Braiding in Composites Free

Over the past decade, the use of braided reinforcements has become known as a cost-effective method to produce high-performance, high-volume composites (Ref 1, 2). Braid allows for the creation of composite parts with an efficient fiber architecture and provides composite part manufacturers with a material that is easily handled and that results in fewer processing steps and less waste than woven alternatives (Ref 1, 2).

Braid has applications in a variety of fields, including aerospace, automotive, medical, recreational, and civil engineering. Within these fields, braiding technology has been used for the creation of jet engine fan cases, aircraft fuselage frames, jet engine stator vanes, composite bridge repairs, high-pressure air beams, automotive wheels, rocket nozzles, aircraft propellers, and prosthetics.

A maypole braiding machine is a standard braiding machine used to create braided sleevings (Fig. 1). A typical maypole-style braiding machine (Fig. 2) is comprised of a set of standard parts or features that include a circular track plate, a gear train located on or below the track plate, a set of spool or bobbin carriers, a forming device, and a take-up mechanism (Ref 2, 3). The individual gears of the gear train are referred to as horn gears (Ref 2). These horn gears (Fig. 3) are comprised of a spur gear bottom and a slotted gear top (Ref 2). The slotted gear top may be referred to as a horn disk (Ref 3). Each horn gear assembly rotates in either the counterclockwise or clockwise direction, and horn gears adjacent to one another will rotate in opposition, as depicted by “Gear rotation A” and “Gear rotation B” in Fig. 3.

The movement of a single spool carrier from one horn gear to another is defined by a sinusoidal slotted path in the track plate as well as the particular slots of the horn disk with which the spool carrier interacts. A typical horn disk is comprised of four slots, and spool carriers may be placed in half of these slots. In a typical maypole braiding machine there are two sets of spool carriers, one that travels clockwise around the braiding machine, and one that travels counterclockwise around the braiding machine.

To form a braided structure, bobbins, or spools, are filled with material and are placed onto the spool carriers. The material is then threaded through a series of yarn guides attached to the spool carrier. The spool carriers not only support the bobbins but also apply tension to each individual tow, or fiber bundle, as the braid is being formed and control the amount of fiber released as the spool carrier moves around the braiding machine track. Typical spool carriers use springs to control the tension applied to each tow. Spool carriers also generally are comprised of a ratcheting mechanism that controls the amount of material released from the spool.

As spool carriers move around the sinusoidal path of a standard maypole braiding machine, such as that illustrated in Fig. 2, they move nearer and further from the vertical axis, or center, of the braiding machine. The radial change in position of spool carriers allows for the intertwining of tows. Additionally, the yarn length supplied by each carrier must change in relation to each radial position such that each tow remains under tension. This yarn length is controlled by the spool carrier mechanisms.

The structure of a braid formed by a maypole-style braiding machine is determined by the relationship of the speed with which the braided structure is taken up and the speed at which the horn gears turn. Additionally, the structure can be affected by the number of spool carriers loaded into the braiding machine. If two of the slots of the horn gear are filled, a 2 by 2 braided structure will be formed. In this 2 by 2, or twill, structure (Fig. 4b), each tow of material passes over two tows and under two tows as the spool carrier moves around the braiding machine track. If only one of the slots of the horn gear is filled, a 1 by 1 plain weave braided structure (Fig. 4a) will be formed. In a 1 by 1 structure, each tow passes over one tow and under another tow in the braided structure.

It is possible to form different braided structures, including 3 by 3, 4 by 4, and so on, by increasing the number of slots in the horn disk. For example, a 3 by 3, or Hercules braid (Fig. 4c), could be formed by increasing the number of horn gear slots from four to six slots.

Formation of a braided structure (Fig. 5) will naturally occur at a formation point above the spool carriers. The fiber interaction below the formation point is referred to as the braid shed. The formation of a braided structure is considered the point in the braiding process at which the intertwining tows have become configured into the desired angles and coverage factor. (Coverage is the degree to which gaps between the braided yarns in a braided structure are observed. High coverage indicates little to no gaps are observed, while low coverage indicates more space between fibers is observed.) A formation device may be placed at the formation point to aid in braid formation. Additionally, a formation device may be used in combination with a mandrel, which would be fixed in place and upon which the braid would form before being pulled up by a take-up device.

While some maypole-style braiding machines are oriented such that the spool carriers move in a plane normal to a vertical axis, other braiding machines may be constructed such that the track plates are oriented in a vertical plane and the braid may be taken up from the braiding machine along a horizontal axis. Both orientations are illustrated in Fig. 2.

A maypole braiding machine may be altered such that a flat braided structure, or tape, may be formed instead of a cylindrical braid. By inserting terminal gears that reverse the direction of the spool carrier travel and prevent the spool carriers from being able to travel the complete circular track of the maypole braiding machine, a flat braided structure can be formed (Ref 3). A simplified example of a tape braiding machine is illustrated in Fig. 6. The terminal gears allow the spool carriers to travel only within a specific arc of the machine track. Within this track, carriers change direction as they pass around the terminal gears. Braiding machines for the formation of braided tapes must be comprised of an odd number of spool carriers, and the terminal gears must contain an odd number of slots (Ref 2). An example of a braided tape structure is illustrated in Fig. 7.

A 3D braiding machine is comprised of multiple adjacent braiding machine tracks, or paths, between which carriers of one track or path may interact with carriers of other tracks or paths to create braided structures with increased thickness. These tracks or paths may form square, rectangular, I-beam, or other more complex shapes than the round shape of the maypole braiding machine (Ref 3). The horn gears comprising 3D braiding machines are generally configured into the same shape as the product they will create (Ref 4). Additionally, the spool carrier paths vary significantly from that of a standard maypole braiding machine (Ref 4). An example of the horn gear configuration and the direction of the movement of each spool carrier for a square 3D braiding machine is illustrated in Fig. 8 (Ref 5). The 3D braided structures provide through-the-thickness reinforcement when used in composites and can be used to form a variety of complex shapes (Ref 6). However, 3D braiding machines are much more complex and costly when compared to standard maypole braiding machines (Ref 2).

Additional examples of 3D braiding machines include 3D radial braiding machines, which have been manufactured by the Atlantic Research Corporation (Ref 2) and Gallagher Engineering (Ref 7). A radial braiding machine is constructed such that the spool axes are oriented toward the center of the braiding machine, similar to the spokes of a wheel, and the multiple-ring track of the braiding machine is located on the inside diameter of a cylinder (Ref 2). An example of a radial 3D braiding machine is illustrated in Fig. 9 (Ref 7).

Braiding machines may range in size from three carriers to more than 800 carriers, as in the example of a mega-maypole braiding machine (Ref 8). To date (2024), the largest existing braiding machine is an 864-carrier machine that is housed at A&P Technology in Cincinnati, OH (Ref 8, 9).

Large braiding machine sizes not only allow for larger-diameter braided structures to be created, but they also allow for high-coverage braids to be created with smaller fiber sizes and smaller unit cells (Ref 8). An example of two braided sleevings of the same diameter, but which were created on two different machine sizes, is depicted in Fig. 10. The sleeving depicted on the left in Fig. 10 was produced on a 96-carrier braiding machine with a 12K carbon tow, while the braided sleeving on the right was produced on a 400-carrier braiding machine with a 1K carbon tow. A large range of braiding machine sizes allows for the tailoring of fiber type, size, and coverage, as well as the refinement of other properties on an application-by-application basis.

Braided structures may be formed with fibers of a wide range of sizes, including from 1K carbon and below fiber sizes to 60K carbon or greater fiber sizes. Additionally, the materials that can be braided vary widely and include glass, carbon, ceramic, metal, thermoplastic, thermoset, aramid, cotton, flax, polyester, and many other fiber types. Further, braided structures may be formed with combinations of these materials. For example, one or more tows of a braided structure may be comprised of glass, while all of the other tows may be comprised of carbon for increased stiffness. Similar hybrids can be created to tune stiffness and strength, while dissimilar materials such as thermoplastics can be added for increased toughness. In an additional example, bundles of two or more tows may be braided together.

Figure 11 illustrates several examples of braided structures with unique tow compositions. Figure 11(a) is a braided structure comprised of very wide tows; Fig. 11(b) is a braided structure comprised of tow bundles that may be twisted or untwisted. Figure 11(c) is a braided structure in which only one fiber position is comprised of a bundle of two tows, while all of the other tows are singular. Figure 11(d) depicts a braided structure in which one set of tows is comprised of very thin tows, while the opposing set of tows is comprised of wide tow bundles. Figure 11(d) also demonstrates a braided structure that may have very little to no crimp.

Crimp is defined as the undulation induced into a tow as a result of the intertwining of tows (Ref 4). This induced undulation into a tow is illustrated in Fig. 12 through the undulation of tow C as it intertwines with tow A and tow B. Braided structures may have varying degrees of crimp based on the sizes and cross sections of the tows within the braided structure. For example, if the cross-sectional area and height of tow A was very small, there would be very little crimp experienced by tow C due to the spread of tow A.

Further, braided structures may have varying degrees of crimp based on the architecture of the braided structure. For example, a 1 by 1 braided structure will have more crimp than a 2 by 2 braided structure. The more tows that a single tow passes over and under within a braided structure, the less the total amount of crimp in the braided structure will be.

A braided structure that is formed only with material oriented at angles relative to the vertical axis, or bias directions, is considered a biaxial braided structure. A biaxial braided structure is depicted in Fig. 13(a). This structure is formed on a maypole braiding machine in which material is loaded only onto the spool carriers.

Biaxial braided structures may be visualized by using the finger-trap analogy. As a finger trap is placed under tension, the diameter of the finger trap decreases such that it traps the fingers inside of it. However, under compression, the diameter of the finger trap increases such that any fingers inside may be released. A biaxial braided structure behaves in the exact same way. A braided biaxial structure comprised of a ±45° construction will decrease in diameter under tension, decrease in angle from the ±45°, increase in thickness, and increase in coverage (Ref 1). The same ±45° biaxial braid under compression will increase in diameter, increase in angle from the ±45°, increase in thickness, and increase in coverage (Ref 1).

The natural conformability of braided structures due to the finger-trap effect allows braid to conform to the surface of a part to which it is applied without wrinkles or pleats in an extremely repeatable way. A baseball bat (Fig. 14) is composed of three distinct regions: the large-diameter barrel, a small-diameter handle, and a variable-diameter throat that transitions from the diameter of the barrel to the diameter of the handle. If a biaxial sleeving is designed such that the nominal diameter of the braided structure at ±45° is the same diameter as the midpoint of the throat, as the braided structure is pulled over the large-diameter barrel of the baseball bat, the braid will conform to the barrel at a high angle. As the braid is pulled over the throat of the baseball bat, the angle will continuously decrease until the midpoint of the throat, where a ±45° angle may be instantaneously achieved, and then the angle will continue to decrease until the braid is pulled over the handle of the baseball bat. As the braid is pulled over the handle, a constant angle lower than the ±45° will be obtained. Additionally, the braid ply or layer will be thickest—and have the highest cover factors—at the barrel and at the handle (Ref 1). The change in the bias angles of tow 1 and tow 1′ along the length of a baseball bat is illustrated in Fig. 14.

As described, a braided structure formed on a traditional maypole braiding machine may be visualized as a cylindrical structure comprising both a diameter and a circumference (Fig. 15). The braided cylindrical structure illustrated in Fig.15 is comprised of a tow, tow 1, intertwined in the counterclockwise direction around the cylinder. Within the braided cylindrical structure illustrated in Fig. 15, tow 1 starts and ends at points vertically, but not circumferentially, displaced from one another. By cutting the cylinder in Fig.15 along a vertical line formed between the vertically displaced starting and ending points of tow 1 and laying the braid flat, a rectangular section of braid is obtained. The width of the rectangular section of braid is defined by the circumference of the original braided cylinder, or the length between points A₁ and B₀, as illustrated in Fig. 15. The height of the rectangular section of braid is equal to the vertical displacement between points A₀ and A₁ on the vertical cutting line. The distance between points A₀ and A₁ is also referred to as the tracer-to-tracer (TTT) measurement.

Tensile or compressive forces applied to a braided structure can result in changes in bias angles and diameter. However, the effects of these forces on a braided structure are highly predictable.

The number of tows, N, making up the braided structure is directly related to machine size and the number of spool carriers used. As previously described, the number of spool carriers within a braiding machine can be altered based on the desired braid architecture. For example, a 96-carrier braiding machine making a 2 by 2 twill braided structure creates a braided structure with 96 total tows, while the same 96-carrier braiding machine making a 1 by 1 plain weave braided structure creates a braided structure with 48 total tows, due to the number of spool carriers used to create each structure.

Examples of unit cells, or the smallest repeating pattern making up a braided structure, in a biaxial and triaxial braided structure as defined in this document are illustrated in Fig. 18.

The height of the unit cell is defined by the length between A′₀ and A″₀, as illustrated in Fig. 19. Figure 19 illustrates two parallel tows, tow 1 and tow 2, adjacent to one another.

The width dimension of the unit cell is illustrated in Fig. 19 as the length between A″₀ and E₀.

This centerline spacing between parallel yarns is illustrated in Fig. 19 as the distance between A″₀ and D₀. The weaving term ends per inch can be calculated by the inverse of the perpendicular distance between parallel yarns calculated for a braided structure.

Equations 1 to 10 may be applied both to braided structures with geometries configured to be oriented along a vertical axis, as in the example of the baseball bat, as well as to parts with geometries that are not oriented along a vertical axis, as in the example of a bridge arch illustrated in Fig. 20.

While the horizontally adjacent unit cells of the braided structure applied to the baseball bat in Fig. 14 may be of the same width and height, radially adjacent unit cells of the braided structure applied to the bridge arch will vary in unit cell height. That variability is due to the change in the radius of curvature (ROC) from the inner and outer radii of the bridge arch. Unit cells located on the innermost ROC of the bridge arch will have the smallest heights, while unit cells on the outermost ROC of the bridge arch will have the greatest heights. These unit cell heights are highly predictable upon application of Eq 2 and 8. However, Eq 2 and 8 must also be coupled with a ratio of the ROC of the bridge arch at a particular radial location to the ROC of the bridge arch at the midline.

Changes in unit cell height along the radial direction of the bridge arch in Fig. 20 will create changes in bias angle. The smallest bias angles will occur on the outermost ROC of the bridge arch, while the largest bias angles will occur on the innermost ROC of the bridge arch. Determination of unit cell height at a particular radial location, or ROC, on the bridge arch leads to the determination of bias angle at that location.

Before determination of the unit cell height and angle on the outermost and innermost ROC of the bridge arch, the unit cell height must be determined at the midline ROC, or midline radius of the arch. To calculate the unit cell height at the midline radius, Eq 8 can be used along with the TTT measurement along the midline radius of the arch, as illustrated in Fig. 20. In the example of the bridge arch in Fig. 20, the TTT measurement is an arc length and not a straight-line displacement.

Finally, Eq 2 may be used to determine the angle at the desired ROC of the bridge arch.

By adding material into the vertical direction of the braided structure such that the material in the bias directions intertwines with the material in the vertical direction, a triaxial braided structure may be formed. A triaxial braided structure is depicted in Fig. 13(b).

By allowing pathways through the center of each horn gear and by providing yarn guides through which to thread material through the center of the horn gears, material in the vertical direction of the braided structure may be added. As the spool carriers move around the braiding machine, the material passing through the center of the horn gear will become inserted within the braided structure; however, this material will not intertwine with other tows in the braided structure and will experience minimal crimp. Therefore, a triaxial braided structure contains material oriented at acute positive and negative angles with respect to a vertical axis, as well as material oriented parallel to the vertical axis or 0° direction.

As illustrated in Fig. 18, a triaxial braided structure is comprised of tows oriented in the 0° direction of a braided structure. An important characteristic of a triaxial braided structure is the distance between axials within a braided structure. The space between potential axial positions is equal to the width of a triaxial unit cell. By knowing the width of a unit cell and the location of axials within a braided structure, the spacing between axials in a triaxial braided structure can be calculated.

Due to the nature of the weaving process, woven products are anisotropic materials, meaning the properties within the material are not balanced in all directions (Ref 3). Due to their anisotropic nature, multiple layers of a woven material that alternate in fiber orientation must be applied to achieve a laminate structure having balanced in-plane properties (Ref 10).

While woven materials are anisotropic in nature, a single braided ply structure can be formed that is quasi-isotropic in nature. A quasi-isotropic material will have properties very close to isotropic materials, in which the properties of the material in all directions are perfectly balanced. A braided structure with a ±60°/0° architecture, or a triaxial braided structure with the bias material oriented at ±60° to the 0° axial direction, is an example of a quasi-isotropic braided structure. A quasi-isotropic braided structure is depicted in Fig. 21. A ±60°/0° quasi-isotropic braided structure will additionally have equal material content by weight in the +60°, −60°, and 0° directions, or 33% of the weight in each direction.

In addition to biaxial and triaxial braided structures, hybrid braided structures (Fig. 13c) may also be created. A hybrid braided structure may contain both biaxial and triaxial regions within the same braided structure. A hybrid braided structure can be created by pulling axial fibers through specific horn gear positions and omitting axial fibers in other horn gear positions. A hybrid braided structure allows for design optimization to put strength where it is required.

Braided fillets are preformed, shaped braids designed to fill open volumes in layups that would otherwise be resin rich (Fig. 22). Fillets can be round rope braids, triangular preforms, or square preforms, depending on the shape needed to fill the part geometry. Traditionally, composite manufacturers added bundles of unidirectional material to combat resin-rich areas, but this often resulted in a mismatch in stiffness between the unibundle and the surrounding composite. Braided fillets can be tuned to match exactly the stiffness and other material properties of a part, minimizing delamination concerns.

The conformability of braid is not limited to the application of continuous sleeves of braid applied either offline or through overbraiding. Cut flat sections of braided fabric may also be applied in molds or on other surfaces. Upon application to a contoured surface, a braided structure will readily conform to the part.

Braided fabrics are produced on standard maypole braiding machines; however, following braid formation, the tubular braided structure is slit open along the axial direction, opened to its full width, and then taken up onto a core as a flat fabric product. Additionally, a tubular braided product may be flattened, and the edges of the tube may be slit away to form two layers of braided fabric. In additional examples, the edges of the tube may remain intact. Braided fabrics may be of biaxial, triaxial, or hybrid architecture, as illustrated in Fig. 13. Braided fabrics may be produced to widths matching standard woven commodity widths.

Biaxial braided fabrics provide efficient layup of continuous ±45° orientation. Traditionally, to create long lengths of ±45° material, 0°/90° woven material is cut on the bias and butt spliced together. The emergence of wide ±45° braided fabrics enables seamless layup of 45° layers, reducing layup labor and improving quality. Furthermore, the bias yarns in biaxial braided fabrics are tensioned the same during manufacture, so properties in each direction are equivalent. Conversely, when producing woven fabrics, the warp yarns are tensioned more than the weft, resulting in more crimp and a knockdown in properties in the weft direction. The mechanical properties of several biaxial braided fabrics are provided in Table 1 (Ref 11).

The 0°, ±60° fabric has equal material by weight in every direction, enabling easy layup and eliminating the need to orient plies. To achieve a balanced laminate with conventional 0°/90° fabrics, four layers or a multiple of four layers is required (Fig. 23) (Ref 10). The 0°, ±60° braided fabric is balanced within a single layer, so the fabricator has the design flexibility to use the exact number of plies necessary to meet strength and stiffness requirements (Ref 10). The quasi-isotropic architecture enables the efficient use of material to minimize part thickness and reduce material and labor costs (Fig. 24) (Ref 10). Benchmarked against a comparable woven laminate, 0°, ±60° braid outperforms woven fabric in tension, compression, and open-hole compression (Ref 12). In an impact event, interlaminar shear stresses are reduced due to the uniform ply architecture, resulting in better energy absorption and higher impact resistance from 0°, ±60° braid (Ref 12).

The available range in braiding machine sizes enables flexibility in design, providing customized solutions to reduce manufacture time and minimize scrap. Fabrics can be made to the exact width needed to meet the geometry of a part, specific areal weights can be produced to provide mechanical properties required at the minimum gage thickness, and architectures can be hybridized across the width of a fabric to increase drapability of a braided fabric. Axial content can be varied within a braided fabric from 10 to 90%. An example of a braided fabric with a high percentage of axial content is shown in Fig. 25. Material types can be hybridized as well depending on the mechanical performance needed; that is, high-modulus carbon can be used in the axial direction to increase stiffness while using standard modulus in the bias to create a cost-effective solution.

Figure 26 depicts a hybrid biaxial/triaxial fabric created for layup of a hat stiffener. The biaxial regions provide conformability, while the triaxial locations provide increased stiffness where needed. An additional example of a hybrid braided structure, for the creation of a flange, is shown in Fig. 27.

The use of braid in advanced composite structures has become increasingly common since the early 1990s (Ref 12). In particular, braided quasi-isotropic fabric has enabled easy lay-up by reducing the number of plies and eliminating the need for ply rotation and has provided increased performance properties for a variety of composite applications (Ref 12).

When evaluating braid mechanical properties, one must consider the method of production of a braided product versus a traditional woven product. A woven product is produced such that the fibers are oriented at 0° and 90° to the longitudinal machine axis, while a braided product is produced such that the fibers are oriented at oblique angles to the longitudinal machine axis. Further, a woven product will never be produced with fibers oriented at any orientation other than 0° and 90°, while biaxial braided structures may comprise bias angles commonly ranging from ±15° to ±75°. The nominal angle that a biaxial or triaxial braided structure may be produced to is dependent on the final application of the braided structure, the diameters to which the braided structure may be applied or required to achieve, the desired performance properties, and the desired coverage.

Current test methods are devised to accurately capture woven product properties, and considerations must be made for braided products. In the case of a 0°/90° woven product, testing will always occur in the fiber direction; however, for a ±45° braided biaxial product, the same testing may occur off-axis and may result in a perceived knockdown in properties. Table 1 reports the mechanical properties of a ±45° biaxial braided product in the off-axis orientation, in which the braid is oriented in the as-produced state, as indicated by the 45° mechanical properties. Additionally, Table 1 reports the same mechanical properties of the same bixaial braided structure oriented in the fiber direction, in which the braid has been rotated to align the fibers in the 0° and 90° directions, as indicated by the 0° and 90° mechanical properties. As observed in Table 1, off-axis testing of a braided structure will result in the appearance of reduced mechanical properties when compared to the same testing in the fiber direction.

The weaving process naturally results in the creation of products with varying amounts of crimp and yarn tension in the warp (0°) and weft (90°) directions. The warp yarns comprising a woven product tend to have less induced crimp, while the weft yarns that intertwine over and under the warp yarns tend to have comparatively more crimp. The differences in yarn tension and crimp between the warp and weft yarns may result in a 10% knockdown of properties in the weft direction, which must be accounted for in part design and layup. Unlike a woven product, a biaxial braided product will demonstrate the same properties in either of the bias directions, as shown in the 0° and 90° mechanical properties of Table 1.

In the case of a ±45° biaxial structure, the braid in testing may be simply rotated to mirror woven product test methods; however, due to the range of bias angles that a braided structure can be produced in, this is not always the case. There may be no other option for testing a braided product than in the off-axis orientation. Additionally, fiber type and resin system may also impact testing data, especially in the off-axis orientation in which the mechanical properties are tested across the resin system.

The mechanical performance of a braided composite laminate can be predicted using classical lamination theory for both biaxial and triaxial constituent plies. For both architectures, the amount of fiber reinforcement in each of the defined fiber directions is a known quantity that is likely described in terms of fiber areal weight, thickness, or percentage of the overall ply. To apply classical lamination theory, each fiber direction within the braided ply is considered to be its own lamina that is oriented at the defined fiber angle and constitutes the known quantity of fiber in each direction.

Unidirectional fiber properties can be used to define each lamina but will result in idealized mechanical property estimates. Similar to the method used to model woven laminates, a knockdown factor will be applied to the unidirectional lamina properties, particularly for the bias fiber directions—to account for crimp in the textile. Therefore, when modeling bias-direction plies of braided structures using unidirectional lamina properties in the same orientation, a 2 to 7% knockdown, depending on the braid construction, should be applied to accurately predict off-axis composite properties. Typical braid constructions call for a 3.5 to 4% knockdown. Unlike woven warp and weft yarns, the bias fibers in a braided structure are tensioned equally, so no distinction is needed for positive and negative bias tow directions. Axial fibers in a triaxially braided ply will be virtually uncrimped, so unidirectional properties are appropriate for the axial fiber lamina.

Biaxial sleeving braid properties and mechanical properties are predictable. When designing using a biaxial sleeving, a design curve is created to predict and verify the off-axis material properties through various angle changes. Design curves for a standard-modulus carbon-fiber sleeving are shown in Fig. 28 and 29.

Also available is a B-basis design allowable through the Federal Aviation Administration-certified Advanced General Aviation Transport Experiments (AGATE) material qualification methodology using an AS4 6K GP biaxial sleeving with PR520 resin (Ref 13, 14). The AGATE methodology produces design curves by testing braid properties at ±30°, ±45°, and ±60° off-axis conditions, then interpolating the remaining properties based on the tested conditions.

As the use of braid in composite parts has increased, so has the testing of braid compared to woven fabrics (Ref 12). This testing has included coupon testing to verify product performance, as well as impact testing (Ref 12).

In a 2017 study conducted to compare braid-reinforced composites to woven-reinforced composites, two quasi-isotropic braids with fibers oriented at ±60°/0° and two woven fabrics with fibers oriented at 0°/90° were compared (Ref 12). Quasi-isotropic laminates were created for all four of the material types in this study (Ref 12). The number of plies and the ply orientation for the creation of the quasi-isotropic laminates for each material are reported in Table 2 (Ref 12). During layup of the panels comprising the ±60°/0° quasi-isotropic braided fabrics, all plies were cut and nested in the same 0° direction, while, due to the anisotropic nature of the woven fabric, the panels comprising the woven 0°/90° material were comprised of plies cut at a 45° orientation and at a 0° orientation (Ref 12). In this study, it was found that quasi-isotropic braid-reinforced laminates showed clear performance benefits compared to the woven fabric quasi-isotropic laminates in both impact testing and mechanical testing (Ref 12). The mechanical test results of this study are reported in Table 3 (Ref 12), and the impact testing results of this study are provided in Table 4 (Ref 12).

The standard ASTM International test methods for the mechanical properties of quasi-isotropic braided structures are listed in Table 5; however, a modified version of ASTM D3039 is recommended for transverse 90° tensile strength testing for quasi-isotropic braided structures of aerial weights greater than 500 g/m² (Ref 15). Additionally, a specific coupon width, within the accepted ASTM International range, is recommended for ASTM D3039 longitudinal 0° tensile strength and modulus testing as well as ASTM D3039 transverse 90° tensile modulus testing for heavyweight quasi-isotropic braided structures (Ref 15).

For ASTM D3039 longitudinal (0°) tensile strength and modulus testing, a straight-sided coupon similar to that illustrated in Fig. 30 is used (Ref 15). For heavyweight quasi-isotropic braided structures with areal weights more than 500 g/m², it is recommended that the minimum width of the coupon used in testing is the value of the unit cell of the braided structure multiplied by four (Ref 15). This minimum width, as illustrated in Fig. 30, is recommended to capture a representative number of unit cells of the heavy quasi-isotropic braided structure for accurate measurement of strength values (Ref 15).

When testing a heavyweight braided structure for transverse (90°) tensile strength, the standard straight-sided coupon does not contain continuous fiber from grip to grip, allowing failures to initiate at the coupon edges, which results in premature tensile strength failure of the braided structure (Ref 15). Therefore, using guidance from ASTM D6856/D6856M-23, a bowtie coupon similar to that illustrated in Fig. 31 is recommended for testing (Ref 15). This bowtie coupon allows continuous fiber to travel from grip to grip for accurate load transfer and yields accurate transverse strengths representative of braid performance in composite structures (Ref 15). However, the modulus cannot accurately be measured with a bowtie coupon (Ref 15). A straight-sided coupon with a minimum width defined by the value of the unit cell of the braided structure multiplied by four is recommended for transverse (90°) tensile modulus testing, as illustrated in Fig. 32 (Ref 15).

The ability of braid to conform to complex part shapes lends braid to a wide variety of applications. While in the example of the baseball bat, the braid was applied in a step separate from the braiding process, braid may also be applied to a mandrel inline for the creation of braided preforms. A few examples of overbraiding production cells are illustrated in Fig. 33 and 34. Examples of overbraided parts are depicted in Fig. 35 to 38. The overbraiding process replaces traditional layup methods for the creation of composite parts. Overbraiding is a low-cost, highly efficient, repeatable, automated, and low-waste-production process for the creation of composite parts.

During the braiding process, a mandrel is inserted into the point of formation of the braided structure. The motion of the horn gears of the braiding machine and the motion of the mandrel are coordinated with one another such that the braid forms around the mandrel. The speed of the motion of the horn gears and the rate at which the mandrel is pulled through the point of braid formation determine the angles at which the braid will be formed onto the mandrel. The overbraiding process can be automated to maintain a constant angle or a constant thickness along the length of the overbraided part. In an example of a braided aircraft engine stator vane (Fig. 35), the angle of the braided structure overbraided onto a vane mandrel can be controlled during the braiding process to create areas of localized thickness and stiffness (Ref 17).

An example of overbraiding technology supporting the creation of preforms of complex geometry includes the braid production defined in U.S. patent 7,793,576B2, titled “Braided Reinforcement for Aircraft Fuselage Frames and Method of Producing the Same.” This patent provides an example of the application of the overbraiding process to curved sections of aircraft fuselage frames comprising varying radii of curvature (Ref 18).

The overbraiding process is not limited to the application of a single layer, or ply, of material. Several layers of a braided structure may be applied in succession to a mandrel. Additionally, braided layers may be applied to specific regions of a mandrel to create localized buildups.

Advancements in overbraiding technology have granted manufacturers of large composite structures the architectural benefits of braided reinforcements as well as the economic efficiencies found in standard braiding. The use of braided reinforcements for large structural preforms has increased dramatically, signifying an eagerness to apply the structural advantages found with braid as well as the cost-efficiencies discovered in the fabrication of large composite parts (Ref 8). This rapid accumulation of experience is demonstrated by the selection of overbraiding by the U.S. Air Force to enable high-rate production of military aircraft (Ref 19). The overbraiding of an aircraft fuselage for the construction of military aircraft is depicted in Fig. 37, while the overbraiding of an inlet duct for military aircraft is depicted in Fig. 38.

While some braided fabrics may be taken up onto a core of fixed diameter along the length, similar to the core of a roll of paper towels, in other examples, the braided fabric may be taken up onto a core, or mandrel, of complex geometry. In a case study described in Ref 20, a triaxial braided sleeving was taken up onto a contoured drum following formation of the braided structure (Ref 20). As the braided sleeving in this example was taken up onto the contoured drum, the axials within the braided structure were taken up at different rates, such that the braid conformed to the contoured drum, and the shape of the braid became fixed to the contour of the drum (Ref 20). In large-radius areas of the drum, the axials were taken up at a faster rate and were longer in length than axials in smaller-radius sections of the drum, which were shorter in length and which were taken up at a slower rate (Ref 20). Due to the difference in axial lengths across the width of the braided sleeving, the shape of the sleeving became fixed to the shape of the contoured drum (Ref 20). An example of the formation of a contoured sleeving is illustrated in Fig. 39.

The example of the jet engine fan case described in Ref 20 demonstrates the development of an efficient process for the creation of a composite part of complex geometry, as well as the application of a braided structure to a part that has traditionally been comprised of metal or a layered metal and fabric system (Ref 20). This composite braided fan case is additionally comprised of a quasi-isotropic braided structure, which has uniform lamina stiffness properties in all directions (Ref 20).

Fan cases are one of the heaviest parts of an airplane engine and are used to contain a released fan blade during a “blade-out event” (Ref 20). An additional example of a composite fan case made for the GEnx engine program by GE Aviation has demonstrated a weight-savings of 160 kg (350 lb) per engine and 30% improved blade containment properties than traditional fan containment cases (Ref 21).

Thermoplastic braided materials made from fiber combined with a thermoplastic matrix enable the high-rate, lightweight, and predictable performance demanded by emerging markets. Braided sleevings, fabrics, and preforms can be made using commingled thermoplastic yarns or slit-tape thermoplastics. The slit tapes are made by slitting a unidirectional thermoplastic into narrow widths (3.2 up to 25 mm, or ⅛ up to 1 in.) and then winding these tapes onto modified bobbins designed to accommodate this stiffer material. Examples of a fabric made with thermoplastic materials and a preform made with braided thermoplastic tape are shown in Fig. 40.

Thermoplastic slit-tape braided reinforcements provide near-unidirectional (UD) properties and are offered in architectures that best align with the load path of a structure. Braided tape reinforcements offer superior drape compared to UD tapes and semipreg fabrics while maintaining a combination of near-UD properties and high fiber volumes that have reduced part complexity and weight. Like commingled braids, slit-tape braided materials are provided as fabrics, sleevings, and net-shape preforms and are customized to the width or cross section of a part, minimizing waste. The preformed, preconsolidated battery box shown in Fig. 41 made with slit-tape 0°, ±60° fabric demonstrates the increased conformability enabled by braided thermoplastics through a high-rate stamp preforming process.

There are a number of braid manufacturers in the United States actively developing braided structures for the formation of a wide variety of composite parts. A brief sample list of braiding companies in the United States is provided in Table 6.