Stock material or miscellaneous articles – Hollow or container type article – Polymer or resin containing
Reexamination Certificate
2000-06-30
2002-06-11
Thomas, Alexander S. (Department: 1772)
Stock material or miscellaneous articles
Hollow or container type article
Polymer or resin containing
C428S036910, C156S173000
Reexamination Certificate
active
06403179
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the fabrication of composite structures, and more particularly to the fabrication of hollow structures having non-circular cross sections.
BACKGROUND OF THE INVENTION
Composite materials are widely used in the field of structural applications. Further, nonconducting composite beams have found particular use in boom trucks—trucks that are used to lift a bucket or similar structure containing a person and/or machinery to an elevated position. Typically trucks of this variety are used for building maintenance, repair/installation of high voltage wires, and similar activities. For such uses and many others, the beam must be capable of withstanding considerable loads and stresses. Also, when used as a boom for high voltage repair and/or installation, the beam must have a high degree in dielectric properties, such that the beam is able to withstand a very high voltage while allowing a very low electric current to pass therethrough.
The filament winding process is often employed in the manufacture of composite beams. In addition, it is often desirable to make rectangular shaped beams—typically, square beams—as well as other forms having flat as opposed to rounded sides. In a typical winding operation, a mandrel of the desired shape of the end resulting beam is utilized. A bundle of fibres, generally impregnated with a resin, are wound along the length of the mandrel in a pattern dictated by the winding angle, which is the angle the fibre makes with the longitudinal axis of the mandrel. This angle is, in turn, dictated by the structural requirements of the end product.
Once the desired thickness for the beam walls is achieved, the mandrel overwound with the composite material is left to cure at room temperature—or in some cases at an elevated temperature depending on the resin. Once cured, the mandrel is removed, and the composite beam structure is ready for use. However, the rectangular tubes made from conventional filament winding process have disadvantages compared with round tubes made from the same process.
In the case of round tube winding, the tension force developed in the resin-impregnated fibres during the winding process maintains a uniform radial pressure on the surface of the tube. As the tension force increases, more radial pressure develops, which squeezes the resin out; and that results in a higher fibre content in the finished laminate. The fibre content in the filament wound round tube can be controlled uniformly throughout the laminate. Fibre content is defined as the percentage of fibre by weight as measured in the cured composite beam. In addition, the wall thickness will be consistent as a result of the uniform radial pressure.
However, in rectangular tube winding, the tangential force arising from the fibre tension against the flat surface is much smaller than in the case of a round tube. Furthermore, the pressure does not develop uniformly. At each corner of a rectangular tube, there will be a higher radial pressure. This uneven distribution of pressure creates an uneven fibre content and inconsistent wall thickness. A normal rectangular filament wound beam typically has rounded corners and exhibits a greater wall thickness toward the central portion of the wall on each side of the rectangle, and a much smaller wall thickness towards the corners. A great many remedies have been explored in an attempt to achieve uniform wall thickness.
In the past, generally speaking, there have been two methods to control the external dimensions of a rectangular beam. First, by grinding the crown down to a flattened state, or second, by pressing the crown down to a flattened state in the curing process. Each method encounters the problem of losing strength due to the cutting of continuous fibres and/or the loosening of the tension that was created in the fibres through the winding process. In each case, the final product has a large corner radius with a thinner wall thickness at each of the corners.
The difficulty associated with inconsistent wall thickness is how that inconsistency affects the beam strength. In beams, especially those used as booms such that they act in essence as a cantilever beam, it is important to ensure high bending strength. In turn, it is also important that the beams have a relatively high shear strength. The required strength of each beam will change depending on the intended end use. In turn, the overall strength may be varied by adjusting the winding angle.
In general, when a beam is supported at least at one end and a load is applied to the other end, it bends. In this simple cantilever bending the beam experiences various types of stresses, their extent depending on the location in the beam. The most critical points in the beam are the bottommost corner points. At these points, compressive stress combined with shear stress is at a maximum in the beam. Thus, the strength at these corners is very important, more so than other areas in the beam. If the wall thickness is thinner at these points, higher stresses develop and may result in failure at early stages of loading. In addition, the wall thickness at these corners influences the beam stability against elastic buckling of the bottom wall which is subject to a compressive stress.
There is a need for an improved approach to the fabrication of a composite structure having a non-circular cross section.
DESCRIPTION OF THE PRIOR ART
U.S. Pat. No. 5,238,716 issued to Adachi teaches a composite beam having a hollow cross-section. The beam is made of three distinct structural layers. An inner structural layer comprises a glass filament winding, a middle structural layer comprises four plates placed around the inner structural layer, and an outer structural layer comprises layers of woven and nonwoven fibre material. Each of the inner and outer structural layers have been saturated with resin. Pressure is applied to the second structural layer to remove all entrapped air. The beam has very high compressive and tensile strength. The difficulty in manufacturing such a beam is the requirement for three separate processes to form the three distinct structural layers, making the procedure time consuming and costly.
U.S. Pat. No. 5,505,030 issued to Michalcewiz et al teaches a composite reinforced structure. The reinforced load supporting structure has an inner load supporting structure and an outer exo-skeleton. The inner load supporting structure, typically a column or beam, is enclosed by a layer of at least one distinct piece of preformed engineering material having both a high tensile strength and high modulus of elasticity which forms the exo-skeleton. The exo-skeleton is used to reinforce the load supporting structure and reinforce areas that typically suffer from cracking. An adhesive substance adheres each of the layers (when there is more than one) of the exo-skeleton to the next. The invention allows for reinforcement of generally any size or shape of object. However, the increase in tensile strength of the beam is derived only from the outer exo-skeleton applied to the inner load supporting structure.
U.S. Pat. No. 5,549,947 issued to Quigley et al teaches both the structure and manufacture of a composite shaft. The composite member has a plurality of plies; an interior ply functioning to dampen sudden forces; an intermediate ply functioning as a load carrier; and an exterior ply being abrasion resistant. At least one of the plies has a biaxial or triaxially braided fibre geometry, where one or more of the fibres are helically wound about the circumference of the composite member. The total bending stiffness of the composite member is divided amongst the three plies. The second ply has a share of the bending stiffness that is greater than either of the first or third plies shares. The present invention is intended primarily for tubular shafts and provides a light weight, high strength composite member.
U.S. Pat. No. 5,688,571 issued to Quigley et al is a continuation in part of the previously described patent. This patent teaches a tubular member havi
(Marks & Clerk)
Thomas Alexander S.
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