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Field of Art

[0001]  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 must withstand handling stresses, significant temperature cycling, and, in the event of rapid acceleration or deceleration of the aircraft, shifting cargo which can be thrown under great force against the sides of the container.

Prior Art

[0002]  One of the oldest tasks known to man is how best to transport his possessions from one place to another. From the very first crude 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.

[0003]  A relative 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 other form of transportation, however, air cargo transport demands that its componentry be not only strong, but lightweight, as additional poundage is more costly with air travel. Additionally, safety has its highest priority in the air, as flight is, even more than ocean travel, intolerant of man's foolishness.

[0004]  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, which 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, which 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), as are the attachment means. At 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 holes in the parts they bring together without "slop", as that creates a structure in which the sheets are stressed in shear when the frames are stressed as a single unit, and is therefore stronger.

[0005]  In use, however, the air cargo container will also be subjected to hoop tension or stress ( i.e., perpendicular to its plane) as when the skins must restrain moving cargo. This is, of course, one of the most important if not the most important function of an air cargo container -- to keep its cargo from breaking through its skin and becoming a missile in the event 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 of 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 be able to withstand both significant shear stress and hoop tension.

[0006]  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 form a rigid assembly. Rivets were preferably used to eliminate the "slop" between rivet shanks and the holes formed for the rivets, as rivets are "holefilling" ( i.e., expand to fill the hole). Containers so made give good useful service, as the structures are rigid, are reasonably light in weight and low in cost.

[0007]  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 the transported cargo) makes such use less desirable and eventually not acceptable beyond a certain level. Using higher strength aluminium to solve the problem is actually counterproductive, as the stronger alloys are more brittle and more readily damaged by tearing. Accordingly, there is a need in the art for an improved skin material for air cargo containers.

[0008]  Polycarbonate sheet has many unique 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 modulus of elasticity; the energy from a potentially damage-inducing blow is absorbed by the sheet without damage as though it were a rubber diaphragm. Hence, polycarbonate plastic sheet would theoretically be an ideal replacement for the aluminium skins. Its transparency would allow the contents of the container to be viewed. It is light in weight, only slightly more costly than the aluminium 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 aluminium 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.

[0009]  One such drawback is its very high coefficient of thermal expansion, 0.000067 m/m/°C (0.000037 in/in/°F). This compares to 0.0000234 (0.000013) for aluminium or 0.0000113 (0.0000063) for steel. If the monocoque structure, the air cargo container for example, must operate in the temperature range of -40°C to 60°C (-40°F to 140°F), as occurs in the air cargo container's service environment (at 9144 m (30,000 ft)) versus in the plane's fuselage, on the tarmac, in the hot, desert sun, a typical air cargo-sized panel which is 304.8 cm (120 in) between rivet centres when the panel was manufactured at an ambient temperature of 10°C (50°F) will be 305.82 cm (120.4 in) in length when the temperature is 60°C (140°F) and 303.78 cm (119.6 in) in length when the temperature is -40°C (-40°F). In contrast, the distance between rivet centres of the aluminium structure will be 305.16 cm (120.14 in) at 60°C (140°F) and 304.44 cm (119.86 in) at -40°C (-40°F) as the coefficient of linear expansion for aluminium is far less. Thus, conventional wisdom 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 0.66 cm (0.26 in) on each side of the panel, allowing for a differential expansion between the polycarbonate sheet and the aluminium frame of 1.32 cm (0.52 in) total.

[0010]  The resultant structure would, however, be at a severe disadvantage compared to its all-aluminium counterpart. The looseness or "slop" 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.

[0011]  Another disadvantage of the polycarbonate which has heretofore prevented its use in air cargo containers is its very low bearing strength 8.6x10⁸ Pa (12,500 psi) compared to 6.89x10⁸ Pa (100,000 psi) for the aluminium alloys used for air cargo container sheets. In other words, the polycarbonate is 1/8 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 aluminium skins. This would means higher costs for the fasteners and the labour for installation, in addition to the heavy, costly frame structures. The resultant structure would be too heavy and costly to compete with the all-aluminium container.

[0012]  There has heretofore been yet another disadvantage to the polycarbonate's use on air cargo containers; namely, its susceptibility to stress-induced 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". These include a variety 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. If the residual operational stresses are kept low, for example, under 1.38x10⁷ Pa (2000 psi), and 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 "Catch-22" 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.

[0013]  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 component. For example, in the reference book published by the principal manufacturer of polycarbonate sheets, the means and methods displayed for attaching the sheets specify loosely torqued bolts 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. Accordingly, there has existed a need in the art for a means for rigidly attaching polycarbonate sheet material to a metal structure element in a way to allow the polycarbonate to act as a structural component while, at the same time, eliminating or substantially alleviating the material's tendency to crack or craze under stress.

[0014]  US-A-4 833 771 discloses an assembly in accordance with the prior art portion of claim 1 and a method of attaching a polycarbonate sheet to a metal structural member as referred to in the prior art portion of claim 14. With this prior proposal, a relatively complicated securement means is provided in that specially shaped corrugated edges need to be provided at the edges of the polycarbonate sheet with clamping means engaging within these corrugations to secure the polycarbonate sheet to the structural member. This can lead to localised stresses at the junction of the sheet with the corrugated edge thereof and also provides complications in mounting the sheet to a frame, as in an air cargo container.

[0015]  It has been discovered that by providing the novel attachment means of this invention as defined in claim 1 and by using the method as defined in claim 14, the polycarbonate can 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 amount 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 uses 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 length of the attachment area. Conventional bolt or rivet means are used to attach this assembly together under sufficient torque to prevent slippage.

[0016]  In an alternative 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.

[0017]  This invention solves each of the aforementioned drawbacks which had previously prevented the use of the polycarbonate sheets as a structural element; such as the skin in commercial air cargo containers. After the clamping bolts or rivets are torqued 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 is spread over a 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 costs of assembly.

[0018]  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 hold 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 structure 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 this invention.

[0019]  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.

[0020]  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.

Description of the Figures

[0021]  Figure 1 is a plan view showing the polycarbonate sheet assembled to the metal structural member.

[0022]  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.

[0023]  Figure 3 is a similar cross-sectional view, showing the alternate embodiment of the bolting strip, here in the untorqued or flexed position.

[0024]  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 view of the attachment strip is exactly the same as that shown in Figure 2, except that it is thinner and therefore lighter in weight.

[0025]  Figure 5 shows an air cargo container in which 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

[0026]  Referring to Figure 1, the components of the attachment means are the structural metal member 10 (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.

[0027]  Looking at Figure 2, the structural member 10 is commonly L-shaped and will have 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 12 is caused to overlap a portion of the structural member 10, such that an attachment area (as defined in Figure 1 by the area bounded on the top by line 20, on the bottom by line 22, on the left by line 24 and on the right by line 26) 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.

[0028]  The first embodiment of the attachment strip 14 as shown in Figure 2 is, in both the torqued and untorqued 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 stresses resulting from the torquing 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 1.59cm (5/8-inch) in diameter. Taking into account the 0.64cm (1/4-inch) diameter hole 18, the entire compressive force would therefore be concentrated upon approximately 16.65sq cm (2.58 square inches) of the polycarbonate sheet. If the rivet 16 is tightened to a torque of approximately 0.55 metre-kg (48 inch-pounds) (which is typical with some air cargo containers), the resultant force on the polycarbonate sheet is 2.05x10⁷ Pa (2,976 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 0.95cm (3/8-inch) wide, and the rivets 16 are affixed on 6.35cm (2 1/2-inch) centers, the effective area under compression for each rivet 16 is approximately 12.10sq cm (1.875) square inches which results in a stress of 2.8x10⁶ Pa (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 1.1 metre-kg (96 inch-pounds) which with the attachment means here described, would result only in 4.29x10⁶ Pa (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 6.89x10⁶ Pa (1,000 lbs per square inch) tension or compression.

[0029]  In Figures 3 and 4, the alternate embodiment of the attachment strip is depicted in cross-section. 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 on the upper portion of the strip 40 will cause it to straighten, bringing legs 44 flush against the sheet 12, and accordingly provide uniform compression loads over the entire attachment area, as shown in Figure 4. This alternate embodiment is used when the torque loads are high and the 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 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.

[0030]  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 cross-members 60 are added for strength and stability.