Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
W~ 91/77934 ~ ~ ;~ ~y ~ t~ ~ hCT/U~97/03577
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oESrzorr
Method and Apparatus for Structural Attac~ hment
of Polycarbonate P1_astic Sheet to_
Su~portinet Strena~k:h Members and
Air Carao container Utilizina Same
Backaround
Field of Art
This invention pertains to a method and apparatus by
which a metal structural element and a polycarbonate sheet
are attached together under torque by means of an
attachment strip. It is believed that 'the invention will
find at least a first primary use in air cargo containers
wherein polycarbonate sheets are used as the siding or
'skin°~ of the containers and mint withstand handling
stxesses, significant temperature cycling, and, in the
e'~ent of rapid acceleration or deceleration of the
aircx°aft, shifting cargo which can be thrown under great
force against t:he sides of the container.
Prior Art
Dne of the oldest tasks known to man is how best to
transport his possessions from one place to another. ,From
the very first cxude bags made of animal hide to the space
shuttle, man has been engaged in a continuous attempt to
develop means to transport cargo farther, faster, safer,
cheaper and easier.
A x~elat:ive newcomer in this millennia of
transportation is the aircraft, and although relatively
new, it is now a major player in transporting the property
of man: More than any a~l:her farm of transportation,
however, air cargo transport demands that its componentry
be not only strong, bwt lightweight, as additional
poundage is more costly with air travel , Additionally, ; .
safety has its highest priority in the air, as flight is,
a 67
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even more than ocean travel, intolerant of man's
foolishness.
Therefore, the transportation of cargo by air
requires, like no other, that the often elusive goals of
strength, light weight and safety be accomplished in a
single structure. For the transportation of cargo by air,
the industry has come to rely almost exclusively on the
all-aluminum cargo container, rahich is first loaded with
cargo and is then itself loaded onto the aircraft. This
modern air cargo container is a monocoque structure,
comprising a rigid frame to which a sheet material,
generally referred to as the °°skin", is attached to the
'°bones" of the frame. In these monocoque structures, the
skin is load- carrying, sharing the stresses with the
frame structure. The loads go from the frame to the skin
or from the skin to the frame via their attachment means,
~rtaich can be rivets, bolts, etc. In construction and at
rest, the skin is usually stressed in shear (meaning along
the plane of the sheet rather than perpendicular to it),
2~ as are the attachment means. ~,t the attachment points the
holes in the sheets and frame are formed as close to the
diameter of the fasteners as is practical 'to make the
structure as rigid as possible. Clearance between the
holes and the fasteners creates '°slop" between the parts
and therefore reduces the rigidity of the structure as
relative movement between the sheet and frame create a
"loose," and 'therefore weak, assembly. The ideal
fasteners completely fill the hales in the parts they
bring together without "slop'°, as that creates a structure
in cahich the :sheets are stressed in shear when 'the frames
are stressed as a single unit, and is therefore stronger.
In use, however, the air cargo cantair~er will also be
subjected to hoop tension or stress (i<~., perpendicular
to its plane) as when the skins must restrain moving
cargo. This is, of course, one of the most important it
not the most important function of an air cargo container
-- to keep its cargo from breaking through its skin and
VVO 91/1793n f(.'~'/~J5")I/4f3~i17
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becoming a missile in the :vent of crash-generating
deceleration forces on the aircraft or in the event of
turbulence inducing either severe acceleration or
deceleration forces. In those flight-threatening events
during which accelerations or decelerations occur, the
cargo moves against the skins caf the container which are
thusly stressed in hoop tension which is transferred to
the frame, then to the floor locks, then to the floor of .
the aircraft and eventually to the airplane itself.
Hence, the skin material of the container must toe able to
withstand both significant shear stress and hoop tension.
For obvious reasons, the ideal air cargo container is
light in weight, low in cost and capable of withstanding
not only the stress encountered inflight, but also the
day-to-day rigors of service -- i.e., cargo crates being
thrown against the walls, being bumped and.jostled -- all
without being damaged. The best prior art devices used
aluminum frames and skins with the sections being riveted
together to farm a rigid assembly. Rivets wire preferably
used to eliminate the ~~slop°~ between rivet shanks and the
holes formed for the rivets, as rivets are ~tholefilling'~
(i.e., expand to fill the hale). Containers so made give
good useful service, as the structures are rigid, are
reasonably light in weight and low in cost.
The main problems these all--aluminum devices
encountered in service were with the aluminum skins as
they are subject to denting and tearing. Rough use and
sharp-cornered boxes take their toll on 'the skins and
often produce tears and dents. When torn, the containers
are not serviceable as they are no longer '~airworthy" and
must be taken out of service and patched before they can
be used again. Furthermore, torn skins present a hazard
to loading crews and the cargo as the sharp edges cut
indiscriminately. The aluminum skins can be made more
resistant to such damage by making 'them thicker and more
resistant to tearing, but then weight increases and the
cost of flying dead weight (i.e., other than the weight of
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the transported carg~) makes such use less desirable and
eventually not acceptable beyorGd a certain level. 'gsing
higher strength aluminum to solve the problem is actually
counterproductive, as the stronger alloys are more brittle
and more readily damaged by tearing. Accordingly, there I ,
is a need in the art for an improved skin material for air
cargo containers.
polycarbonate sheet has many unic,~xe qualities making
its use desirable in many industrial applications. :It is
transparent. It can be struck heavily without being
dented, torn or broken. This is because of the material's
very low modules of elastici~typ the energy from a
potentially damage-inducing blow is absorbed by the sheet
without damage as though it were a rubber diaphragm.
~ience, polycarbonate plastic sheet would theoretically be
an ideal replacement for the aluminum skins. Its
transparency would allow the contents of the container to
be viewed. Lt is light in weight, only slightly more
costly than the aluminum alloys used and capable of
accepting the rough rigors of service without being dented
or torn, as it, is much more resistant to tearing or
denting than is aluminum of comparable thickness and
weight. The polycarbonate has substantial drawbacks to
its use, however, which until now rendered it not feasible
for use as a structural element and certainly not as the
skin in a monocoque structure such as an air cargo
container.
One such drawback is its very high coefficient of
thermal. expansion, .000037 inches per inch per degree
Fahrenheit. 'this compares to .000013 for aluminum or
.0000063 for steel. If the monocoque structure, the air
cargo container for example, must operate in the
temperature range of -40 ° F to + 1~ 0 ° F, as occurs in the
aa.r cargo container's service environment (at 30,000 feet
versus in the plane's fuselage, on the tarmac, in 'the hot,
desert sun), a typical air cargo-sized panel which is 120
inches between rivet centers when the panel was
W~ 91 /17934 ~ ~ ~ e~ ~ Z~ ~ ~~'/'~~91 /x'3577
manufactured at an ambient temperature of 50°F will be
120.4 inches in length (120 inches x 90°F temperature
differential x .000037 coefficient) when the temperature '
is 140°F and 119.6 inches in length when 'the temperature
- 5 is -40°F. In contrast, the distance between rivet centers
of the aluminum structure will be 120.14 inches at 140°F
and 119.86 at -40°F as the coefficient of linear expansion
for aluminum is far less. Thus, conventional wisdam has
in the past dictated that in order for the polycarbonate
sheet to be compatible within 'this type environment the
holes would have to be oversized in diameter (or slotted)
by .26 inches (120.4 - 120.14 + 119.86 °° 119.6) on each
side of, the panel, allowing for a differential expansion
between the polycarbonate sheet and the aluminum frame of
.52 inches total.
The resultant structure would, however, be at a
severe disadvantage compared to its all-aluminum
counterpart. The looseness or "slap" of the fasteners in
the holes would prevent the sheet and the frame from
acting as a load-sharing single unit. Therefore air cargo
containers using polycarbonate sheets and conventional
attachment means would have to bear the shear loads in the
frame alone, which would have to be made larger in order
to be stronger, and would therefore be excessively heavy,
Another disadvantage of the polycarbonate which has
heretofore prevented its use in air cargo containers is
.its very low bearing strength, 12,500 psi compared to
100,000 psi fox the aluminum alloys used for air cargo
container sheets. In other words, the polycarbonate is
one-eighth as strong in bearing. To compensate, the
polycarbonate skin would have to be attached to the frame
at many more locations than is necessary with aluminum
skins. This would mean higher costs for the fas'terlerS and
the labor for installation, in addition to the heavy,
costly frame si:ructures. The resultant s~tru,cture would be
too heavy and costly to compete with 'the all-aluminum
container.
dV~D 91/1793A 4~ , ~, ~.7 ~.~ ~~ ~ ~~[.'T/~.1~)1/03577
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There has heretofore been yet another disadvantage to
the polycarbonate's use on air cargo containers; namely,
its susceptibility to stress~i:nduced and crazing agent
induced cracking or crazing. When there are residual
stresses in polycarbonate, the, material is subject to
cracking, particularly in the presence of "crazing
agents°'. Theca include a varieay of materials including
hydrocarbons, jet fuel cleaning' materials, etc., many of
which are used near the air cargo containers. A cracked
polycarbonate sheet is non-serviceworthy as once cracked,
the cracks spread very easily. One crack and the part
must be taken out of service. Tf the residual operational
stresses are kept low, far example, under 2000 psi, arid
the materials are kept free of "crazing agents,'° the
material is relatively free of this incipient cracking
problem. As explained above, however, this creates a
classic °°C;atch~-2.2°' situation in that an unstressed
sheet
would require such a heavy frame that the resultant
container would be unuseable, whereas riveting the sheet
to the frame so that the overall container is unitarily
stressed creates a crack-inducing environment, as high
stresses are created under the head of the rivet and
against the inside of the hole by the expanding rivet
shank.
Because of these disadvantages, the use of
polycarbonate has heretofore been restricted to
applications where it "floats°' in its frame, as in signs
and aircraft windows, and has not been used as a genuine
structural companent. For example, in the reference hook
published by the principal manufacturer of poiycarbonate
sheets, the means and methods displayed for attac2xing the
sheets specify loosely torqued baits in oversized holes
with a silicone cushion. Certainly, polycarbonate sheet
material has not heretofore proven to be an acceptable
substitute for the aluminum "skin" on a monocoque airline
cargo structure because no acceptable means for attaching
the polycarbonate to the aluminum frame was known.
W~ 91/17934 fa;'t'lUS9t/435J7
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Accordingly, there has existed a need in the art for a
means for rigidly attaching polycarbonate sheet material
to a metal structural element in a way to allow the
polycarbonate to act as a structural component, while at
_the same eliminating or substantially alleviating the
material's tendency to crack or craze under stress.
Summary of Invention
Tt has been discovered that by providing the novel
attachment means of this invention, the polycarbonate can
ZO be attached to the metal structural elements in a non~
slip manner which does not induce cracking or crazing of
the polycarbonate. The means of attachment comprises
having the polycarbonate sheet overlap the metal
structural member by a substantial amaunt to create an
attachment area. Rather than attaching the,polycarbonate
to the metal by conventional, inter-spaced bolts or
. rivets, the device of this invention eases an attachment
strip which is essentially a u-shaped channel member
having a width not substantially less than the width of
the attachment area and which extends substantially the
entire lErngth of the attachment area. conventional bolt
or rivet means are used to attach this assembly together
under sufficient torque to prevent slippage.
In an alternate embodiment intended for high-torque
applications, the bolting strip is flexed slightly in the
untorqued condition, the face of which is then brought
,, flush against the polycarbonate sheet in 'the torqued .
condition.
ThlS lr1VE31'1'~1(~T ~nl~s~c nani, ...p a-~,... _.s_~_____~~ .
drawbacks which had previously prevented the use of
polycarbonate sheets as a structural element. such as the
skin in comms:rcial air cargo containers. After the
clamping bolts or rivets are tarqued in place, the
strength of 'the resultant assembly is the sum of the
strength of the sheet in bearing and the friction induced
by the clamping. The force of clamping i.s spread over a
Va~O 9 a / 17931
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broad area, not just under the fastener (as under the
washer of a bolted joint or under the rivet head in a
riveted joint) such that the joint is protected from high
incipient stress levels and consequent cracking due to
crazing from stresses and crazing agents. Also, because
the large attachment strip spreads the attachment force
over a large area and hence provides sufficient friction,
the holes through which the bolts or rivets are inserted
in the sheets can be over-sized so as eliminate the
possiblility of creating excessively high localized stress
levels within the hole itself. Being rigidly clamped to
the frame by the attachment strip, however, the assembly
still works as a single unit sharing the stresses, as does
the riveted all~aluminum structure, wherein the sheets are
stressed in shear and hoop tension and the frame in
bearing. As the strength due to friction is substantial,
fewer fasteners are required for the clamping system than
for an exactly comparable all-aluminum structure,
therefore reducing the casts of assembly.
a0 It has also been found that the use of this invention
also overcomes the drawback inherent in the great
difference between the coefficient of thermal expansion of
'the polycarbonate sheet and the aluminum frame.
Specifically, it was found that the high clamping forces
achieved with this invention bald the polycarbonate sheet
so tightly in the frame that when the temperature is
reduced the sheets do not shorten, Instead, as the
temperature drops, the sheets pull inwardly, but the
clamps are sufficiently tight to prevent slippage and the
sheets become stretched tightly in the frame s'truc'ture as
a head of a drum and 'the sheet thickness actually gets
thinner rather than the overall length of the sheet
reducing! The low elastic modulus of the the '
polycarbonate permits the tightening of the sheet in the
frame without pulling loose from the clamped assembly of
:his invention. '
CA 02063392 1998-03-16
9
Highly torqued bolts are required to clamp the
polycarbonate sheets to the frames in certain structures to
overcome stress due to heavy handling or extreme temperature
cycling. Although there is clearance between the bolt shanks
and holes in the polycarbonate sheet (to avoid high localized
stress) there is no "slop" in the structure; the high friction
forces make the assembly act as a unit which permits a lighter
and less costly frame structure acceptable for air cargo use.
In sum, it is now possible for the first time to use
polycarbonate as a structural material in a monocoque
structure, rigidly attaching it to the metal frame and thereby
loading it in both shear and hoop tension and using all of the
benefits the material offers, without subjecting the
structures to the dangers of cracking due to the residual
stresses and crazing agents, and still having a container that
exhibits the strength and light-weight of its all-aluminum
counterpart. The novel attachment means by which this is
accomplished and the novel air cargo container utilizing
polycarbonate sheet as a structural component are described
and depicted hereinafter in detail.
In accordance with the present invention, there is
provided an assembly comprising a metal structural member to
which a polycarbonate sheet is attached by attachment means,
said attachment means comprising: a) said polycarbonate
sheet overlapping said structural member along substantially
the entire length of the polycarbonate sheet to create an
attachment area; b) an attachment strip having a width
substantially the same as the width of said attachment area,
60724-2027
CA 02063392 1998-03-16
9a
and having a length substantially the same as the length of
said attachment area; c) said attachment strip having a
channel formed therein on the side adjacent said polycarbonate
sheet; and d) fasteners passing through said structural
member, polycarbonate sheet and attachment strip for holding
those elements rigidly together.
In accordance with the present invention, there is
further provided an assembly comprising a metal structural
member to which a polycarbonate sheet is attached by
attachment means, said attachment means comprising: a) said
polycarbonate sheet overlapping said structural member such
that the area of overlap is approximately 1~ inches along
substantially the entire length of said polycarbonate sheet;
b) an attachment strip having substantially the same length
and width as said area of overlap; c) said attachment strip
having a channel formed in one side thereof adjacent said
polycarbonate strip such that as assembled the entire side of
said attachment strip will not be in contact with said
polycarbonate sheet, and only an area approximately ~ inch
wide and being the same length as said attachment strip on
either side of said channel will contact said polycarbonate
sheet; and d) bolts on approximately 2~ inch centers inserted
through apertures in said structural member, said
polycarbonate sheet and said attachment strip and torqued to
not more than 100 inch-pounds.
In accordance with the present invention, there is
further provided a method for attaching a polycarbonate sheet
to a metal structural member comprising the steps of: a)
60724-2027
CA 02063392 1998-03-16
9b
overlapping said polycarbonate sheet onto said structural
member such that an attachment area is formed; placing an
attachment strip having a length and width approximately equal
to that of said attachment area against the attachment area,
said attachment strip having a channel formed therein on the
side adjacent said polycarbonate sheet such that only a
portion on the outer edges of said strip come into contact
with said sheet; c) installing fasteners into spaced and
aligned holes through the assembly of said metal structural
member, said polycarbonate sheet and said attachment strip;
and d) tightening the fasteners to create a non-slip union
between said member and said sheet.
Description of the Figures
Figure 1 is a plan view showing the polycarbonate
sheet assembled to the metal structural member.
Figure 2 is a cross-section, taken along line 2-2 in
Figures 1 and 5 showing the polycarbonate sheet "sandwiched"
between the metal structural member and the attachment. Here,
a rivet is shown rather than a conventional bolt.
Figure 3 is a similar cross-sectional view, showing
the alternate embodiment of the bolting strip, here in the
untorqued or flexed position.
Figure 4 shows the alternate embodiment of the
bolting strip in Figure 3, but in the torqued position. It is
noted that in this condition, the cross-sectional
60724-2027
WO 91 / 17934 ~ , d7 ~ ~ 4~, Y''C T/1J~91 /03x7 l
~l~~cle.~~r.~~r
view of the attachment strip i~; exactly the same as that
shown in Ffigure 2, except that it is ~tha_nner and therefore
lighter in weight.
Figure 5 shows an air cargo container in which
5 polycarbonate sheets are rigidly attached as the skin and
as a structural component using the attachment strip
assembly depicted in Figures 1 and 2.
Description of the Preferred Embodiment
Referring to Figure 1, the components of the
10 attachment means are the structural mytal member l0 ( it
can be either steel or aluminum preferably); the
polycarbonate sheet 12; the attachment strip 14
(preferably of the same material as the member 10) ; and
the rivets or bolts 16, which are inserted through
approximately-sized (so as to avoid intra-hole stress)
holes 18.
Looking at Figure 2, the structural member 10 is
commonly L-shaped and will nave another polycarbonate
sheet 10 (not shown) attached to its opposite side, In
Figures 2 through 4, it is seen that in this assembly the
polycarbonate panel l2 is caused to overlap a portion of
the structural member 10, such that an attachment area (as
defined in Figure l by the area bounded on the top by line
20, on the battom by line.22, on the left by line 24 and
on the right by line 2H) is created. It will be
understood that Figure 1 is '°cut-away°' on the top and
bottom. The actual assembly extends for a considerable
distance and the area of overlap and hence the attachment
area will also continue for substantially the entire
length of the polycarbonate sheet 12.
The firsts embodiment of the attachment strip 14 as
shown in Figure 2 is, in both the torqued and untorc~ued
condition, planar on all major surfaces, and has a channel
28 formed centrally on the side adjacent to the
polycarbonate sheet 12, such that :Legs 30 are created.
This is provided to relieve and distribute the compressive
WO 91/1793A c~ ~ ~ ~-~ '~ (~~ ~ feCT/U~9~/03~77
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stresses resulting from the tarquing of the bolt or rivet
16. Instead on being concentrated under the bolt or rivet
head, substantial contact areas are provided not only
adjacent to the rivet, but also linearly therebetween. If
the rivet 16 were attached directly to the polycarbonate
sheet 12 (in other words, without the attachment strip
14), the compressive forces under the rivet head would
extend outwardly to about 5/8-inch in diameter. Taking
into account the 1/4-inch diameter hole 18, the entire
compressive force would therefore be concentrated upon
approximately 2.58 square inches of the polycarbonate
sheet. If the rivet 16 is tightened to a torque of
approximately 48 inch-pounds (which is typical with some
air cargo containers); the resultant force on the
polycarbonate sheet is 2,376 pounds per square inch. This
amount of stress is very prone to cause crazing. If,
using this invention on the other hand, the legs 30 of the
attachment strip 14 are each 3/8-inch wide, and the rivets
16 are affixed on 2 1/2-inch centers, the effective area
under compression for each rivet 16 is approximately 1.875
square.inches which results in a stress of 410 pounds per
square inch. This amount of stress does not promote
crazing. In fact, the torque on the rivets 16 could be
increased ~to 96 inch-pounds which, with the at~taehment
means here described, would result only in 622 pounds per
square inch of stress on the polycarbonate sheet 12.
There would not be a danger of crazing at this stress
level since polycarbonate is susceptible to crazing in the
presence of crazing agents at stress levels over 1,000 per
square inch tension or compression,
In Figure:> :3 and 4, the alternate embodiment of the
attachment str~.p is depicted in cross-~seetion. Here, the
strip is pre-formed in a flexed or concave shape. As in
the previously embodiment, a central channel 42 is formed
on its underside to create legs 44. The torque forces
pressing downward an the upper portion of the strip 40
will cause it to straighten, bringing legs 44 flush
WO 91/17934 c° 4 ~n ~~ -1 ~''~
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against the sheet 12, and accordingly provide uniform
compression loads over the en~:ire attachment area, as
shown in Figure 4. This alternai~e embodiment is used when
the torque loads are high and th.e strips are made thin to
save cost and weight. If the higher torque loads were
applied to a thin, flat strip, 'there is danger of stress
concentration on the inner edges of channel 42. This
stress concentration could provide an uneven load on the
polycarbonate sheet, thereby subjecting the sheet at
l0 certain points to increased stress and a possibility of
crazing failure. It will be appreciated that with this
invention the amount of torqued applied to the rivet
should be closely controlled. The size of hole 18 should
be sufficiently large; and 'the torque on the rivet
sufficiently low to prevent intra-hole stress.
As mentioned, it is believed that the use of the
attachment strip assembly previously described will find
a first utility in monocoque air cargo containers, such as
that shown in side view in Figure 5. It comprises the
metal (preferably aluminum) base 50, to which a frame 52
of metal (preferably aluminum) structural members 54 are
attached by conventional rivet, bolt or welding means {not
shown), and to which the polycarbonate sheets 56 are
attached using the assembly described and shown above.
The attachment strip 14 is shown in shadow. A door (not
shown) is provided in the front panel section of the
container. As can be seen, the packages in the container
are visible through the polycarbonate sheet. Corner
gussets 58 and crass-members 60 are added for strength and
stability.
Although specific embodiments of dais invention have
been set forth above, it should' be apparent to those
skilled in the art that other modi.ficatio.ns upon those
embodiments would be possible without departing from 'the
inventive concepts hereinafter claimed. Accordingly, this
patent and the protection provided by it are not limited
to the specific embodiments set forth above, bLnt are of
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the full larea~h and scope of each of 'the appended claims
nr their equivalence.
