Composites consist of one or more discontinuous phases, such as filament reinforcements, embedded in a continuous phase, such as a thermosetting resin matrix. Composite materials offer a way to improve mechanical properties such as strength, stiffness, toughness and high temperature performance. Properties of composites are strongly influenced by the properties of their constituent materials, their distribution and the interaction among them. In describing a composite material, besides specifying the constituent materials and their properties, one needs to specify the geometry of the reinforcement with reference to the system. The geometry of the reinforcement may be described by the shape, size and size distribution. The composite material of the present invention comprises continuous filaments as the reinforcement or discontinuous phase and a hardenable liquid matrix as the continuous phase. The geometry of the reinforcements employed in the present invention can be described as continuous filaments of uniform diameter having manufactured lengths of several hundred meters and filament diameters typically ranging from 7 microns (0.00028 in.) to 25 microns (0.001 in.). The concentration of the filament reinforcements comprising the composite material of the present inventions typically ranges from 45 to 60 percent by volume. The composite material of the present invention most closely resembles the class of composites known as unidirectional composites.
An important mechanical property and design parameter of unidirected composite materials is herein referred to as the "transverse shear strength". This property is the shear strength of a bundle of filament strands held together by a hardened matrix. It is also referred to as the "across strand" shear strength, the value of which can be determined by the ASTM test method D-732 which uses a punch type shear tool and a composite structure test specimen measuring 2".times.2" and comprising a single thickness of filament strands oriented approximately parallel to each other. The composite structure test specimen is clamped between the punch holder so that the filaments are oriented at right angles to the punch face. Shear strength tests performed by ASTM D-732 method show that the transverse shear strength of a single ply of a unidirected twined strand composite comprising sinusoidally oriented glass filament reinforcements impregnated with a hardenable polyester resin matrix ranges from 172 to 241 MN/m.sup.2 (25,000 to 35,000 PSI). This substantially exceeds the interlaminar "in plane" shear strength of conventional reinforced thermosetting plastics determined by ASTM D-3846.
It is necessary to distinguish a unidirectional "ply" from a unidirectional "laminate" if the discoveries disclosed in the present invention are to be clearly understood. Prior art unidirectional laminates, such as comprise prior art filament wound and pre-preg ribbon layered composites, are constructed not from individual filaments but from "tows" or "strands" containing numerous filaments. It is well known that each filament strand or tow contains hundreds and often thousands of individual filaments which, especially in the case of the glass filament strands called "roving", are not exactly parallel to each other but are twisted in a loose untangled manner after being coated with a dissolvable fiber size that converts the bundle of filaments into a strand to provide what is referred to as strand integrity. These strands of glass filaments are wound onto a collet to form a primary package or "cake" to facilitate their use in filament winding and pultrusion operations. Typically these "cakes" or roving packages are hollow cylinders that enable the strands to be fed by pulling from the center or interior of the package in order to eliminate mounting and rotating the roving package while the filament strand is fed or pulled from the roving package. Such center pull operations impart a further twist to the filaments contained in the strand. The amount of strand twist is governed by the roving package size as well as the "way wind" number used in making the package. Generally the "way wind" number is between 2.7 and 4.1 which means that for a 6 inch inside diameter cake the filaments are twisted completely 360.degree. at least once for each meter of strand pulled from the roving package. Unless corrected, this strand twist serves to reduce by as much as half the optimum tensile strength attainable from unidirectional laminates fabricated from untwisted strands. This is due to the fact that in a twisted strand the individual filament lengths resisting a tensile load are not exactly equal and thus only the shortest length filaments are those that primarily resist a tensile load. Because carbon and aramid filament reinforcement is from five to thirty times more expensive than glass filament reinforcement the strands of carbon and aramid filament are made, packaged and dispensed in a manner that reduces the twist and strength loss of carbon and aramid filament strands.
An idealized unidirectional composite is one which consists of parallel untwisted continuous filaments embedded in a matrix. A prior art "laminate ply" results when two or more unidirectional layers are stacked in a specified sequence of orientation to fabricate a composite structure. Each layer of unidirectional twined strands discussed in the present invention is referred to as a "ply" to distinguish it from prior art stacks of thin laminates which are conventionally referred to as a "laminate ply". A "ply" is herein defined as made from one or more tensioned and unidirected approximately parallel "twines". A "twine" is herein defined as comprising three or more unidirected filament strands twisted together to provide a sinusoidal wave-like configuration to each strand. A ply made from twined strands typically has a thickness to filament diameter ratio of at least fifty whereas a typical prior art unidirectional composite "laminate" comprises untwisted parallel strands and has a thickness to filament diameter ratio ranging from ten to forty. The ideal orientation of the sequence of plies made from unidirectional sinusoidally configured twined strands described in this invention is either 0.degree. or 90.degree. with respect to each other with a manufacturing deviation that approximates plus or minus 10.degree..
Prior art unidirectional composite tubular laminates made by conventional filament winding apparatus and method are fabricated by use of strand feed, impregnation and strand placement techniques that endeavor to minimize the twist of filaments and filament-containing strands and thereby minimize the concomitant loss of laminate tensile strength resulting from twisted filament strand reinforcements. The unidirectional twined strand composite plies of the present invention are constructed by methods and with apparatus that intentionally increases rather than decreases twisting of the unidirectional filament reinforcements to provide the desired sinusoidal orientation of the strand filaments. The reason is most clearly understood when it is realized that, although important, tensile strength is not the only property required of a unidirectional composite, especially if the composite is to serve as a spring or a frequently flexed structure. This invention teaches that the sinusoidal wave-like arrangement of twined filament strands not only greatly increases the stiffness of a unidirectional composite ply structure made therefrom but also greatly facilitates the fabrication of tubular composites constructed from two or more biaxial plies.
Prior art composite tubular structures subjected to free-end closure pressure stress (ASTM D-1598 and D-2992) are constructed in a manner that requires the tubular wall structure to simultaneously resist longitudinal and circumferential stresses. In composite pressure vessels, such as closed-end pipe, the circumferential hoop stress, S.sub.C, is always double the longitudinal end load stress, S.sub.L, and is calculated from the formula S.sub.C =PD/2t.sub.c ; S.sub.L =PD/4t.sub.c where "P" is the internal pressure to which the tube is subjected, "D" is the outside diameter of the tube, "t.sub.c " is the proportional thickness of the tube wall material which resists the circumferential stress, and "t.sub.L " is the proportional thickness of the tube wall material which resists the longitudinal stress. This invention teaches that a single "CIRC" ply of circumferentially disposed twines having a thickness, "t.sub.c ", and constructed upon an impermeable tubular membrane can comprise the tube wall material used to resist the circumferential tube hoop stress, "S.sub.C ", and that a single "LONGO" ply of longitudinally disposed twines having a thickness "t.sub.L " and constructed upon the CIRC ply can comprise the tube wall material used to resist the tube end load longitudinal stress, "S.sub.L ".
Prior art composite tubular structures used as pressure vessels exhibit a change in overall length and diameter that greatly depends upon the behaviour of the matrix material bonding together the individual laminate plies. Theoretically, the change in tube diameter ".DELTA.D" of such prior art structures having a diameter, "D", can be calculated from the formula: .DELTA.D=.epsilon..sub.c D where .epsilon..sub.c =.sup.S c/E.sub.c and is the circumferential strain value produced by the hoop stress, "S.sub.c ", in a composite tube material having a tensile modulus equal to "E.sub.c ". In a simular manner the change in tube length, ".DELTA.L" of such prior art structures having a length, "L", can theoretically be calculated from the formula .DELTA.L=.epsilon..sub.L L where .epsilon..sub.L =.sup.S L/E.sub.L and is the longitudinal strain value produced by the longitudinal stress, "S.sub.L ", in a composite tube material having a tensile modulus equal to "E.sub.L ". Unfortunately, the tensile modulus values E.sub.c and E.sub.T for prior art composite tube laminate ply materials are unpredictably influenced by the tensile modulus values and the Poisson ratio values of the matrix materials used to bond together the laminate plies. For this reason, the location and magnitude of the changes in length and diameter of prior art composite laminate tubes, especially those constructed of a multitude of helically disposed laminate ribbons of unidirectional filament strands, cannot be reliably predicted or calculated.
The present invention teaches that by use of twines of helically configured strands which are placed adjacent to each other and separated by a compatible interface material having a tensile strength less than or equal to the hardened bonding matrix used to impregnate and bond together the helically configured twine strands, the location and magnitude of the changes in diameter and length of a pressurized composite tube structure can be reliably predicted and calculated.
The structural integrity of prior art composite structures made from multiple layers of unidirected filament laminates is governed by the integrity of the matrix material used to bond the laminates together. For this reason prior art composite structures degrade in performance over time as the bonding strength of matrix material is reduced by the micro fractures between the matrix layer and the laminate resulting from cyclic stresses. The micro fractures in the laminate bonding matrix not only reduce the peel strength and interlaminar shear strength of the matrix but also promote filament bundle wicking by exposing edges and surfaces of the laminate to liquids, vapors or gases.
This invention teaches that a substantial increase in composite structure durability results when the structure is made of twine plies which comprise helically configured strands. The twine strand helical configuration disclosed in this invention provides a means for composite structures to be independent of the interlaminar shear strength, peel strength, and micro fractures associated with the matrix material used to impregnate and bond together the filament reinforcements, and thereby experience substantially less degradation in stiffness and other physical properties.
With the advent of high pressure composite pipe which can be rapidly and mechanically joined to provide a permanently sealed connection it is no longer necessary for pipe engineers to depend exclusively upon welded steel pipe as the most reliable and economical method to transport water, oil, gas and slurry products. In addition to such features as high strength to weight and long term resistance to cyclic fatigue and corrosion, composite pipe has an extremely smooth inner surface which reduces fluid flow friction and thus lowers the cost to pump product through the pipe.
The single most important feature that governs economic comparisons among pipe of equivalent linear foot cost and performance is the method used to join and seal the pipe. Steel pipe is most economically joined and sealed by welding rather than by use of bolted flanges or threaded couplings. Composite pipe on the other hand is most economically joined and sealed by use of mechanical couplings rather than by use of field-bonded connections. The speed and ease by which modern composite pipe can be coupled and sealed as well as uncoupled and removed provides it an economic merit that compares favorably with mechanically coupled steel pipe.
Modern composite mechanical couplings provide a rapid and reliable method of connecting composite pressure pipe. Seals made of modern elastomer materials provide a sealing permanence and integrity equivalent to that of bonded or welded connections. Composite mechanical couplings which use threaded joints or bolted flanges are more expensive than those which employ shallow non-bolted flanges. For this reason, increased attention has been given to the use of coupling structures which engage grooves machined in the ends of composite pipe. Such coupling structures generally comprise inwardly flanged members such as employed by Victaulic type clamps. Flexible steel cable or plastic rod used as flexible keys engage shallow recessed flanges or key ways machined into composite pipe joint ends provide another commonly used method of mechanically coupling composite pipe. Prior art mechanical couplings which employ shallow flanges provided by recessed grooves machined in the conventional laminate ply composite pipe joint ends are limited in joint tensile strength to the matrix dependent interlaminar "in plane" shear strength of the composite material and for this reason have limited the attainable joint strength of prior art composite pipe joints and mating composite mechanical coupling structures. The coupling structures of the present invention provide a means to overcome prior art coupling strength limits.
A structural flange is a protuberance that enables the transfer of a load from one body to another. For stress analysis purposes a structural flange can be treated as a short cantilever beam permanently attached to a body which resists the load imposed on the flange. Flanged structures seldom act singly but generally perform cooperatively with another flanged structure to provide a coupling and exchange of load between separate structures. Flanged structures are most commonly employed to transfer tensile, compressive and torsion loads. Torsion loads applied to prior art composite structures are primarily limited by the low shear strength of the composite matrix material used to bond a torqued flange to the surface of the torque-resisting body. The torque resisting coupling of the present invention overcomes such prior art limitations by making use of the high transverse shear strength of unidirected longitudinal twined strand cords. Flanges which primarily resist tensile and compressive loads imparted to the flange face behave as uniformly loaded cantilever beams. These tensile and compressive loads impart bending moment stress as well as shear stresses to the material connecting the flange to the load resisting body. This invention teaches that flanges attached to an integral composite cantilever spring are superior to prior art coupling structures by providing flange construction which not only reduce the bending moment stress at the flange connection but also increases the strength of the flange connection. An objective of the present invention is thus to teach how a composite flange attached to a composite structure can be constructed to lower the bending moment stress imposed at the flange connection while concomitantly increasing the strength of the flange connection to provide a flanged composite structure superior to prior art composite couplings.
Prior art composite pipe couplings have been developed which employ segmented spring-loaded curved square shaped keys that engage grooved composite pipe joint ends to provide an automatic quick connect type coupling. Such spring action is provided by independent members contained within a socket-end groove constructed within a composite pipe joint. Such spring-action type composite couplings employ flange members that act independently as movable shear keys and are constructed separately from the spring members. Such prior art automatic couplings are limited in scope and application by the spring member reliability as well as the shear key and grooved flange in-plane shear strength.
A cantilever spring is a structural member which exhibits a predictable deflection when subjected to a known load and which returns to its original position when the load is removed. The present invention teaches a method of making and using a composite cantilever spring which possesses greater fatigue life and spring stiffness for a given spring thickness than prior art composite springs.
Prior art composite springs, which usually are not flanged, comprise multiple layers of thin laminates containing unidirectional filaments. The stiffness and thus the spring constant of such composite laminate cantilever springs is greatly dependent upon the tensile strength, and the in plane interlaminar shear strength of the matrix material which bonds together the individual laminate plies. For this reason the tensile strength and the spring constant of prior art composite laminate cantilever springs is matrix dependent since such springs can not efficiently utilize this stiffness and strength of the filament reinforcements comprising the material from which they are constructed.
I have discovered that when cords of sinusoidally twined matrix-impregnated strands of continuous filaments are arranged in parallel fashion on a forming surface and individually tensioned prior to being shaped or formed, a unitary ply of composite material is produced from which stiff high performance composite cantilever springs can be made. Such composite springs have been found to exhibit a substantially higher spring constant and stiffness than prior art composite multiple laminate springs of identical thickness and configuration.
I have further discovered that an array of independent composite cantilever spring members with predictable stiffness can be produced when a single ply of twined longitudinal filament strands is impregnated with a hardenable liquid matrix and formed into a tubular composite structure having a polygonal cross section that is slotted at least partially along the vertices of the tubular polygon in a direction parallel to the tubular axis to provide a straight hinge line for each spring member.
Prior art methods for making composite tubular structures comprising tensioned longitudinal filament reinforcements generally employ a sequence of overlapping laminates where each laminate comprises a single thickness of filament strands aligned parallel to each other. Such methods are time consuming, complicated and expensive when used to construct composite tubular structures requiring a longitudinal laminate ply wall thickness greater than that attainable with a single filament strand. The present invention teaches a method to construct tubular composite structures having a single ply wall thickness governed by the cross section area of a twine of strands.
Prior art composite multiple ply structures which resist high temperature and which will not burn are generally made with a single liquid matrix that possesses the desired heat resistance and non-burning properties. Prior art methods employed to fabricate such non-burning high temperature composites generally require the use of non-combustible additives which tend to lower the viscosity of the liquid matrix and thereby inhibit a thorough impregnation of the filament reinforcement strands and thus reduce the composite material strength. To overcome this problem prior art non-burning composites commonly comprise a liquid polymer matrix blended with liquid halogen-containing fire-retardant additives. Such composites, when subjected to fire or extreme heat not only decompose and lose strength but release deadly toxic halogen-containing gases that not only impede fire fighting operations but may cause fatalities among persons exposed to such fire-produced gases.
I have discovered a non-combustible liquid matrix material that, while in a liquid uncured state, is compatible with most conventional uncured combustible thermosetting polymer materials. A composite structure which possesses greater resistance to degradation from fire or heat can be fabricated when filament strand reinforcements impregnated with a combustible thermosetting resin are twined or otherwise intimately combined with other filament strand reinforcements that are impregnated with the compatible non-combustible liquid matrix.
Prior art composite couplings such as described in U.S. Pat. No. 4,385,644 employ non-bevelled composite flanges comprising a longitudinal filament ply sandwiched between two annular composite rings containing circumferentially oriented continuous filament strands one of which rings serves as the flange load face. Composite coupling flanges having this construction are unable to resist tensile strength coupling loads that exceed the interlaminar shear strength of the resin matrix bonding the sandwiched plies together. Such non-bevelled inwardly facing composite ring coupling flanges are further unable to act as flanged composite spring members that flex and thereby assist coupling assembly as well as provide the longitudinal assembly force required to sufficiently compress an elastomeric gasket to accomplish a face seal between abutting pipe joint ends. When experiencing tensile end loads such prior art segmented composite couplings do not act to secure and lock an encircling composite sleeve structure so as to prevent disassembly when subjected to longitudinal stress.
Prior art annular composite sleeves used to assemble and encircle segmented composite coupling structures are not divided to enable easier coupling assembly as well as impose a compressive radial force upon only the outer faces of each coupling flange.
Prior art tubular composite laminates are generally single wall structures which are stiffened by use of sandwiched sand-resin mixtures or structural foam. Such structures are poorly suited to serve as beams or structural members subjected to bending stresses since they depend upon the interlaminar shear characteristics of the matrix material used to bond the laminates to the foam or filler material sandwiched between them. Such prior art tubular composite structures poorly resist delamination between inner and outer walls due to thermal stresses which serve to change the lengths of the inner wall and outer walls.
The following summarize the objectives of this invention to overcome the limitations of prior art composite coupling structures:
(a) To provide a superior coupling to connect composite panels and tubular structures. PA1 (b) To provide a composite spigot and socket coupling structure that is an integral structural constituent of composite pressure pipe. PA1 (c) To provide a composite coupling structure able to easily connect high pressure pipe. PA1 (d) To provide a composite mechanical coupling that is able to make and maintain a compression pressure seal between connected pipe joint faces. PA1 (e) To provide a high temperature composite material that resists deterioration and loss of strength when exposed to fire. PA1 (f) To provide composite beam, truss and panel structures that can be easily joined or disconnected. PA1 (g) To provide a composite coupling structure that resists disassembly when subjected to tensile end loads. PA1 (h) To provide apparatus and methods for making a wide range of composite coupling structures having predictable characteristics of strength and sealing capability.
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