Overview of Recent Developments in 3D Structures

Transcription

1 Overview of Recent Developments in 3D Structures Michael McClain, Senior R&T Engineer Organic Matrix Composites Jonathan Goering Divisional Chief Technology Officer Albany Engineered Composites 112 Airport Drive Rochester, NH Three dimensional woven composite structures are slated to provide significant weight savings on the next generation of engines for commercial aircraft. The inherent toughness of these structures, the weight savings and the ability to weave near net shapes were key drivers in their selection for these applications. This discussion will provide an overview of the 3D weaving process as well as a summary of recent work in the area of 3D composite and preform fabrication. On-going work currently focuses on using the technology to fabricate beams and frames. Of particular interest is the fabrication of sine wave beams that do not require the darting associated with prepreg type structures. 1 Introduction 3D woven composites, molded using the Resin Transfer Molding (RTM) process, are being utilized and considered for a wide variety of aerospace applications, including landing gear braces, fan blades, and fan cases. In all of these components, the strength and performance of the final products is inherently connected to the 3D woven structure of the parts. For this reason, this paper will begin with background on the 3D weaving process. 2 3D weaving Jacquard weaving, also known as three-dimensional (3D) weaving, is a textile process that can be used to fabricate complex, net-shape 3D fiber preforms. For the purposes of this paper, the term 3D woven preform is used to describe multi-layer textiles that have fibers that interlock two or more layers of preform. Just as with 2D woven fabric, the basic 3D woven preform has fiber oriented primarily in two orthogonal directions (the warp direction at 0 and the weft direction at 90 ). The difference, however, is that 3D preforms can have a tailored through- 1

2 thickness (z) component. As depicted in Figure 1, warp weaver refer to the warp fibers or tows that have this through thickness component. The ovals represent the weft fibers (directed into the plane of the page) and the green lines depict warp stuffers. Warp stuffers are used to increase stiffness along the warp direction of the preform. Z Figure 1 Tow definition in a 3D preform Orthogonal fiber architecture Z Warp Typical 3D fiber architectures Ply to ply interlock fiber architecture Z Warp Through thickness angle interlock fiber architecture Warp Z Unreinforced plane Conventional laminate with 2D Fabric plies Conventional laminated fabric Warp Figure 2 Schematics of 3D woven and conventional 2D laminate fiber preforms (weft direction into page) 2

3 Figure 2 shows some samples of more common 3D weaving architectures. Again, the ovals represent fibers oriented in the weft direction, while the warp fibers are represented by the continuous lines weaving over and under the weft fibers. Note that though theree are unreinforced planes between the fabric layers in the 2D fabric laminate illustration in Figure 2, these planes do not exist for the 3D fiber architectures. The z-direction reinforcement that the warp weaver tows provide is referred to as continuous, through thickness reinforcement. This type of reinforcement: interlocks the preform layers in the z-direction improves through-thickness strength and stiffness improves final part damage tolerancee Jacquard weaving machines that perform 3D weaving allow engineers to control where and how fiber tows are placed within a 3D woven preform. These machines are typically controlled electronically and may be programmed for a number of different fiber architectures, operating with minimal intervention. With this ability to control how fibers are woven together, one can incorporate geometric features - such as changes in thickness, integral stiffeners, or bifurcations - directly into the preform. The term bifurcation refers to a local area of a 3D preform without through thickness reinforcement that can be separated up to a point; this separation enables the preform to later have corners and joins through which continuous fibers run. The as woven preform in Figure 3 has two bifurcations; which open to a point and formss a flange. When molded, the resultant composite I-beam would have continuous fiber woven throughout the joint between the web and the flanges. Figure 3 Construction of straightforward 3D woven I-beam using bifurcations 3

4 This ability to control fiber architecture and preform geometry can be used to produce near net shape articles that are ready for molding with minimal trimming or hand work (see Figure 4). Preform geometries can vary from simple cross sectional shapes, such as Pi s or T s [1], to extremely complicated shapes, such as the airfoil geometries used in jet engine fan blades. To produce similar geometries using a 2D laminated process often requires ply cutting and lay-up steps that increase cost and can introduce fiber damage, distortion, or stacking errors. Figure 4-3D woven, near net shape preform that was woven as a single part with Jacquard weaving (left) and molded using RTM (right). One can also engineer the through-thickness and in-plane properties of 3D woven preforms by varying the tow size/type and local fiber volume. In conjunction with the geometric changes, this enables a designer to vary the section properties (i.e. stiffness and/or strength) along the length of the preform, which translates into tailored properties in the finished composite structure. 3 3D Woven Composites Some advantages of 3D woven composites, as compared to traditional 2D laminated structures, include: improved toughness in the final molded part due to continuous through thickness reinforcement, reduced fabrication labor, reduced scrap, and lowered overall part cost due to the ability to produce near net shape preforms, and elimination of delamination as a failure mode The primary disadvantage is that the main acreage of a 3D woven composite does not contain off-axis fiber to support off-axis loads. To address this, off-axis properties are often improved by other means, perhaps through the addition of structural stiffeners or by increasing the thickness of the part. These types of choices can, however, result in increased weight. 4

5 Figure 5 shows one such composite structure, producedd via resin transfer molding (RTM) and a single 3D woven preform. The stiffeners, one main longitudinal and three shorter transverse, are integrally woven into a skin to form a stiffened panel using the bifurcation weaving methods previously presented in this paper. As a result, there iss continuous fiber running from the skin into each stiffening element, thus eliminating the need for any secondary bonds or fasteners at these joints. This continuous fiber reinforces what would be a resin rich bond line at the interface of the stiffener and flat acreage in a conventional laminated composite. New developments in weave design enable the fiber to also reinforce the joints between the stiffeners. Figure 5 Stiffened panel utilizing a 3D woven fiber preform A similar approach can be used to make a straightforward 3D woven I-beam preform. First, a multilayer fabric is woven with bifurcations along the edges, as was depicted in Figure 3. The I cross sectional shape is formed by folding the two halves of each bifurcation outward by 90º. While straightforward, this particular I-beam concept contains limitations. In particular, the lack of off-axis fiber in the web diminishes the load carrying capacity when compared to a beam with off-axis fibers in the web. To overcome this limitation, a techniquee for including off axis stiffening elements was developed. This technique is based on weaving a bifurcation between two nterlocking layers oriented along a diagonal line rather than being oriented parallel to one of the principal fiber 5

6 directions. An example of this type of bifurcation is shown in Figure 6. In the preform shown, there is continuous fiber running from the skin into each of the four sections of stiffener. Figure 6 Integrally woven off-axis stiffener To improve the shear carrying capability of a beam, it is possible to combine the offf axis bifurcation techniquee shown in Figure 6 with the edge bifurcation shown in Figure 3. An example of a molded beam that utilizes this preform concept is shown in Figure 7. Important features of this part s preform include continuous fiber linking the caps to the web, the web to the off-axis stiffeners, and the off-axis stiffeners to the caps. Again, this eliminates the possibility of resin rich areas at these interfaces. It is possible to vary both the percentages of warp and weft fiber, as well as the numbers of layers in each area of this preform. This ability to tailor the weave architecture throughout a part allows designers to optimize composite performance in various regions of a single part for various purposes; one section may be woven to supportt high shear loads and another areaa may be woven to accommodate bending loads. In the design used for the part in Figure 7, axial and bending loads are carried primarily by the 0º fiber in the caps, through thickness compression loads are carried by the 90º fiber in the web, and shear loads are carried by the off-axis stiffeners. The beam includes off-axis stiffening members on both sides of the web, offset from one another by 90 degrees. In other words, the stiffenerss are orthogonal to one another on opposing sides of the web instead of planar, though it is possible to make the stiffeners planar if the situationn so warrants. 6

7 Figure 7 Prototype truss beam with integral off-axis stiffeners; Fabricated using a single 3D woven preform and resin transfer molding A recent development in 3D woven preforms is the ability to produce longer segments. Figure 8 shows the longest part woven and molded to date, 26.5 feet (8.1 m) long, by Albany Engineered Composites (AEC). For scale, the person holding the part is approximately 6 feet (1.83 m) tall. This particular component enabled development of new methods for weaving, handling and molding parts of this length. This part was woven and processed into final preform shape a at AEC s Rochester, NH facility. The preform was then shipped to AEC s Boerne, TX facility for molding. A total of six parts were successfully molded using toughened resin in six attempts using resin transfer molding. The volume fraction was in the anticipated range (~58%) while the void content was measured at less than 1%. Another recent development in 3D woven preforms is the ability to weave Pi preforms that are continuous around corners. While the prototype displayed in Figure 9 shows an approximate 90 degree corner, large radii bends and other angles are possible. Additionally, it is possible to change the thickness and width of the legs and base flange. It is also possible to change the width between the upright legs to accommodate different thicknesses in the inserted component (i.e. bulkhead, web, closeout, etc.). 7

8 Figure feet long molded composite part formed from a single 3D woven preform (with off-axis stiffeners) and resin transfer molded Figure 9 Corner pi preform with continuous fiber 8

9 4 Multi-Component Preforms For those situations where off-axis fiber is necessary, 3D woven Pi preforms have been proposed for use as joining elements in advanced composite structures [2]. Typically, these preforms have been used as inserts that help join a web to a skin, as shown in Figure 10. The upstanding legs of the Pi allow loads to be transferred between the web and skin through double lap shear, and the Pi s flange helps to distribute pull off loads over a larger area. Figure 10 Typical application of a 3D woven Pi preform to join 2D laminated web and skin via RTM In addition to using the flange of the Pi as a means for attaching a web to a skin, it can also be used directly as the cap in a beam. Using this multi-component preform approach, the flange of the Pi can be engineered to have a high percentage of axial fiber to help carry axial and bending loads, while the web laminate (which the Pi caps onto) can be tailored to carry transverse compression and shear loads without buckling. Very simple and efficient I-beams using this approach have been demonstrated [3]. Beams with straight, curved, constant cross section and variable cross section geometries (i.e. beaded) have also been fabricated using 3D woven Pi preforms as the caps and conventional laminates as the webs. Figure 11 shows a beaded web demonstrator that was fabricated using this approach. 9

10 Figure 11 Beaded Web Demonstrator Some recent work to further improve the performance of an I-beam constructed using a multi component preform has focused on using a sine wave web along the beam s length (depicted in Figure 12). The sine wave shape provides geometrical stiffening that allows the web thickness to be reduced while the beam s performance otherwise remains the same. The beam s construction utilizes three relatively simple pieces, as shown in Figure 13. The preform weaving becomes somewhat more complex to form Pi caps with legs that follow sine wave geometry. Since the weaving process is automated, however, the cost impact is minimized. A beam such as this could be further customized as well. In this particular case, the geometry of the preform is constant. It is possible, however, to alter the preform s geometry (i.e. thickness of flange and/or upright legs, distance between upright legs for pad-ups, amplitude, wavelength, etc.) to tailor the beam s design as needed. The laminated web can include bias plies (±45º plies) to carry shear loads, and 0º/90º to carry transverse compression loads. It is worth noting, however, that the percentage of 0º/90º material can be reduced because of the geometric stiffening provided by the sine wave shape. Assembly of the final preform is very straightforward. The web is laid up as a series of simple rectangular pieces and formed into the sine wave shape. The caps are simply cut to length and placed on either edge of the web laminate. The entire preform can then be placed into a tool and resin transfer molded. Figure 13 shows one such molded prototype beam. 10

11 Figure 12 Components in a sine wave web beam preform Figure 13 Prototype sine wave web beam 11

12 5 Conclusion and Future Work 3D woven composites offer a number of unique strengths, most of which arise from Jacquard weaving s ability to weave near net shape preforms with continuous through thickness reinforcement. The topics presented here represent a number of recent development areas: Method for fabricating (weaving through molding) longer 3D composite components (up to 26.5 feet without any known restrictions to extend to greater distances) Multi-component preforms for beam structures, to improve off-axis load bearing capacity Unique preform design and arrangement that permit off axis fibers. Future work will examine methods to further reduce costs through automated processes in weaving and molding. New joining concepts will also be explored to realize the full potential of 3D woven preforms in composite structures. 6 References 1. Goering, J., and McClain, M., Recent Developments In 3D Woven Pi Preforms, American Society for Composites, 22 nd Technical Conference, Seattle, WA USA, September 17-19, Bersuch, L., Benson, R., and Owens, S., Affordable Composite Structures For Next Generation Fighters, 43rd International SAMPE Symposium, Long Beach, CA USA, McClain, M., Goering, J., and Rowles, C., Web Stiffened Stretch Broken Carbon Fiber Frame Fabrication, SAMPE 2009, Baltimore, MD USA, May 18-21,