so that when viewed at a
certain angle the cells will have a uniform or identical
cross-section at any position perpendicular to the view angle. That
is, a cross section taken at any position would be identical to one
taken at any other position. For example, a honeycomb cell
structure viewed in plan or from directly above will provide a
uniform cross-section at any depth through the cells. This can be
translated to curved shapes such as the ovoid shape necessary to
form a helmet, for example. When viewed at any particular point
looking inwards towards the centre of the interior, the cells will
appear identical to those viewed from another point also looking
inwards towards the centre of the interior.
[0096] It should be noted that other types of structure, formed as
stretch dominated structures, will also provide the same
advantages. For example, 3-D stretch-dominated structures such as a
truss structure or a structure similar to a crystal lattice
structure can also be formed, which will provide the same impact
absorption benefits.
[0097] As shown in FIGS. 3 and 4 the hollow cell stretch dominated
structure 1 used in a first embodiment of the present invention is
a unitary material formed into a honeycomb structure. It is
preferred that the cells are hexagonal, as hexagonal cells 2 such
as those used in the hollow cell structure 1 tessellate and so form
a structure where each cell wall is common with an adjacent cell. A
grid formed from hexagonal cells also provides a balance between
overall grid density (the total amount of material), and the
layout/location of the cell wall material and the empty space which
the cell walls encompass. That is, tessellation is achieved with
the cell walls distributed over a given planar or curved surface as
evenly as possible, with no overloaded focal areas, or over-large
uncovered areas.
[0098] Hexagonal honeycomb can be thought of as a stretch dominated
structure by applying the Maxwell criterion:
M.sub.honeycomb=30-3.times.12+6=0
[0099] FIG. 4 shows a section of a periodic lattice of hexagonal
cells, showing the positions of the joints j and struts s for this
stretch-dominated structure.
[0100] In practical use, and when experiencing an impact, the
honeycomb structure will experience both in-plane and out-of-plane
loading. Stretch-dominated structures such as the hexagonal hollow
cell structure 1 are generally used in a planar or sheet form,
either flat or curved, and the impacts received by the hollow cell
structure have a primary force component directed into the plane
perpendicular to the point of impact. That is, in the opposite
direction to out-of-plane arrow 3 in FIG. 1. However, as noted
there will frequently be a force component at an angle to this, and
the theory behind this is discussed in detail in Appendix C.
[0101] The impact absorption properties of a stretch-dominated
structure such as the hollow cell structure 1 are determined by the
material used to form the structure, and the specific geometry of
the structure: i.e. cell size, cell wall thickness, cell width and
cell length as shown in FIG. 4.
[0102] If used in an impact-absorbing structure such as a helmet,
the lattice is designed so that the axial part of the cell is
always perpendicular to the surface of the head. This is important
as the crush strength of honeycomb significantly diminishes as the
impact angle increases away from perpendicular to the axial part of
the cell.
[0103] If the cell dimensions are known, a value of hollow cell
relative density (or volume fraction solid) can be calculated using
the equation shown below:
p * p s = { h l + 2 2 * ( h l + sin .theta. ) cos .theta. } t l = 2
3 t l ( 1 - 1 2 3 t l ) = 2 3 t l ##EQU00002##
[0104] For the particular embodiments of the honeycomb structure 1
of the invention described, h is assigned a value of 1, .theta. has
a value of 30 (degrees), and the ratio of cell wall thickness (t)
to cell length (l) is significantly small.
[0105] A proposed use for the hollow cell structure 1 would be in
bicycle helmets. The material used to create the hollow cell
structure 1 in this embodiment is Nylon 11 and ST Elastomer. This
is a readily available material, which is lightweight, easily
formed and malleable, and is therefore suitable or at least
analogous to the type of material that would be used for
mass-manufactured helmets.
[0106] The hollow cell structure 1 was manufactured by additive
manufacturing. The process is briefly described in Appendix B.
[0107] Tests were carried out as detailed in Appendix A, and
Appendix D, with the objective of determining how varying the
relative density of the honeycomb hollow cell structure 1 (this
type of structure also known as `out-of-plane honeycomb`) would
affect the hollow cell structure 1 when subjected to impact
testing. As shown in Appendix A, the relative density was varied
between 0.1 and 0.33 by changing the cell size (s) from between a
minimum of 6 mm and a maximum of 20 mm, with the wall thickness
maintained at a constant 1 mm.
[0108] The results indicate that an acceptable range of optimum
relative densities lies between 0.125 and 0.175 for this material
and for the particular cell/lattice size and shape used during
testing, for the reasons outlined in the `Results from Impact
Testing` section of Appendix A, and Appendix D. The results
indicate that the cell size, cell wall thickness, cell width and
cell length can be freely varied relative to one another, and as
long as the relative density lies between 0.03 and 0.17, then the
structure will provide optimised impact absorption properties.
[0109] As outlined in the `background` section, helmet design is
generally a trade-off between the overall weight of a helmet, and
the impact-absorbing properties. Based on the results of the tests
detailed in Appendix A and Appendix D, a helmet such as helmet 5
shown in FIGS. 3 and 4, constructed using a structure the same as
or similar to the inner impact resistant liner 7 (formed as a
hexagonal hollow-cell stretch-dominated structure) covered by an
outer shell 6, formed from nylon 12 or a similar material, will
provide a lightweight structure capable of meeting and exceeding
the relevant standards for impact absorption, in particular BS EN
1078. The test results indicate the elastic-plastic honeycomb has a
3.times. greater EPV than a typical expanded polystyrene helmet.
This is clearly shown by the plots of the experimental results
shown in FIGS. 13 and 14.
[0110] The reasons can be summarised as follows: [0111] Stretch
dominated structures rely on `hard` modes of deformation through
compression and tension. A long plateau force is achieved, as the
stress response softens as the area increases. [0112] Stretch
dominated structures have higher specific strength for the same
relative density, so it is possible to increase the densification
strain, as the relative density is lower in a stretch dominated
structure. [0113] All stretch dominated structures have similar
mechanical responses. Therefore, an impact-absorbing structure can
be formed from any appropriate material and at any shape and size
(e.g. all honeycomb topology and materials), and will still provide
the advantages as outlined above.
[0114] As briefly noted above, where 2D hollow cells are referred
to in this specification, this indicates a three-dimensional
structure, with the cells of the structure formed in such a way as
to have depth, but so that when viewed at a certain angle the cells
will have a uniform or identical cross-section at any position
perpendicular to the view angle. That is, a cross section taken at
any position would be identical to one taken at any other position.
For example, a honeycomb cell structure viewed in plan or from
directly above will provide a uniform cross-section at any depth
through the cells. It should also be noted that when a structure is
referred to as `stretch-dominated`, this is according to the
Maxwell criterion as outlined herein. It should further be noted
that the phrases `relative density` and `volume fraction solid`
essentially have the same meaning and are used interchangeably
within this specification.
* * * * *