U.S. patent application number 12/052021 was filed with the patent office on 2013-11-21 for lightweight transparent armor window.
The applicant listed for this patent is Carsten Weinhold. Invention is credited to Carsten Weinhold.
Application Number | 20130305912 12/052021 |
Document ID | / |
Family ID | 40512120 |
Filed Date | 2013-11-21 |
United States Patent
Application |
20130305912 |
Kind Code |
A1 |
Weinhold; Carsten |
November 21, 2013 |
LIGHTWEIGHT TRANSPARENT ARMOR WINDOW
Abstract
The invention relates to a lightweight transparent armor
laminate comprising layers of borosilicate glass, layers of
transparent glass-ceramics and a polymer spall layer of
polycarbonate and/or polymethyl methacrylate. The layers are bound
by polyurethane and/or polyvinylbutyral interlayer films.
Inventors: |
Weinhold; Carsten;
(Scranton, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Weinhold; Carsten |
Scranton |
PA |
US |
|
|
Family ID: |
40512120 |
Appl. No.: |
12/052021 |
Filed: |
March 20, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60975661 |
Sep 27, 2007 |
|
|
|
Current U.S.
Class: |
89/36.02 ;
428/220 |
Current CPC
Class: |
B32B 17/10045 20130101;
Y10T 428/31518 20150401; Y10T 428/31601 20150401; B32B 2333/12
20130101; B32B 17/10119 20130101; B32B 17/10761 20130101; B32B
17/1077 20130101; F41H 5/0407 20130101; Y10T 428/24942 20150115;
B32B 2369/00 20130101; Y10T 428/31507 20150401 |
Class at
Publication: |
89/36.02 ;
428/220 |
International
Class: |
F41H 5/04 20060101
F41H005/04 |
Claims
1. A transparent multi-layer laminate comprising: a strike face
layer of soda-lime or borosilicate glass; a first glass-ceramic
layer; an intermediate layer of soda-lime or borosilicate glass; a
second glass-ceramic layer; and a polymer spall layer as a rear
layer of the laminate, wherein each of said strike face layer, said
first glass-ceramic layer, said intermediate layer, said second
glass-ceramic layer, and said polymer spall layer are bound to an
adjacent layer by one of a plurality of polymer interlayers, and
wherein the overall thickness of the laminate is less than 80
mm.
2. The laminate of claim 1, wherein said first glass ceramic layer
and said second glass-ceramic layer are each independently a
lithium-alumo-silicate glass ceramic or a lithium-disilicate glass
ceramic.
3. The laminate of claim 1, wherein said polymer spall layer is
polycarbonate, polymethyl-methacrylate, or a laminate of
polycarbonate and polymethyl-methacrylate.
4. The laminate of claim 1, wherein each of said polymer
interlayers is a polyurethane or polyvinylbutyral film.
5. The laminate of claim 1, wherein the intermediate layer is 14 to
25 mm thick.
6. The laminate of claim 1, wherein each of said first
glass-ceramic layer and said second glass-ceramic layer is 6-14 mm
thick.
7. A glass based transparent armor comprising the laminate of claim
1.
8. The laminate of claim 5, wherein the intermediate layer is two
individual layers bound together by a polymer interlayer.
9. A transparent multi-layer laminate comprising: a) a borosilicate
outer strike face layer; b) at least three glass-ceramic layers; c)
an intermediate layer of soda-lime or borosilicate glass; and d) a
polymer spall layer; wherein layers b) and c) are disposed between
said strike face layer a) and said polymer spall layer d), wherein
each of said layers a), b), c), and d) are bound to adjacent layers
by one of a plurality of polymer interlayers, and wherein the
overall thickness of the laminate is less than 80 mm.
10. The laminate of claim 9, wherein said outer strike face layer
is 3-6 mm thick glass.
11. The laminate of claim 9, wherein each of said at least three
glass ceramic layers b) are independently a lithium-alumo-silicate
glass ceramic or a lithium-disilicate glass ceramic.
12. The laminate of claim 9, wherein said polymer spall layer d) is
polycarbonate, polymethyl-methacrylate, or a laminate of
polycarbonate and polymethyl-methacrylate.
13. The laminate of claim 9, wherein said each of said plurality of
polymer interlayers is a polyurethane or polyvinylbutyral film.
14. The laminate of claim 9, wherein the strike face layer a) is
3-6 mm thick, each layer b) is 6-14 mm thick, layer c) is 14 to 25
mm thick, said polymer spall layer d) is 10-20 mm thick, and each
of said plurality of polymer interlayers is 10-80 mil thick.
15. The laminate of claim 9, wherein the overall thickness of the
laminate is less than 70 mm.
16. A transparent armor comprising a laminate of claim 9.
17. The armor of claim 16, which can defeat an 0.30 cal. AP-M2
projectile at an impact speed of up to 2750 fps.
18. A method of securing a space from a projectile, comprising
placing an armor according to claim 17 between said space and said
projectile.
19. A transparent multi-layer laminate comprising: a strike face
layer of soda-lime or borosilicate glass; at least three
glass-ceramic layers; an intermediate layer of soda-lime or
borosilicate glass between two of said glass ceramic layers; and a
polymer spall layer as a rear layer of the laminate, wherein each
of said strike face layer, said at least three glass-ceramic
layers, said intermediate layer, and said polymer layers are bound
to an adjacent layer by one of a plurality of polymer interlayers,
and wherein the overall thickness of the laminate is less than 80
mm.
20. A glass based transparent armor comprising the laminate of
claim 19.
21. The laminate of claim 19, wherein said intermediate glass layer
is 14 to 25 mm thick.
22. The laminate of claim 19, wherein each glass-ceramic layer is
6-14 mm thick.
23. The laminate of claim 19, wherein the overall thickness of the
laminate is less than 70 mm.
24. A method of securing a space from a projectile comprising
placing an armor according to claim 20 between said space and said
projectile.
25. The laminate of claim 19, wherein each of said at least three
glass ceramic layers are each independently a
lithium-alumo-silicate glass ceramic or a lithium-disilicate glass
ceramic.
Description
[0001] This application claims the benefit of the filing date of
U.S. Provisional Application Ser. No. 60/975,661 filed Sep. 27,
2007.
[0002] Commercially available, glass-based transparent armor
typically consists of multiple glass and polymer layers, which are
laminated together to form a relatively thick composite. The
resulting composite must be transparent and essentially free of
optical distortion while providing maximum protection against
ballistic impact of projectiles and fragments at minimum weight and
minimum cost. Of particular interest are transparent laminates,
which restrict the destruction caused by the projectile locally to
ensure maximum residual vision and provide protection against
multiple hits.
[0003] To successfully stop a projectile, impact resistant
transparent laminates typically engage various defeat mechanisms,
including projectile fragmentation and mass removal by projectile
erosion. Systems employing transparent ceramic materials such as,
for example, transparent spinel, sapphire, or AlON, show superior
ballistic performance over traditional glass-based systems, but are
often not available in larger sizes and volumes. Currently, the
cost per square inch for these systems is typically more than 5
times higher than for glass-based systems offering comparable
protection.
SUMMARY OF THE INVENTION
[0004] According to one aspect of the invention there is provided a
multi-layer transparent laminate having a plurality of layers bound
together by polymer interlayers. The multi-layer laminate has an
outer soda-lime or borosilicate glass strike face layer, a
plurality of glass-ceramic layers, at least one internal soda-lime
or borosilicate glass layer, and a polymer spall layer. The
glass-ceramic and internal soda-lime or borosilicate glass layers
are disposed between the strike face layer and the spall layer.
Overall the thickness of the composite is preferably less than 80
mm, whereas the overall areal density is preferably less than 30
psf.
[0005] According to another aspect of the invention there is
provided a multi-layer transparent laminate having a plurality of
layers bound together by polymer interlayers. The multi-layer
laminate has an outer glass-ceramic strike face layer, at least one
additional glass-ceramic layer, at least one internal soda-lime or
borosilicate glass layer, and a polymer spall layer. The
glass-ceramic and internal soda-lime or borosilicate glass layers
are disposed between the strike face layer and the spall layer.
Overall the thickness of the composite is preferably less than 80
mm, whereas the overall areal density is preferably less than 30
psf.
[0006] According to another aspect of the invention there is
provided a multi-layer transparent laminate having a plurality of
layers bound together by polymer interlayers. The multi-layer
laminate has a soda-lime or borosilicate glass layer disposed
between two glass-ceramic layers and a polymer spall layer. Overall
the thickness of the composite is preferably less than 80 mm
[0007] Thus, the present invention relates to a multi-layer
transparent laminate having a plurality of layers joined together
by polymer interlayers. All layers are commercially available. The
multi-layer laminate may comprise an outer soda-lime or
borosilicate glass strike face layer, a plurality of glass-ceramic
layers, at least one internal soda-lime or borosilicate glass
layer, and a polymer spall layer. The glass-ceramic and glass
layers are disposed between the strike face and the spall layer.
The overall thickness of the composite is preferably less than 80
mm.
[0008] Present State-of-the-Art glass-based systems provide
single-hit protection against armor piercing projectiles (STANAG
Level 3 or similar) at oblique impact at areal densities of about
30 psf. In comparison to other glass-based impact-resistant
laminates, the designs disclosed provide multi-hit protection
against 0.30 cal. AP-M2 or similar projectiles at impact speeds of
up to 2750 fps at a thickness of less than 80 mm and an areal
density of less than 30 psf. Single-hit protection against the same
threat is achievable at an areal density of less than 25 psf by
removing one of the glass ceramic layers. The composites are
useful, for example, as transparent armor structures in military
and security vehicles as well as for windows in secured buildings
applications.
[0009] Preferably the strike face is a 3-6 mm thick layer of
BOROFLOAT.RTM. glass. Preferably, at least three glass-ceramic
layers are disposed between the strike face and the spall layer.
Each glass-ceramic layer is from about 6-14 mm thick. Preferred
glass ceramics are lithium-alumo-silicate glass ceramics such as
SCHOTT's ROBAX.RTM. or ZERODUR.RTM., or a lithium-disilicate glass
ceramic such as ALSTOM's TRANSARM.RTM.. Alternatively, the outer
strike face layer may be a glass-ceramic layer.
[0010] The internal soda-lime or borosilicate glass layer is from
about 14-25 mm thick and is disposed between two of the
glass-ceramic layers. The internal soda-lime or borosilicate glass
layer can be monolithic, or a multi-layer laminate consisting of
individual layers with thicknesses between about 6-19 mm, most
preferably between about 7-14 mm and bound together by a polymer
interlayer. Preferred glasses are borosilicates such as PYREX.RTM.
or BOROFLOAT.RTM..
[0011] The spall layer is preferably made out of polycarbonate,
polymethyl-methacrylate, or a combination thereof. Preferably, the
spall layer has a thickness in the range of about 10-20 mm, and
consists of a single layer of polymethyl-methacrylate laminated to
a single layer of polycarbonate, e.g., by a polymer interlayer.
[0012] All laminate layers are joined together with polymer
interlayers. Each interlayer may range from about 10 to 80 mil
thick in the finished laminate. Most preferably the polymer
interlayer is polyurethane or polyvinylbutyral. Select interlayers
may be reinforced, for example by incorporating a tear-resistant
PET film. Optionally, a thin glass layer may be laminated to the
backside of the spall layer to protect the polymer surface against
degradation including scratches and chemical attack by window
cleaning agents.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Various other features and attendant advantages of the
present invention will be more fully appreciated as the same
becomes better understood when considered in conjunction with the
accompanying drawings, in which like reference characters designate
the same or similar parts throughout the several views, and
wherein:
[0014] FIG. 1 shows an isometric view of a multi-layer laminate of
Glass/Glass Ceramic and Polymers with an optional thin glass sheet
laminated to backside of the spall layer;
[0015] FIG. 2 shows a cross-section through one edge of the
multi-layer laminate according to the invention; the multi-layer
laminate is surrounded by a gasket, which is surrounded by a frame
(not depicted) made out of a high-strength aluminum alloy;
[0016] FIG. 3 shows a cross-section through a preferred embodiment
of the multi-layer laminate according to the invention. A preferred
thickness designation (mm) for each layer is indicated;
[0017] FIG. 4 shows another preferred embodiment, where the
monolithic internal glass layer is replaced by a double-layer glass
laminate. A preferred thickness designation (mm) for each layer is
indicated;
[0018] FIG. 5 shows another preferred embodiment, where the
polycarbonate layer is replaced by a thin-glass/polycarbonate
laminate. A preferred thickness designation (mm) for each layer is
indicated;
[0019] FIG. 6 shows various views of a small-size (250 mm.times.250
mm) window with gasket;
[0020] FIG. 7 shows critical areal densities for traditional
glass-based systems depending on impact obliquity as disclosed in
the US Military Specification MIL-G-5485D (22 Feb. 1993);
[0021] FIG. 8 shows common failure modes and defeat mechanisms in
ballistic impact situations;
[0022] FIG. 9 shows the typical appearance of a ballistic test
coupon mounted on an oversized polycarbonate backing before and
after the ballistic impact test; and
[0023] FIG. 10 shows the appearance of 0.30 cal. AP-M2 cores after
impact with various laminates according to the invention.
[0024] FIG. 11 shows the strikeface of sample No 3 in Example 3
after the test. No bulging or other deformation of the spall layer
is observed; and
[0025] FIG. 12 shows the back spall layer of sample No 3 in Example
3. No bulging or other deformation of the spall layer was
observed.
DETAILED DESCRIPTION OF THE INVENTION
[0026] The abbreviations used herein shall mean the following
unless otherwise specified.
[0027] 0.30 cal. AP-M2 Projectile type/designation; Armor Piercing
M2
[0028] ALSTOM Company name
[0029] CTE coefficient of thermal expansion
[0030] DOP Depth-of-Penetration (ballistic test)
[0031] fps Feet per second
[0032] mil one thousands of an inch (1 mil=25.4 microns)
[0033] mm millimeter
[0034] PC Polycarbonate
[0035] PMMA Polymethyl methacrylate
[0036] PU Polyurethane
[0037] PVB Poly-Vinylbutyral
[0038] psf pounds per square foot
[0039] SCHOTT Company name
[0040] TPU thermoplastic Polyurethane
[0041] Vs Striking velocity
[0042] Vr Residual velocity
[0043] Vs/Vr Striking versus residual velocity (ballistic test)
[0044] The strike face is the side of the laminate that is most
likely to encounter the initial impact of a projectile. The
preferred material for the strike face is a borosilicate glass,
most preferably BOROFLOAT.RTM. from SCHOTT Germany. Preferred is a
glass layer having a thickness of more than about 3 mm but less
than about 6 mm, which is able to withstand the impact of debris in
every-day use (e.g., rock strikes, etc.). Alternatively, soda-lime
glass or a polymer sheet or multiple tear-resistant films with
scratch-resistant coating can also be used for the strike face
material. Alternatively, a glass-ceramic can also be used for the
strike face material. In the disclosed design, the strike face
layer has multiple functions. First and foremost, it was found that
the use of a high-surface quality material in combination with
standard polymer interlayers enable the use of glass-ceramic
material as-rolled, i.e. without grinding and polishing, to achieve
an essentially distortion-free, transparent view. Mechanically, the
strike face protects the surface of layer 1 against scratches, and
acts in combination with layer 1 and layer 2 to slow-down and
destabilize (i.e. tip or turn) the projectile in order to induce
fragmentation by side-impact.
[0045] The adhesive interlayers are preferably made from a material
such as polyvinyl butyral (PVB) or polyurethane (PU). The
interlayers are optically transparent, provide strength and add
only a minimal thickness and weight to the overall laminate.
[0046] Polyurethane resins provide not only good bonding to glass
but also provide excellent internal strength. Polyurethane resins
are much lighter than glass and have been found to expand and
contract at rates close to that of standard glass, thus leading to
minimal cracking or delamination during thermal expansion and
contraction of the laminate. Trade names for suitable polyurethane
films include: Huntsman KrystalFlex.RTM., and Deerfield
DureFlex.RTM..
[0047] Polyvinyl butyral (or PVB) is also an excellent choice for
interlayer. It provides bonding between the laminate layers.
Polyvinyl butyral is a resin usually used for applications that
require strong binding, optical clarity, and adhesion to many
surfaces, toughness and flexibility. It is prepared from polyvinyl
alcohol by reaction with butyraldehyde. The major application is
laminated safety glass for automobile windshields. Trade names for
PVB-films include: BUTACITE.RTM., SAFLEX.RTM., S-Lec.RTM. and
TROSIFOL.RTM..
[0048] In a preferred embodiment, preferably each interlayer film
thickness is around 25 mil to accommodate thermal expansion
mismatches between the layers and to accommodate uneven gaps caused
by thickness variations and/or surface figure deviations of the
individual layers. In certain layers, 50 mil or 75 mil thick
interlayer films may replace the 25 mil interlayer films.
Alternatively, to increase multi-hit performance, one or more of
the interlayer films may be an optical TPU laminates incorporating
a PET film, such as, for example STEVENS SECURSHEILD.RTM.. In
general each polymer interlayer performs a specific function.
Interlayer 1 acts to bond the strike face to a first layer (e.g.,
ROBAX). Preferably, the interlayer is a soft material having good
adhesion to BOROFLOAT.RTM. (preferred strike face) and ROBAX.RTM.
(preferred first layer). Interlayer 1 accounts for the slight
difference in thermal expansion between the layers and enables
flexing of the strike face upon impact to destabilize the
projectile. Interlayer 1 can be reinforced with a tear-resistant
film to keep comminuted material in the laminate. Interlayer 2 acts
to bond a first layer to a second layer. If, for example, both
layers are ROBAX.RTM. and have the same thermal expansion, then
preferably the chosen interlayer is hard with good adhesion to
ROBAX.RTM., such that both layers behave together like a monolithic
piece upon impact. The ballistic function of interlayer 2 is to
arrest cracks and to hold comminuted material in place thus slowing
and/or deflecting the projectile. Interlayer 3 acts to bond a
second layer to a third layer. If both layers have a slightly
different thermal expansion, then preferably the interlayer is soft
and has good adhesion to both materials. The ballistic function of
interlayer 3 is to arrest cracks, hold comminuted material in
place, and promote slowing and/or deflection of the projectile.
Interlayer 4 acts to bond a third layer to a fourth layer. If both
layers have a slightly different thermal expansion then preferably
the interlayer is soft and has good adhesion to both materials. The
ballistic function of interlayer 4 is to arrest cracks, hold the
comminuted material in place and to promote slowing and/or
deflection of the projectile. Interlayer 5 bonds the back of a
fourth layer to the Spall-Layer. To account for the difference in
thermal expansion (about one order of magnitude), a thicker,
preferably soft interlayer is preferably used.
[0049] Additional suitable materials for the interlayer include
transparent thermoplastics or thermosets such as
acrylonitrile-butadien-styrene (BS), acetyl resins, cellulose
acetate, cellulose acetate butyrate, cellulose acetate propionate,
cellulose tri-acetate, acrylics and modified acrylics, allyl
resins, chlorinated polyethers, ethyl cellulose, epoxy,
fluoroplastics, ionomers (e.g., Dupont Surlyn A.RTM.), melamines,
nylons, parylene polymers, transparent phenolics, phenoxy resins,
polybutylene, polycarbonates, polyesters, polyethylenes,
polyphenylenes, polypropylenes, polystyrenes, polyurethanes,
polysolphones, polyvinyl-acetate, polyvinyl butyral, silicones, as
well as styrene-acrylonitride and styrene-butadiene copolymers.
[0050] If the spall layer is a combination of PMMA on PC then an
interlayer 6 is used to bond the two spall layers together. This
interlayer has to be stiff and provide good adhesion. The
combination of PMMA on PC is known to improve the ballistic
performance of systems due to the stiffening of PMMA under high
strain rates with PC providing a more stretchable support. Further,
the gradual change in acoustic impedance provides a better
impedance-match to the last layer.
[0051] If the spall layer is protected by a thin glass layer
against abrasion or chemical attack, then an interlayer 7 with a
thickness of 50 mil or 75 mil is used to bond the thin glass sheet
to the back of the polycarbonate. To improve mechanical
performance, the thin glass sheet might be chemically
strengthened.
[0052] Additional suitable materials for the spall layer include
transparent thermoplasts or thermosets such as
acrylonitrile-butadien-styrene (BS), acetyl resins, cellulose
acetate, cellulose acetate butyrate, cellulose acetate propionate,
cellulose tri-acetate, acrylics and modified acrylics, allyl
resins, chlorinated polyethers, ethyl cellulose, epoxy,
fluoroplastics, ionomers (e.g., Dupont Surlyn A.RTM.), melamines,
nylons, parylene polymers, transparent phenolics, phenoxy resins,
polybutylene, polycarbonates, polyesters, polyethylenes,
polyphenylenes, polypropylenes, polystyrenes, polyurethanes,
polysolphones, polyvinyl-acetate, polyvinyl butyral, silicones, as
well as styrene-acrylonitride and styrene-butadiene copolymers.
[0053] In general, ballistic performance of a system is improved if
the interlayer is strong enough to hold comminuted material in
place and prevent ejection; in that case, the tightly packed,
broken material is typically able to provide about 70% of the
penetration resistance of intact material.
[0054] The multi-layer laminate of the present invention preferably
contains at least three glass-ceramic layers each of which is
preferably from about 6-14 mm thick. Glass-ceramic materials
exhibit a unique microstructure, and share many properties with
both glass and more traditional crystalline ceramics. They are
formed as a glass, and then made to crystallize partly by heat
treatment. Unlike sintered ceramics, glass-ceramics have no pores
between crystals. Some well-known brands of glass-ceramics are
PYROCERAM.RTM., CERAN.RTM., NEOCERAM.RTM., EUROKERA.RTM., or
MACOR.RTM.. The preferred glass-ceramic of the present invention is
ROBAX.RTM. glass ceramic from SCHOTT, which can be in the glassy or
the ceramized state. Alternatively, it can be replaced by other
glass ceramic materials such as ZERODUR.RTM. from SCHOTT,
TRANSARM.RTM. from ALSTOM, CLEARCERAM.RTM. from OHARA,
KERALITE.RTM., PYROCERAM.RTM., PYROCERAM III.RTM. AND VISION.RTM.
from CORNING, NEOCERAM.RTM. from NEG, and CDM glass Ceramic.
Mechanically, the glass-ceramic layers act to slow-down and/or
catch projectile fragments as well as provide support to the
neighboring layers.
TABLE-US-00001 TABLE 1 Typical Properties of Select Glasses and
Glass-Ceramics Sound Velocities Young's Density VI ong V shear
Modulus Poisson's CTE Knoop Glass Type [g/ccm] [m/s] [m/s] [G Pa]
Ratio [10.sup.-6/K] Hardness Sodalime Glass 2.5 5850 3450 73 0.23
8.9 480 PPG Starphire 2.5 5550 3400 72 0.22 9.03 SCHOTT BOROFLOAT
2.23 5550 3400 62 0.2 3.25 480 SCHOTT Robax 2.53 6650 3850 94 0.25
0 +/- 0.5 540 SCHOTT Robax glassy 2.43 6200 3750 83 0.2 520 SCHOTT
Zerodur 2.53 6220 3700 82 0.22 560 ALSTOM Transarm 2.45 7000 4250
105 0.2
[0055] The internal soda-lime or borosilicate-glass layer is
positioned within the laminate between two glass-ceramic layers,
and may comprise one or more individual layers. Borosilicate glass
is less dense than ordinary glass and has a very low thermal
expansion coefficient, about one-third that of ordinary glass. This
reduces material stresses caused by temperature gradients, thus
making it more resistant to breaking. Due to the smaller CTE
mismatch to the neighboring glass-ceramic layers, their lower
density and their ballistic properties, borosilicate glasses are
preferred. Due to its optical quality and transparency,
BOROFLOAT.RTM. is the preferred borosilicate glass, however other
borosilicate glasses such as ENDURAL.RTM. or BOMEX.RTM. are also
contemplated. In certain applications soda-lime glass may be used.
Mechanically, the internal soda-lime or borosilicate layer provides
support to the preceding layers, and acts to slow-down and/or catch
projectile fragments. Preferably, the internal soda-lime or
borosilicate glass layer comprises two individual sub-layers
laminated together and is from is 14 to 25 mm thick. Each
individual sub-layer of may be from 6-19 mm thick, most preferably
between about 7-14 mm. The sub-layers are bound together by a
polymer interlayer.
[0056] The spall layer, which entraps and/or catches shattering
material, may be polycarbonate, polymethyl-methacrylate, or
preferably a laminate of polycarbonate and polymethyl-methacrylate
bound together via a polymer interlayer. The spall layer is
preferably from about 10-20 mm thick. Polymethyl methacrylate
(PMMA), or poly (methyl 2-methylpropenoate) is the polymer of
methyl methacrylate. The thermoplastic and transparent plastic is
sold by the trade names PLEXIGLASS.RTM., PLEXIGLAS-G.RTM.,
R-CAST.RTM., PERSPEX.RTM., PLAZCRYL.RTM., LIMACRYL.RTM.,
ACRYLEX.RTM., ACRYLITE.RTM., ACRYLPLAST.RTM., ALTUGLAS.RTM.,
POLYCAST.RTM. and LUCITE.RTM.. It is often also commonly called
acrylic glass or simply acrylic.
[0057] Polycarbonate is lightweight and highly fracture resistant
particularly when compared to silica glass. This polymer also is
highly transparent to visible light and is sold by the trade names
LEXAN.RTM. from General Electric, CALIBRE.RTM. from Dow Chemicals,
MAKROLON.RTM. from Bayer and PANLITE.RTM. from Teijin Chemical
Limited. Most preferably, the spall layer is a laminate of
polycarbonate and polymethyl-methacrylate bound together via a
polymer interlayer. The polycarbonate layer provides a stretchable
support to the PMMA layer, which undergoes stiffening/hardening at
high strain rates.
[0058] In certain circumstances it is desirable to bond an
additional glass layer to the outer surface of the spall layer.
This allows the transparent laminate to be cleaned using solvents
or abrasive cleaners without substantial degradation of the optical
properties of the laminate.
[0059] The laminate may also incorporate other conventional
functional thin layers to provide coloring, optical, anti-glare,
anti-dirt, anti-scratch, and anti-frost functions. Additionally, a
network of antenna conductors or heating wires and/or any
peripheral cladding of enamel or opaque paint may also be added to
the laminate. Glass and glass-ceramic layers are typically not hard
enough to cause the erosion of armor-piercing projectiles or
projectile cores. In order to defeat an armor-piercing round like,
for example, 0.30 cal. AP-M2 at 2750 fps, one has to engage
different failure/defeat mechanisms by selecting the thickness and
the sequence of the materials employed accordingly. In multi-layer
glass/glass-ceramic/polymer systems one typically observes
different failure modes for each layer: brittle fraction,
plugging/cone fracture, radial fracture and fragmentation for glass
and glass-ceramic layers; ductile hole growth for polymers like
polycarbonates; and radial fracture and brittle fracture for
polymers like polymethyl-meth-acrylate.
[0060] In certain embodiments, the thickness of the individual
layers maybe important to consider. As a rule-of-thumb, the thinner
the layer(s), the smaller is the diameter of the destruction zone
perpendicular to the projectile path. However, in general, the
ballistic performance will suffer, if the layers are too thin or
too thick for the given material. If the layers are too thin,
individual layers can break from the back face of each layer in
rapid succession upon or shortly after impact, thereby decreasing
resistance against the projectile which passes through already
destroyed layers. If, on the other hand, the layers are too thick
for the given material the failure wave traveling in front of the
projectile comminutes material in advance over a greater distance,
thereby decreasing resistance against the projectile.
[0061] In certain embodiments the sequence of the various layers
can be an important factor to consider. In the wrong sequence, the
kinetic energy loss induced by preceding layers is not high enough
so that subsequent layers are able to hold-up to the progressing
projectile. Sequence is also important for projectile
destabilization (tipping, turning) and to induce projectile
fragmentation by side-impact and deformation (blunting, etc.).
[0062] The experimental results obtained with small samples on a PC
backing as support show that samples with BOROFLOAT.RTM. perform
best when BOROFLOAT.RTM. is positioned in the middle of the
laminate lay up. It was found, that systems incorporating a
sequence of ROBAX.RTM. layers in the glassy or the glass-ceramic
state have the ability to erode the tip of 0.30 cal AP-M2 steel
cores (see FIG. 10). Furthermore, if the thickness and the sequence
of the individual layers are selected right, the projectile will
deviate from the original trajectory, and is fragmented inside the
laminate.
[0063] In a preferred embodiment the multi-layer laminate according
to the invention has the following layers:
TABLE-US-00002 # Of layers/layer designation Layer composition
Layer thickness One layer/layer a Strike face layer 3 mm Three
layers/layer b Transparent Glass 8 mm each ceramic Layers One
layer/layer c Borosilicate Glass layer 21 mm One layer/Spall layer
d Polymethylmethacrylate 18 mm (PMMA) laminated to Polycarbonate
(PC)
[0064] In another preferred embodiment the multi-layer laminate
according to the invention has the following layer sequence:
TABLE-US-00003 Layer Material Thickness a) Strike face BOROFLOAT
.RTM. 3 mm-6 mm b) ROBAX .RTM. 8 mm-12 mm b) ROBAX .RTM. 8 mm-12 mm
c) BOROFLOAT .RTM. 18 mm-25 mm b) ROBAX .RTM. 8 mm-12 mm d) Spall
Layer- 12 mm-18 mm Polymethylmetaacrylate laminated to
Polycarbonate
[0065] In another preferred embodiment the multi-layer laminate
according to the invention has the following layer sequence:
TABLE-US-00004 Layer Material Thickness a) Strike face BOROFLOAT
.RTM. 3 mm-6 mm b) ROBAX .RTM. 8 mm-12 mm b) ROBAX .RTM. 8 mm-12 mm
c) BOROFLOAT .RTM. 9 mm-11 mm c) BOROFLOAT .RTM. 9 mm-11 mm b)
ROBAX .RTM. 8 mm-12 mm d) Spall Layer- 12 mm-18 mm
Polymethylmethacrylate laminated to Polycarbonate
[0066] In another preferred embodiment the multi-layer laminate
according to the invention has the following layer sequence:
TABLE-US-00005 Layer Material Thickness a) Strike face BOROFLOAT
.RTM. 3 mm-6 mm b) ZERODUR .RTM. 8 mm-12 mm b) ZERODUR .RTM. 8
mm-12 mm c) BOROFLOAT .RTM. 9 mm-11 mm c) BOROFLOAT .RTM. 9 mm-11
mm b) ZERODUR .RTM. 8 mm-12 mm d) Spall Layer- 12 mm-18 mm
Polymethylmethacrylate laminated to Polycarbonate
[0067] In another preferred embodiment the multi-layer laminate
according to the invention has the following layer sequence:
TABLE-US-00006 Layer Material Thickness range a) Strike
face-BOROFLOAT .RTM. 3 mm-6 mm b) ALSTOM TRANSARM .RTM. 8 mm-12 mm
b) ALSTOM TRANSARM .RTM. 8 mm-12 mm c) BOROFLOAT .RTM. 9 mm-11 mm
c) BOROFLOAT .RTM. 9 mm-11 mm b) ALSTOM TRANSARM .RTM. 8 mm-12 mm
d) Spall Layer- 12 mm-18 mm Polymethylmethacrylate laminated to
Polycarbonate
[0068] In another preferred embodiment the multi-layer laminate
according to the invention has the following layer sequence:
TABLE-US-00007 Layer Material Thickness range a) Strike face PYREX
.RTM. 3 mm-6 mm b) ROBAX .RTM. 8 mm-12 mm b) ROBAX .RTM. 8 mm-12 mm
c) BOROFLOAT .RTM. 18 mm-25 mm b) ROBAX .RTM. 8 mm-12 mm d) Spall
Layer- 12 mm-18 mm Polymethylmethacrylate laminated to
Polycarbonate
[0069] In another preferred embodiment the multi-layer laminate
according to the invention has the following layers:
TABLE-US-00008 # of layers/layer designation Layer composition
Layer thickness One layer/layer a Glass ceramic Strike 3 mm face
layer At least 2-layers/layer b Transparent Glass 8 mm each ceramic
Layers One layer/layer c Borosilicate Glass layer 21 mm One
layer/Spall layer d Polymethylmethacrylate 18 mm (PMMA) laminated
to Polycarbonate (PC)
[0070] In another preferred embodiment the multi-layer laminate
according to the invention has the following layer sequence:
TABLE-US-00009 Layer Material Thickness a) Strike face ROBAX .RTM.
8 mm-12 mm b) ROBAX .RTM. 8 mm-12 mm c) BOROFLOAT .RTM. 18 mm-25 mm
b) ROBAX .RTM. 8 mm-12 mm d) Spall Layer- 12 mm-18 mm.
Polymethylmethacrylate laminated to Polycarbonate
[0071] In another preferred embodiment the multi-layer laminate
according to the invention has the following layer sequence:
TABLE-US-00010 Material Thickness ROBAX .RTM. 8 mm-12 mm BOROFLOAT
.RTM. 18 mm-25 mm ROBAX .RTM. 8 mm-12 mm Spall Layer- 12 mm-18 mm.
Polymethylmethacrylate laminated to Polycarbonate
[0072] In another preferred embodiment the multi-layer laminate
according to the invention has the following layer sequence:
TABLE-US-00011 Material Thickness ROBAX .RTM. 8 mm-12 mm
Borosilicate 18 mm-25 mm ROBAX .RTM. 8 mm-12 mm Spall Layer- 12
mm-18 mm. Polymethylmethacrylate laminated to Polycarbonate
[0073] In another preferred embodiment the multi-layer laminate
according to the invention has the following layer sequence:
TABLE-US-00012 Material Thickness TRANSARM .RTM. 8 mm-12 mm
BOROFLOAT .RTM. 9 mm-11 mm BOROFLOAT .RTM. 9 mm-11 mm TRANSARM
.RTM. 8 mm-12 mm Spall Layer- 12 mm-18 mm. Polymethylmethacrylate
laminated to Polycarbonate
[0074] The multi-layer transparent laminate of the present
invention can be made by conventional methods such as, for example,
by assembling the interlayers and layers in the desired sequence,
and feeding them through an autoclave to apply heat and pressure.
Alternatively, the multi-layer transparent laminate of the present
invention can be made by the methods taught in WO93/22136, which is
hereby incorporated by reference.
[0075] Without further elaboration, it is believed that one skilled
in the art can, using the preceding description, utilize the
present invention to its fullest extent. The following preferred
specific embodiments are, therefore, to be construed as merely
illustrative, and not limitative of the remainder of the disclosure
in any way whatsoever.
[0076] In the foregoing and in the following examples, all
temperatures are set forth uncorrected in degrees Celsius and, all
parts and percentages are by weight, unless otherwise
indicated.
EXAMPLES
[0077] A combination of DOP (Depth-of-Penetration) and Vs/Vr
(strike velocity versus residual velocity) is conducted to
determine the kinetic energy loss, the critical areal density and
the limit thickness for monolithic layers under different failure
modes and defeat mechanisms. The results of these tests are then
used as a guideline to determine thickness ranges for individual
layers, as well as the number of layers needed to successfully
defeat a 0.30 cal. AP-M2 projectile at impact speeds of up to 2750
fps.
[0078] The projectile is launched against the target with a
powder-actuated, universal gun. The projectile speed is measured
using two sets of lightscreens; reported are the individual speeds
as well as the average.
[0079] To engage different failure and defeat modes, the small-size
samples are mounted against different types of backings.
[0080] In the first type of tests, the sample is mounted with epoxy
on a rigid, semi-infinite backing. To determine the influence of
the impedance mismatch between sample and backing, tests are
conducted against RHA Steel as well as an aluminum alloy. In both
cases, the monolithic sample itself is confined with a
tight-fitting frame. The sample is uniformly supported, and fails
mainly in compression. The depth-of-penetration of the projectile
into the rigid backing is measured. In the second type of test, the
sample is mounted either (a) with a polymer film on an oversized
polycarbonate backing, or (b) with epoxy on an oversized backing
made from a high-strength aluminum alloy. In both cases, the
monolithic sample is unconfined. The backing flexes upon impact,
and the samples fails in a combination of compression and tension.
Measured is the residual velocity of the projectile by using a
high-speed camera. FIG. 9 shows the typical appearance of a
monolithic sample mounted on an oversized polycarbonate backing
before and after ballistic testing; the sample successfully
defeated a 0.30 cal. AP-M2 round.
[0081] By using a combination of these tests it is possible to
estimate the kinetic energy loss of the projectile for different
impact scenarios, and derive for a given threat thickness ranges
for individual layers as well as layer combinations which serve as
a starting point for the design and optimization of a multi-layer
system.
Example 1
[0082] The following arrangement of laminate layers provides
protection against 0.30 cal. AP-M2 projectiles at speeds of up to
2750 fps. The window, without frame, exhibits an areal density of
30 psf or less. [0083] Strike face: SCHOTT BOROFLOAT with a
thickness between 4 mm to 6 mm [0084] 1st Interlayer: 25 mil PU
(Polyurethane) film (Huntsman PE501 or similar) [0085] 1st Layer
(b): SCHOTT ROBAX Glass Ceramic with a thickness between 7 mm to 8
mm [0086] 2nd Interlayer: 25 mil PU film (Huntsman PE501 or
similar), or 25 mil PVB (Polyvinylbutyral) film [0087] 2nd Layer
(b): SCHOTT ROBAX Glass Ceramic with a thickness between 7 mm to 8
mm [0088] 3rd Interlayer: 25 mil PU film (Huntsman PE501 or
similar), or 25 mil PVB film (Polyvinylbutyral) [0089] 3rd Layer
(c): [0090] Option A: single-layer SCHOTT BOROFLOAT with a
thickness between 18 mm to 21 mm [0091] or [0092] Option B:
double-layer SCHOTT BOROFLOAT with a thickness of each individual
layer between [0093] 9 mm to 11 mm, bonded with 25 mil PVB or PU
film [0094] 4th Interlayer: 25 mil PU film (Huntsman PE501 or
similar), or 25 mil PVB film (Polyvinylbutyral) [0095] 4th
Layer(b): ROBAX Glass Ceramic with a thickness between 7 mm to 8 mm
[0096] 5th Interlayer: 75 mil PU film (Huntsman PE501 or similar)
[0097] 1st Spall layer: 6 mm or 9 mm PMMA (Polymethyl-methacrylate)
[0098] Bonding layer 25 mil PU film, or similar [0099] 2nd Spall
layer: 6 mm, 9 mm or 12 mm PC (Polycarbonate) [0100] Optional: 2 mm
thin glass sheet bonded with 75 mil PU film to the 2nd spall layer
for scratch protection
Example 2
Ballistic Test Results
[0101] Small, multi-layer test coupons (100 mm.times.100 mm) are
mounted on a 12''.times.12''.times.12 mm thick polycarbonate
backing. The samples are tested in a configuration similar to the
one shown in FIG. 9. The test projectile is 0.30 cal. AP-M2 at the
indicated nominal speed (2250 fps or 2750 fps). Based on the test
results, the design described in Example 1 is derived combining the
two multi-layer sequences marked with an asterisk in Table 2 to
accommodate for scaling.
TABLE-US-00013 TABLE 1A Ballistic Test Results - 2250 fps impacts
by 0.30 cal. AP-M2 Shot Weights (Grains) Velocity Data (ft/sec)
Penetration Number Projectile Propellant No. 1 No. 2 Average
Description/Pen. into PB 9-17564 162.8 34.4 2283 2281 2282 No
Penetration 9-17565 163.4 34.4 2259 2259 2259 No Penetration
9-17566 163.1 34.4 2254 2254 2254 No Penetration 9-17567 163.5 34.4
2222 2222 2222 No Penetration 9-17568 163.3 34.4 2284 2284 2284 No
Penetration 9-17569 163.4 34.4 2257 2257 2257 No Penetration
9-17570 163.6 34.4 2281 2280 2281 No Penetration 9-17571 163.4 34.4
2268 2267 2268 No Penetration
TABLE-US-00014 TABLE 1B Ballistic Test Results - 2750 fps impacts
by 0.30 CAL AP-M2 Shot Weights (Grains) Velocity Data (ft/sec)
Penetration Number Projectile Propellant No. 1 No. 2 Average
Description/Pen. into PB 9-17572 162.8 44.5 2759 2759 2759
Penetration, 300pages (1) 9-17573 162.8 44.5 2761 2761 2761
Penetration, 600 pages (1) 9-17574 162.8 44.5 2717 2716 2717 No
Penetration 9-17575 162.4 44.6 2751 2751 2751 Penetration, 450
pages 9-17576 163.6 44.7 2770 2769 2770 Penetration, 1100 pages
9-17577 164.0 44.7 2768 2768 2768 Penetration, 1800 pages 9-17578
162.5 44.7 2746 2745 2746 No Penetration 9-17579 163.2 44.7 2759
2759 2759 Penetration (2) (PB) Denotes phone book and value
indicates the No. of pages penetrated. 1 PB = ~1200 pages. Single
center tile impacts at two velocities: 2250 .+-. 30 fps and 2750
.+-. 30 fps. (1) Denotes the recovered projectile was fractured,
not intact. (2) Denotes the projectile was thrown to the far left
exiting the target and missed the phone books.
TABLE-US-00015 TABLE 2 Layer Sequence vs. Result of Ballistic
Impact Test Layer Sequence Areal Shot Impact Design (Strikeface
first) Density Number Velocity Result 01 ROBAX-8 mm/ROBAX-8
mm/ROBAX-8 mm/ 19 9-17564 2282 PP ROBAX-8 mm/PC-12 mm 9-17572 2759
CP 02 ROBAXG-8 mm/ROBAXG-8 mm/ROBAXG-8 mm/ 19 9-17565 2259 PP
ROBAXG-8 mm/PC-12 mm 9-17573 2761 CP 03 * ROBAX-8 mm/BOROFLOAT-21
mm/ROBAX-8 mm/ 21 9-17566 2254 PP PC-12 mm 9-17574 2717 PP 04
BOROFLOAT-21 mm/ROBAX-8 mm/ROBAX-8 mm/ 21 9-17567 2222 PP PC-12 mm
9-17575 2751 CP 05 BOROFLOAT-21 mm/ROBAX-8 mm/ROBAX-8 mm/ 25
9-17568 2284 PP ROBAX-8 mm/PC-12 mm 9-17576 2770 CP 06 BOROFLOAT-21
mm/ROBAXG-8 mm/ROBAXG- 25 9-17569 2257 PP 8 mm/ROBAXG-8 mm/PC-12 mm
9-17577 2768 CP 07 ROBAXG-8 mm/ROBAXG-8 mm/ROBAXG-8 mm/ 25 9-17570
2281 PP BOROFLOAT-21 mm/PC-12 mm 9-17578 2746 PP 08 * ROBAX-8
mm/ROBAX-8 mm/BOROFLOAT- 21 9-17571 2268 PP 21 mm/PC-12 mm 9-17579
2759 CP ROBAX SCHOTT ROBAX - ceramized ROBAXG SCHOTT ROBAX - glassy
PC GE LEXAN Polycarbonate backing BOROFLOAT SCHOTT BOROFLOAT
[0102] Samples are considered to pass if and only if the spall
layer is not pierced by fragments (i.e., penetration). In the
tables, CP denotes "Complete Penetration". If at least one fragment
pierced the spall layer then the layer failed. PP denotes "Partial
Penetration". If the projectile penetrated into the laminate and
was stopped, and no fragments pierced the spall layer, the sample
passed. The ROBAX 8 mm/BOROFLOAT 21 mm/ROBAX8 mm samples passed at
both impact velocities (9-17566, 9-17574). The ROBAX 8 mm/ROBAX 8
mm/ROBAX 8 mm/BOROFLOAT 21 mm samples (9-17570, 9-17578) induced
different defeat modes. One preferred design is a combination of
ROBAX 8 mm/ROBAX 8 mm/BOROFLOAT 21 mm (9-17579), which almost
passed and induced core fragmentation by tipping the projectile and
ROBAX 8 mm/BOROFLOAT 21 mm/ROBAX8 mm (9-17566, 9-17574). The
combination of both designs is desired in order to achieve
comparable ballistic performance of the full-scale window, and to
achieve multi-hit capability (three shots placed in a 120 mm
triangle).
Example 3
Multi-Hit Example
[0103] Three 500 mm.times.500 mm test coupons for multi-hit testing
are prepared, and tested against an 0.30 cal. AP-M2 round at
nominal 2750 fps; the shot pattern is a 120 mm triangle, the shot
sequence is 12 O'clock, 4 O'clock and 8 O'clock. The nominal areal
density of the samples is 29 psf; due to slight variations in the
thickness of the individual glass and glass-ceramic layers, the
areal density of the samples as manufactured is 29.7 psf (samples 1
and 2) and 30 psf (sample 3). The samples have the following
structure:
TABLE-US-00016 500 mm .times. 500 mm, edges sealed, frameless
Sample 1 Strikeface 6 mm Borofloat Interlayer Huntsman PE-501 - 25
mil Ply 01 8 mm Robax Glass-ceramic Interlayer Huntsman PE-501 - 25
mil Ply 02 8 mm Robax Glass-Ceramic Interlayer Huntsman PE-501 - 25
mil Ply 03 21 mm Borofloat Interlayer Huntsman PE-501 - 25 mil Ply
04 8 mm Robax Interlayer Huntsman PE-501 - 75 mil Spall-Layer 9 mm
PMMA laminated to 9 mm PC (Lexan) Sample 2 Strikeface 6 mm
Borofloat Interlayer Huntsman PE-501 - 25 mil Ply 01 8 mm Robax
Glassceramic Interlayer PVB - 25 mil Ply 02 8 mm Robax
Glass-Ceramic Interlayer Huntsman PE-501 - 25 mil Ply 03 21 mm
Borofloat Interlayer Huntsman PE-501 - 25 mil Ply 04 8 mm Robax
Interlayer Huntsman PE-501 - 75 mil Spall-Layer 9 mm PMMA laminated
to 9 mm PC (Lexan) Sample 3 Strikeface 6 mm Borofloat Interlayer
Huntsman PE-501 - 25 mil Ply 01 8 mm Robax Glassceramic Interlayer
PVB - 25 mil Ply 02 8 mm Robax Glass-Ceramic Interlayer Huntsman
PE-501 - 25 mil Ply 03a 11 mm Borofloat Interlayer Huntsman PE-501
- 25 mil Ply 03b 11 mm Borofloat Interlayer Huntsman PE-501 - 25
mil Ply 04 8 mm Robax Interlayer Huntsman PE-501 - 75 mil
Spall-Layer 9 mm PMMA laminated to 9 mm PC (Lexan)
Test Results (Multi-Hit Test)
TABLE-US-00017 [0104] Shot Weights (Grains) Velocity Data (ft/sec)
Penetration Sample Number Projectile Propellant No. 1 No. 2 Average
Description/Sample 1 9-18294 163.3 47 2749 2747 2748 No
Penetration, 12 O'clock 9-18295 163.2 47 2769 267 2768 Penetration,
4 O'clock 9-18296 162.5 47.5 2778 2777 2778 Penetration, 8 O'clock
2 9-18297 162.8 47 2730 2729 2730 No Penetration, 12 O'clock
9-18298 163.4 47.2 2774 2772 2773 No Penetration, 4 O'clock 9-18299
163.7 47.2 2756 2755 2756 No Penetration, 8 O'clock 3 9-18300 163.4
47.2 2740 2737 2739 No Penetration, 12 O'clock 9-18301 162.7 47.2
2778 2777 2778 No Penetration, 4 O'clock 9-18302 163.5 47.2 2798
2798 2798 No Penetration, 8 O'clock
Whereas sample No 1 failed on the 2.sup.nd and the 3.sup.rd shot,
samples No. 2 and No. 3 according to the present invention
withstood all three shots. FIG. 11 shows the strike face of sample
No. 3 after the test, FIG. 12 shows the back (spall layer) of
sample No 3. For samples No. 2 and No. 3, no bulging or other
deformation of the spall layer is observed.
[0105] The entire disclosures of all applications, patents and
publications, cited herein are incorporated by reference
herein.
[0106] Additionally, the following five references provide
background and general knowledge to one skilled in the art and are
incorporated herein by reference. [0107] 1. NATO AEP-55 Volume 1
(Edition 1) February 2005 (PROCEDURES FOR EVALUATING THE PROTECTION
LEVELS OF LOGISTIC AND LIGHT ARMOURED VEHICLES FOR KE AND ARTILLERY
THREATS). [0108] 2. US Military Specification MIL-G-5485D (22 Feb.
1993). [0109] 3. Horsfall et al. A Comparison of the Performance of
Various Light Armour Piercing Ammunition, Journal of Battlefield
Technology, Vol 3, No 3, November 2000. [0110] 4. Moy, P. et al.
Dynamic stress-strain response and failure behaviour of PMMA.
Proceedings of the ASME International Mechanical Engineering
Conference, Washington, D.C., November 2003. [0111] 5. Kinloch A.
I. Fracture Behavior of Polymers, Applied Science Publishers, New
York, N.Y.
[0112] The preceding examples can be repeated with similar success
by substituting the generically or specifically described reactants
and/or operating conditions of this invention for those used in the
preceding examples.
[0113] Without further elaboration, it is believed that one skilled
in the art can, using the preceding description, utilize the
present invention to its fullest extent. The preceding preferred
specific embodiments are, therefore, to be construed as merely
illustrative, and not limitative of the remainder of the disclosure
in any way whatsoever.
[0114] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this invention
and, without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions.
* * * * *