WO2002032657A1

WO2002032657A1 - Wavy crossply composite structures

Info

Publication number: WO2002032657A1
Application number: PCT/US2001/042771
Authority: WO
Inventors: William F. Pratt
Original assignee: Pratt William F
Priority date: 2000-10-16
Filing date: 2001-10-15
Publication date: 2002-04-25
Also published as: AU2002224455A1

Abstract

Sinuous or wavy composite structures (7) as a capable replacement for crossply laminates. By combining wavy composite laminate (5, 6) in various waveforms, offsets, angular orientation and material combinations, it is possible to provide axial, torsion, or shear properties equivalent to unidirectional materials but without the limitations related to fiber discontinuity, labor costs for fabrication, and weakness at seams where laminates overlap. Several examples of both way crossply laminates (5, 6) and unidirectional crossply laminates are analyzed and compared.

Description

DESCRIPTION

WAVY CROSSPLY COMPOSITE STRUCTURES

TECHNICAL FIELD

The present invention relates to fiber reinforced resin matrix composites, and more particularly, to improved crossply laminate structures made from wavy composite materials. Such materials and structures made from wavy composites have enhanced structural properties and represent a greatly enhanced method of manufacturing crossply laminates.

BACKGROUND ART

Fiber reinforced resin matrix composites have been used for decades to provide stiff, strong, and lightweight structures to a wider field of applications in aerospace, sports, automotive, marine, civil infrastructure, and consumer markets. Favorable characteristics include high stiffness and strength to weight ratios, corrosion resistance, and tailorable structural properties. Disadvantages primarily involve cost, especially labor costs in fabricating practical structures.

Because of its anisotropy, fiber reinforced composites can be used to tailor the properties of the structure to the expected loads in a very efficient manner, especially if structural loads are limited to bending only, axial loads only, or torsion loads only, etc. When applied loads on a structure are uncertain or are known but involve many modes (i.e. axial, bending, and torsion for example) then in these cases, careful design and multiple fiber orientations are necessary to prevent failure of the structure. In these cases, multiple fiber orientations can give the structure of the composite properties that range from anisotropic to quasi-isotropic to isotropic, depending on the materials or laminate structure (Hyer 1997).

There are three basic methods used to obtain isotropy or near-isotropy in composite structures. These three methods use special materials such as chopped fiber mats and woven fiber cloth, filament winding, and ply stacking of oriented cloth or unidirectional prepreg. Each method has its advantages and disadvantages. Fiber mats or sprayed chopped fibers are easy to use and give near isotopic properties but cannot be used to tailor the stiffness and strength properties of composites to efficiently resist loads and are not typically used in other than light loading conditions.

Woven fiber cloth can be made in a number of fiber orientations, weights, and weaves, and can be tailored to a degree but are typically more expensive, usually require manual lay-up fabrication methods, and can limit the flexibility of the design if the fiber orientation is not optimal for the particular application. Additionally, woven cloth fibers are typically cut, interrupting fiber continuity especially where primary orientation of the fiber must be changed abruptly.

Filament winding methods offer continuity of fibers and automation as primary advantages but can present significant difficulties in obtaining the necessary fiber orientations for efficient structural properties. In these cases, it is customary to interrupt the fiber continuity to add layers of unidirectional pre-preg to accomplish the desired structural properties. Additional disadvantages include significant investment in capital and other fabrication issues. Ply stacking of unidirectional and/or woven cloth offers the most flexible method of making composite structures that meet the desired properties of the structure. Advantages include not only flexible manufacturing, but include ease in compaction, more uniform properties, and minimal investment in capital equipment. Disadvantages include interruption of fiber continuity, and high labor costs (Reinfelder, Jones et al. 1998)

Therefore, a need exists for a method and a composite structure that provides multiple fiber orientations, has the characteristics of ply-stacked composite structures, can be automated, and minimizes or eliminates the interruption of fibers.

The following terms used herein will be understood to have their ordinary dictionary meaning as follows:

Composite: made up of distinct parts. In the general sense, refers to any fiber reinforced material but especially any cured fiber reinforced matrix structure. Crossply, crossply lay-up, or crossply laminate: Two or more laminae made from unidirectional pre-preg arranged in such a manner that the primary direction of the fiber or strong material direction in the layers differ in orientation, or "cross" each other.

Fiber: a thread or a structure or object resembling a thread. A slender and greatly elongated natural or synthetic filament. (Includes metal fibers)

Lamina(te): a thin plate ....: layer(s)

Matrix: material in which something is enclosed or embedded.

Offset: In the context of this invention, means a generalized lead or lag of one waveform relative to another, similar to a phase angle in electronic engineering.

Off-axis: In the context of this invention, means a rotational difference of the strong axis between one laminae waveform relative to another or some reference.

Pre-preg: Fiber reinforced, resin matrix impregnated materials where the matrix is partially cured and ready for use. A special "uncured" case of the more general term "Composite",

Resin: an uncured binder, especially an uncured polymer binder or matrix used to bind fibers or fibrous materials; the matrix component of an uncured pre-preg.

Viscoelastic: having appreciable and conjoint viscous and elastic properties. Note: a special case of the term "viscoelastic" is "anisotropic viscoelastic", which is a viscoelastic material reinforced with fibers that give the material anisotropic properties. When the term viscoelastic is used in the text it should be construed to encompass this special case. Wavy crossply, wavy crossply lay-up, or wavy crossply laminate:

Two or more wavy fiber laminae arranged in such a manner that the primary direction of the fiber or strong material direction in the layers differ in orientation.

Wavy: The pattern of fiber lay that has a sinusoidal look, especially a sinuous wavy fiber in the plain of a laminate; the wave pattern need not be periodic or uniform. The following publications, incorporated herein by reference, are cited for further details on this subject.

1. Pratt, W. F. (2001). "Continuous wave composite viscoelastic elements and structures," Patent 6,287,664. US. 2. Pratt, W. F. (1999). "Patterned Fiber Composites, Process, Characterization, and Damping Performance," Ph.D. Dissertation. Provo, Utah, Brigham Young University, 195 pgs. (Note: not yet publicly released as of November 2000).

3. Pratt, W. F. (2000). "Method of making damped composite structures with fiber wave patterns," US Patent 6,048,426. US & PCT, Brigham Young University. 4. Darrow, Burgess, "Reinforced Web," 1931, US Patent No. 1,800,179.

5. Dolgin, Benjamin P., "Composite Passive Damping Struts for Large Precision Structures," 1990, US Patent No. 5,203,435.

6. Dolgin, Benjamin P., "Composite Struts Would Damp Vibrations," NASA Technical Briefs, 1991, Vol. 15, Issue 4, p. 79. 7. Hyer, M. W. (1997). "Stress Analysis of Fiber-Reinforced Composite Materials," The McGraw-Hill Companies, Inc.

8. Mellor, J. F. (1997). "Development and Evaluation of Continuous Zig-zag Composite Damping Material in Constrained Layer Damping," Masters Thesis, Provo, Utah, Brigham Young University. 9. Olcott, D.D., (1992). "Improved Damping in Composite Structures Through Stress Coupling, Co-Cured Damping Layers, and Segmented Stiffness Layers," Ph.D. Thesis, Provo, Utah, Brigham Young University,

10. Reinfelder, W., C. Jones, et al. (1998). "Fiber reinforced composite spar for a rotary wing aircraft and method of manufacture thereof, US Patent 5,755,558. US, Sikorsky Aircraft Corporation. l l . Trego, A. (1997). "Modeling of Stress Coupled Passively Damped Composite Structures in Axial and Flexural Vibration," Brigham Young University, Ph.D. Thesis, Provo, Utah, Brigham Young University.

Pratt (reference 1) proposed the use of wavy composite contemplated by Dolgin (references 5 and 6) as constraining layers for a soft viscoelastic damping material in several combinations of wavy composite, viscoelastic, and conventional materials. Additionally, Pratt proposed the use of "wavy pre-preg for use with or without a separate viscoelastic layer" but did not teach or further amplify the construction or benefits. Pratt (reference 2, page 92) proposed, constructed, and tested balanced wavy composite crossply samples (without viscoelastic layers) for the purpose of determining the properties of wavy composite.

Darrow (reference 4) proposed a device for obtaining a permanent sinuous waveform in metallic wires for the production of rubber tires but never contemplated modern fibers, resin systems and composite structures. Dolgin (reference 5) proposed a specialty composite structure made from opposing chevron and sinusoidal patterned composite lamina constraining a viscoelastic layer. In reference 6 Dolgin stated that the production of wavy sinusoidal pre-preg should be possible but did not describe any process or apparatus. Neither reference taught or cited any method of constructing or using wavy or chevron patterned composites as replacements for unidirectional pre-preg based wavy crossply laminates.

Hyer (reference 7) is a good all-around and current basic composite book that covers the properties of composites, especially unidirectional pre-preg based crossply laminates. Wavy composite is not mentioned at all.

Mellor (reference 8) proposed the use of standard bi-directional cloth in a zig- zag (chevron) pattern contemplated by both Dolgin and Olcott as a constraining layer for viscoelastic materials. No use of this concept as a structural material in a wavy crossply structure was contemplated, mentioned, or taught.

Olcott (reference 9) predated Mellor and proposed, fabricated, and tested the chevron patterns used as constraining layers for viscoelastic damping layers contemplated by Dolgin. Olcott did not contemplate use of wavy or chevron patterned laminae without the use of viscoelastic layers.

Reinfelder, et al, (reference 10) discussed the construction of a rotary wing spar for use on a helicopter. It is a good example of the superiority of crossply laminates and is an example of an application that could benefit from the use of wavy crossply laminate structures. Trego (reference 11) extended the Finite Element Analysis model proposed by Olcott (reference 9) and built several chevron based constrained layer damping tubes to validate the model. No mention of using wavy composites in wavy crossply lay-ups was made or proposed. Crossply lay-ups, as discussed by Reinfelder, et al, and Hyer, typically involve the use of unidirectional pre-preg with fiber orientations designed to maximize the desired structural properties. For example, if a tube is to be loaded in the longitudinal or axial mode, most if not all of the unidirectional fibers would be oriented in the longitudinal (or 0°) direction for maximum stiffness. Some small percentage of total fibers in the tube may be oriented perpendicular to these fibers for hoop strength, to prevent separation, or to prevent buckling, but such fibers would not resist longitudinal loads. Such tubes are easy to make by cutting an appropriate length of unidirectional pre-preg from a roll and rolling the composite onto a mandrel. No fibers (for the 0° layers) are cut or interrupted. Loads are resisted best when fibers are not cut. If cut, loads between such fibers are transmitted through the matrix or resin and stiffness and strength can be considerably reduced. A tube with all or mostly 0° fibers would be very efficient in resisting longitudinal loads but would not resist any significant torque or bending loads because such loads would be resisted primarily by the shear strength of the matrix and not by fibers. A better design for resisting torque loads in a tube would be to add additional layers of fibers oriented at angles to the longitudinal axis so that the fibers would spiral around the tube. Such fibers would provide the primary resistance to torque loads and would provide resistance to shearing loads along the neutral axis during bending similar to a truss like structure. To avoid cutting the fibers (except at the ends of a tube) the unidirectional pre-preg would have to be spirally wound on the tube. More typically, the unidirectional materials are cut at an angle from a larger sheet and the "off-axis" rectangle of material thus created is rolled on to the tube as is done for the longitudinal fiber plies. This leaves a series of cut fibers that spiral around the tube ending on a discernable seam that runs the length of the tube. This represents a potentially significant weakness in the crossply laminate. If several such layers of opposing "off- axis" plies are used , the normal practice is to offset the ending and beginning of such plies so that the seams of each layer are offset. (Reinfelder, et al, 1998). DISCLOSURE OF INVENTION

The present invention is directed to an improved composite structure and method for manufacturing the same from wavy fiber pre-preg materials. Generally, characteristics of the structure and methods include: • Two or more wavy laminae used in opposing patterns or offset patterns in a composite structure, where the laminate properties created have variable crossply characteristic.

• Laminate properties that can be tailored by the stacking sequence, waveform, offset, axis orientation, and material used. • Wavy crossply structures that can be laid down by tape laying machines and apparatus with as little as one axis of control.

• Wavy crossply structures that minimize the interruption of fibers thereby making the laminate stronger and less prone to failure.

Wavy composite pre-preg can be used to create virtually seamless crossply-like laminates with little or no interruption of fibers. This is simply accomplished by combining two or more wavy composite plies using opposing waveforms in its simplest form, or by using combinations of opposing and offset wavy composite waveforms to form the laminate. Such a laminate displays the properties of both unidirectional and crossply characteristics in that it can efficiently resist both axial and transverse shearing loads.

The fact that such a structure, which has fibers oriented in multiple directions, can be laid down with standard automation equipment (with as little as one axis of control) makes the structure and method economical. This is in contrast to laminates and methods used to make conventional unidirectional pre-preg based crossply laminates that cannot readily be automated. Additionally, experience has shown that wavy pre-preg can be more easily draped over contoured surfaces and tooling, further easing fabrication.

Finally, there is a finite maximum width to pre-preg (typically 60 inches maximum) that often causes laminators to have to splice and overlap sheets of unidirectional pre-preg to form large laminae. This is especially true for off-axis unidirectional laminae. This introduces seams that often represent a significant weakness in the laminate (see Figure 5, items 12-14). Wavy composite can be easily spliced together across the width without the need to interrupt the edge fibers (see Figure 5, item 1 and 15). Since the present invention discloses how wavy composite laminae can be combined to produce crossply laminate structures, it is now possible to create such structures with a minimum of interruption of fiber continuity and without overlapping seams. In fact, wavy crossply tubes have been made that exhibit no discernable seam, have no interruption of fibers (except at the ends of the tubes), and which display classic crossply laminate characteristics.

The present invention also relates to the use of wavy composite and unidirectional composite in crossply lay-ups in the generalized fabrication of tubes, wing spars, rotary wing spars, and similar structures.

BRIEF DESCRIPTION OF DRAWINGS

A more complete understanding of the present invention and the features and advantages thereof may be had by reference to the following detailed description of the invention when considered in conjunction with the following drawings:

Figure 1 is an exploded perspective view of a balanced wavy crossply lay-up used to perform stiffness and strength tests on wavy composite material according to reference 2.

Figure 2 is an exploded perspective and corresponding side view of a wavy crossply lay-up with a unidirectional ply interposed for the purpose of improving laminate properties according to a generalized embodiment of the present invention.

Figure 3 is a top-down side view of different wavy crossply lay-ups for the purpose of improving laminate properties according to a generalized embodiment of the present invention. Figure 4 is a top-down view of different unidirectional crossply lay-ups for the purpose of comparison to the present invention.

Figure 5 is an illustration of the advantages of joining adjacent wavy composite plies to make wider laminae compared to typical methods used to join adjacent off-axis unidirectional materials to create wider laminae. Figure 6 illustrates the ease with which wavy composites can be used to construct efficient crossply-like layups compared to unidirectional ply methods. Figure 7 is an example of a using wavy composite offset by an angle with respect to a reference axis, with improved laminate properties according to a generalized embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A single layer of wavy composite has a fiber lay that oscillates between a negative maximum angle and a positive maximum angle in a pre-determined pattern. As a result, the individual laminae will vary in stiffness and displacement characteristics along its length as the angle of the fiber changes. Thus where the angle is 0° relative to the length of the waveform, the laminae will have the characteristics of a 0° unidirectional composite, and where the angle diverges from 0° the laminae will have the characteristics of an off-axis unidirectional laminae. If several opposing wavy composite laminae are joined together in a symmetric lay-up (see Figure 1), the laminate will exhibit quasi-isotropic properties at any point along the length. Refer to Figure 3. Items 5 and 6 refer to opposing pairs of wavy composite laminae. Where the angle is at a ± maximum (item 10), the properties of the laminate will be like a ± unidirectional crossply. These areas where the angles are at a ± maximum will resist in-plane shear loads effectively but will have a lower longitudinal or lengthwise stiffness. Where the angle is at 0° (item 11) the laminate will have the properties of a 0° unidirectional lay-up (Pratt, 1999). These areas (where the angle of the fibers is 0° relative to the longitudinal direction) will not resist shear loads effectively but will have a greater lengthwise stiffness. These localized differences in stiffness can be overcome by combining two pairs of wavy composite laminae into a single laminate as is shown for item 7. The structure of the wavy laminate enclosed by item 7 is hereafter termed "wavy crossply laminate."

Thus, for one pair of opposing wavy laminae, the angle will be at a ± maximum but the second pair of opposing wavy laminae will have a fiber angle that is at 0° or nearly 0° relative to the general direction of the laminate. This gives the laminate an equivalent unidirectional lay-up of four total layers where two of the layers are unidirectional plies with a ± fiber orientation, and the other two layers were equivalent to two 0° unidirectional laminae. The difference is that the construction of a unidirectional version of a crossply laminate cannot be easily automated; the construction of a wavy crossply laminate can be automated. In the process of characterizing the properties of damped wavy composites, several sample tubes constructed from wavy composite with constrained viscoelastic layers were compared to equivalent undamped unidirectional crossply tubes. It was found that damped wavy tubes took significantly less time to fabricate. As a result, and in an effort to save labor time, several undamped tubes were manufactured using the lay-up shown in Figure 3, item 5 (one pair) as a replacement for the undamped unidirectional tubes because of the ease with which wavy tubes could be fabricated. The realization of the superior handling, excellent stiffness and strength, and significant time savings led to additional discoveries which are the subject of the present invention. The following discussion further amplifies the advantages of using wavy composite pre-preg in wavy crossply lay-ups. Figure 4 illustrates three typical unidirectional crossply lay-ups that use several unidirectional pre-preg plies (item 2) to build a laminate with quasi-isotropic properties. In reference 10, Rienfelder, et al showed how to construct a rotary wing spar using different materials and ply stacking methods and cited superiority of consolidation of the laminate, uniformity, and tailorability of the laminate combinations as advantages. They further cite the disadvantages of fiber winding techniques for this application as "relate[d] to difficulties associated with expanding/urging the fibers against the mold surfaces of the matched metal mold." It can be concluded that in some applications, fiber plies or laminae made from unidirectional materials cut off-axis will be preferable to the use of fiber winding methods and that continuity of fibers of such plies will be interrupted. The authors state that such plies were butt-joined together and that such joints from additional plies above and below in the stacking sequence were offset so that adjacent butt-joints would not coincide. This concept is shown in general in Figures 5 and 6. Pre-preg made with unidirectional fibers comes in finite widths, typically up to 1.5 meters in width, in very long lengths. The same is true with cloth pre-preg, and wavy composites. Because of the limited widths of pre-preg, there is a finite length of off-axis material that can be cut from a roll of unidirectional pre-preg. In Figure 5, several unidirectional plies (12) are cut from a long continuous sheet of unidirectional material. The plies are then butt- joined (13) and overlapped (14) with other plies from the same cut of material. The weakest link in the laminate is the seam between butt-joined plies (13) where fiber continuity is interrupted. Thus to obtain a viably strong laminate with an off axis orientation, a minimum of two laminae are required.

Wavy composites do not have this limitation. As shown in Figure 5, two or more plies (1) of wavy pre-preg can be placed adjacent to each other to create a wider laminae (15). When using concepts shown in Figure 3, the wider wavy laminae can be combined in different ways to form a laminate with the desired properties. To further amplify the advantages of wavy composite, refer to Figures 3 thru 6.

In order to make a crossply laminate from unidirectional pre-preg similar to concepts shown in Figure 4 with a ply stacking sequence of 0°/+45°/-45°, it would be necessary to cut the off-axis plies as shown in Figure 5 (12). Several such plies would be butt-joined and overlapped as shown in Figure 6 (13) and (14). While the 0° ply (16) is continuous in this drawing, there are significant interruptions of fiber continuity in the off-axis plies. All of this is typically done by hand and is very labor intensive.

If, however, the designer were to use wavy composite, the equivalent of the 0°/+45°/-45° laminate could be completed using wavy plies offset as shown in Figure 3 item 7. If hand labor is used, the steps required to accomplish the desired laminate are greatly reduced. Instead of having to cut separate laminates as shown in Figure 5, (12), the fabricator uses successive wavy layers of any desired length to form the wavy crossply laminate. In fact, it is very possible for this process to be automated by feeding (for example) four spools of wavy composite through a pair of pinch rollers with the patterns offset appropriately, to create a continuous roll of wavy crossply as shown in Figure 3, (7). The beginnings of such a process are shown in Figure 6 (bottom) where the second wavy composite layer (1) is overlaid on the first layer (15). Figure 6 shows the next layer of wavy composite (1) offset to the right of the base layer (15) for clarity. In actual practice both layers (1 and 15) could begin at the end; this figure is only illustrative of the concept. Of course there are many methods and mechanism whereby wavy layers can be combined to form wavy crossply laminates including successive passes by a single axis laminator, multiple feed rolls, etc, and even manual labor methods. All will be significantly easier and more economical than current methods of producing unidirectional pre-preg based crossply laminates. The present invention includes a structure such as is shown in Figure 1 where several layers of wavy composite (1 and 3) are used to build a "balanced" wavy composite laminate.

The present invention also includes a laminate consisting of a mix of wavy composite layer(s) (items 1 and/or 3) and unidirectional layers (2) as shown in Figure 2. In this figure the laminate consist of two opposing wavy layers (1 and 3) and one transverse unidirectional ply (2). This is a very lightweight, efficient lay-up that has been used to produce undamped tubes with excellent stiffness and improved strength properties. It has been found, that the lamination of at least one transverse ply (2) and at least one wavy ply (1 or 3) provides greater strength. When a single layer of wavy composite is pulled to failure (along the strong or longitudinal axis), the failure generally occurs in the matrix where the angle of the fiber is at a ± maximum. The matrix splits between the fibers. If fibers are added generally perpendicular to the wavy fiber lay, then the laminate failure occurs at a much high stress level because the transverse fibers resist the transverse stresses efficiently. It is also possible to orient two or more wavy layers transversely to each other as is shown in Figure 3 item 4. Such a laminate structure would be useful in providing quasi-isotropic properties to sculpted surfaces where special properties or laminating issues are important. This can also be accomplished by making the wavy composite layer from a woven fabric where the fill fibers or fibers that run transversely across the wavy fibers are essentially straight. The use of wavy woven fabric creates a wavy lamina that is essentially equivalent to a wavy layer (1) and a transverse unidirectional layer (2) combined and is a special subset of the definition of "wavy".

The most useful configuration is shown as items (5)-(9) in Figure 3. Items (5), (6), and (8) represent pairs of opposing wavy composite laminae joined together. The two (or more) wavy laminates need not "oppose" each other (e.g. have a one-half waveform phase lag or offset) to conform to the meaning of the present invention. It may be useful to cause a more or less than half waveform phase lag as demonstrated in the area enclosed by (7) of Figure 3. Combining two or more "pairs" of wavy laminae need not be joined together along their longitudinal axes but may be laid at some off-axis angle with respect to each other as is shown in Figure 3, (5) and (8). This will give a unique crossply effect as shown in the area enclosed by (9). Although the pairs of wavy laminates (Figure 3, (5) and (8)) are shown essentially perpendicular to each other, it is also possible to vary this angle more or less to accomplish unique laminate properties. For example, it may be desirable to place the pairs of wavy laminates at a -fc 30° angle or some other angle with respect to a reference line as is commonly done with unidirectional plies. These different angular orientations are contemplated by this invention.

To further illustrate the capability of wavy crossply laminates, the following table documents the equivalent axial stiffness of several different configurations of wavy crossply laminates using (for example) a typical carbon fiber-resin combination to represent the material properties of both unidirectional and wavy composite. Table 1 shows the configuration of each laminate. Each laminate is defined by the words "unidirectional", or "wavy crossply", or "wavy crossply & unidirectional" defining the materials used in the lay-up. This is shown in the "Laminate" column of the table. The laminate configuration is further defined by the angle of the plies relative to the longitudinal direction of the sample tube used to model the lay-up. This is shown in the "Configuration" column of the table. For example, "0°" means all fibers are oriented at zero degrees to the reference, or run longitudinally in the tube. The relative axial stiffness of the laminate is given in the column labeled "Axial modulus." This represents the smeared axial material properties of the lay-up. Axial modulus represents the relative ability of the laminate to resist tension or compression loads, and even bending loads if the neutral axis shear forces are ignored. The "Shear modulus" column represents the ability of the laminate to resist torsion or shear loads.

Table 1

Laminate 1 is a unidirectional fiber composite lay-up that shows the 0 degree properties of the fiber reinforce composite used to model all subsequent lay-ups. Laminate 2 shows the properties of a conventional ± 30 degree unidirectional composite crossply lay-up. Note that the equivalent axial modulus of laminate two is considerably reduced from that of laminate 1, but the equivalent shear modulus is greatly improved over the shear modulus of laminate 1. This is a classic example of how crossply composites lose axial modulus rapidly as the angle of the fiber diverges from zero degrees, but their ability to resist shear loads improves.

As discussed above and shown in Figure 6, a unidirectional crossply laminate is difficult to fabricate with automated means. However, as shown in Table 1 (laminate #2), crossply laminates are useful in providing resistance to both axial and shear loads. If more axial stiffness is required, a unidirectional 0 degree ply can be added. The results of this combination are shown as laminate #3 in Table 1. The axial modulus is improved by 64% relative to laminate #2 but the shear modulus is reduced 26%.

Wavy composite can be used to create wavy crossply laminates equivalent to the unidirectional crossply laminates discussed above. Wavy crossply laminate #5 is equivalent in both axial and shear modulus to unidirectional crossply laminate #2. Likewise, wavy crossply laminate #7 is equivalent in both axial and shear modulus to unidirectional crossply laminate #3. Both wavy crossply laminates are significantly easier to fabricate, do not cut fibers (and therefore do not show any seam), and can be readily automated. The same cannot be said for the two unidirectional crossply laminates.

The remaining entries of Table 1 example only a few of the many different combinations possible by using wavy composite materials. For example, wavy crossply laminate #4 represents the axial and shear modulus of one pair of opposing wavy laminae (Figure 3, (5) or (6)). This combination has a 60% greater axial modulus than the ± 30 degree unidirectional crossply lay-up (laminate #2) but a 57% lower shear modulus. It is exampled here because it represents the simplest wavy crossply laminate. Obviously, it is possible to modify the characteristics of the laminate by changing the waveform, offset, or by orienting the wavy laminae off-axis. This example represents only one combination of parameters and their effects on the stiffness of the laminate thus created.

If greater transverse strength was desired in the crossply laminate, the designer would add an additional layer of unidirectional composite. This is shown in laminates #6 which is a modified version of #4, and in laminate #8 which is a modified version of laminate #7. Both can still be readily automated in fabrication since the 90 degree layers could be added easily. Additionally, 0 degree unidirectional layers can be added to augment the axial modulus without unduly sacrificing the shear modulus. This is shown as laminate #9 in Table 1 and compares favorably with laminates #3, #7, and #8.

The present invention does not limit the waveforms used to identical wave patterns, periods, to a particular waveform (such as a sine wave, cosine wave, etc.), a particular orientation, or to a particular offset. The properties desired in the laminate may require a non-periodic waveform or a combination of waveforms of any type, and unidirectional or woven cloth laminae. The selection of waveforms, materials, orientations, or offsets to use will depend on the properties desired in the laminate. The selection will be obvious to one skilled in the art. The wavy laminates discussed here and illustrated in the figures are for example purposes only.

Finally, the range of possible uses of the example wavy crossply lay-ups shown in Table 1, is potentially limitless. In reference 4, a construction for a rotary wing spar is revealed which uses unidirectional and woven fiber composite layers to provide efficient axial, bending, and torsional stiffness. Although the examples of Table 1 were based upon the analysis of a sample tube, the same or similar wavy composite lay-ups could be used to construct an equivalent spar at a greatly reduced costs. Other applications include automotive, aerospace, and marine drive shafts, composite wing structures of all types, panels, composite I-beams, channels, and virtually an endless combination of possibilities. Composite arrow shafts and golf club shafts would likewise benefit from greatly reduced labor costs in construction. Other applications will be obvious to those skilled in the art. INDUSTRIAL APPLICABILITY

The way in which the Crossply Wavy Composite Structures are capable of exploitation in industry and the way in which the Crossply Wavy Composite Structures can be made and used are obvious from the description and the nature of the Crossply Wavy Composite Structures.

Claims

CLAIMSI claim:

1. A wavy crossply composite structure, which comprises: at least one wavy fiber composite lamina combined with a unidirectional fiber composite.

2. A wavy crossply composite structure, which comprises: at least one wavy fiber composite lamina combined with a woven fiber composite.

3. A wavy crossply composite structure, which comprises: two or more wavy fiber composite laminae combined with opposing orientations.

4. The wavy crossply composite structure as recited in claim 3, further comprising: a unidirectional fiber composite lamina.

5. A wavy crossply composite structure, which comprises: two or more wavy fiber composite laminae combined with opposing and offset orientations.

6. The wavy crossply composite structure as recited in claim 5, further comprising: a unidirectional fiber composite lamina.

7. A wavy crossply composite structure, which comprises: two or more wavy fiber composite laminae combined with opposing, offset, and off-axis orientations.

8. The wavy crossply composite structure as recited in claim 7, further comprising: a unidirectional fiber composite lamina.

9. A wavy crossply composite structure, which comprises: two or more wavy fiber composite laminae combined with opposing and off-axis orientations.

10. The wavy crossply composite structure as recited in claim 9, further comprising: a unidirectional fiber composite lamina.

11. A wavy crossply composite structure, which comprises: two or more wavy fiber composite laminae combined with offset orientations.

12. The wavy crossply composite structure as recited in claim 11, further comprising: a unidirectional fiber composite lamina.

13. A wavy crossply composite structure, which comprises: two or more wavy fiber composite laminae combined with offset and off-axis orientations.

14. The wavy crossply composite structure as recited in claim 13, further comprising: a unidirectional fiber composite lamina.

15. A wavy crossply composite structure, which comprises: two or more wavy fiber composite laminae combined with off-axis orientations.

16. The wavy crossply composite structure as recited in claim 15, further comprising: a unidirectional fiber composite lamina.

17. A wing spar or rotary wing spar constructed with wavy crossply composites.

18. An arrow shaft constructed with wavy crossply composites.

19. A drive shaft constructed with wavy crossply composites.

20. A golf club shaft constructed with wavy crossply composites.

21. A structural member constructed with wavy crossply composites.