Development of Low-Cost

Damage-Resistant Composites

Hiroshi Uchida, Takumi Yamamoto, Hiroki Takashima

Braider Section,

MURATA Machinery, Ltd., Kyoto, JAPAN

Abstract

    Composites will be needed for outer shell structure of aircraft in order to achieve needed weight reduction. Fabrication costs and damage-resistance, however, remain a problem. Murata Machinery, Ltd. and Mitsubishi Heavy Industries, Ltd. have joined together to develop innovative composites that are cost-effective and offer superior damage resistance. This paper discloses the means by which a 5-axis 3-D woven fabric is fabricated using our 5-axis 3-D loom in cooperation with a Resin Film Infusion ("RFI") process. Further, damage-resistance of the fabricated product was disclosed in CAI tests. The 5-axis 3-D woven composites were shown to have very high damage-resistance qualities.

1. Introduction

    New composite materials will be at the heart of major technical breakthroughs in the 21^st century. For example, composites will be needed for the outer shell structure of aircraft in order to achieve needed weight reductions. But before conventional laminated composites can be regularly employed in large primary structures such as the fuselage section of an aircraft, damage resistance must be improved and production costs reduced. It is hoped that research into Z-direction fiber arrangement will yield breakthroughs in damage resistance. NASA's ACT (Advanced Composite Technology) program, for example, is applying stitching, braiding, and knitting technology to 3-D composites, and has achieved significant reductions in cost (20 to 25%) and weight (30 to 35%) in the wing and fuselage sections of commercial aircraft^1-3). Fabrication costs and continuous operation productivity, however, remain a problem.

    Murata Machinery, Ltd. and Mitsubishi Heavy Industries, Ltd. have joined together to develop innovative composites that are cost-effective and offer superior damage resistance. Towards this end, Murata and Mitsubishi have developed a 5-axis 3-dimensional loom (the "5-axis 3-D loom") that performs continuous automatic weaving, and have adapted it for use with a simple molding process.

    This paper discloses the means by which a 5-axis 3-D woven fabric is fabricated using our 5-axis 3-D loom in cooperation with a Resin Film Infusion ("RFI") process. This paper further discloses our test results indicating the mechanical properties (including the tensile, compression, open-hole tensile, and open-hole compression properties) and damage resistance (measured in Compression After Impact, or "CAI," tests) of the fabricated product.

    This paper originates from research first presented in "Study on Low-Cost Damage-resistant Reinforced Outer Shell Structures," (Society of Japanese Aerospace Company, 1998-1999)⁴⁾.

2. 5-axis 3-D Loom

    Figure 1 shows frontal and side-views of the 500mm automatic 5-axis 3-D loom developed jointly by Murata and Mitsubishi. This loom enables fully automated weaving of three-dimensional woven structures. Fibers are arranged in the woven structure at 0°, +/-45°and 90°in the X-Y planes, and are also inserted in the Z-direction. The fibers are woven at 4mm intervals (yielding a 4mm pitch). The machine allows rapid formation of 5-axial 3D structures. Further, the fiber supply devices can be adjusted so as to keep the woven fibers at an optimal tension, and therefore keep them straight. Keeping the fibers at a proper tension also keeps the fibers apart, preventing chaffing and minimizing damage.

Fig. 1   5-axis 3-D loom.

3. Design of the 5-axis 3-D Fabric.

    The 5-axis 3-D fabric woven for this study was designed to meet the requirements for use as a fuselage panel in a commercial aircraft. A carbon fiber (T800H by Toray) of sufficient strength and elastic modulus was used as the in-plane (X-Y-axis) fiber. A carbon fiber (TR40 by Mitsubishi Rayon) with low filament breakage due to buckling was used as the Z-direction fiber.

Table 1   Fiber arrangement data.

                                             In-plane : ±45°/ 0°/90°

    Fiber arrangement data is shown in Table 1. Since the fuselage side panel of an aircraft bears mainly shear load, it is preferable to have a design in which a greater proportion of fibers are arranged at +/- 45°. The appearance of the 5-axis 3-D fabric weave described by Table 1 is shown in Figure 2. It should be noticed that the automatically woven fabric has very little hairiness

Fig. 2  The appearance of the 5-axis 3-D fabric.

4. Resin Film Infusion (RFI)

    RFI is a process whereby a resin film is impregnated in a preform by melting it the preform . The RFI bagging configuration used for the example disclosed herein is shown in Figure 3. A resin film was placed under the 5-axis 3-D fabric to effectively remove the air inside the 5-axis 3-D fabric. The resin film was prepared beforehand by melting bismaleimide (5250-4-RTM, Cytec) at approximately 100°C and precasting into the shape of the panel.

    A cross-section of the fabricated woven composite after RFI is shown in Figure 4. No fatal defects, such as voids, cracks, etc. were observed in the cross-sections, indicating that a good flat panel product (Vf=50%, approx. 1mm thickness) was produced.

Fig. 3   Bagging configuration.

Fig. 4   The cross section of 5-axis 3-D composite.

5. Mechanical Tests

    Tensile and compression tests were carried out according to ASTM D3039 and D3410 standards. Figure 5 shows the results of tensile and compression tests on the 5-axis 3-D woven composite compared to conventional stitched 2D laminated composite. Unusual fracture behavior was not observed in the 5-axis 3-D woven composite, and the fracture strain characteristics were roughly the same as those of conventional 2D laminated composites. This indicates that the tensile and compression properties of the 5-axis 3-D woven composite are comparable to those of a conventional 2-D composite.

    Open-Hole Tensile and compression tests were carried out according to SACMA SRM5 and SRM3 standards. The woven composite specimen was 38.1mm in width, with a 0.167 a/w ratio of hole diameter to width. Figure 6 shows the test results; the ratio of open-hole strength to non-hole strength is approximately 80%, much higher than that of 2D laminar composites.

Fig. 5   Tensile and compression property.

                       (*Expected from reference⁵⁾)

Fig. 5   Open-hole tensile and compression property.

6. CAI (Compression After Impact) Test Results

    In order to conduct the CAI test according to the SACMA method, the tested specimen must be thick enough so that it doesn't buckle. Since the 5-axis 3-D woven composite produced was only 1mm thick, however, a few changes had to be made in order to avoid buckling. A 25.4mm honeycomb board was placed between two 5-axis 3-D composite panels to create a "sandwich structure". Other test conditions were based on SACMA SRM2.

    The relationship between the are of the after-impact damage and the energy level (data was obtained with NDI measurements) are shown in Figure 7. The impact test was also performed on a single composite panel for comparison purposes. The areas of damage on the two composites were much smaller than those on Z-axis reinforced conventional stitch/RFI material, indicating that the 5-axis 3-D composites are highly damage-resistant after impact.

    Results of the CAI test are shown in Figure 8. The composite showed a very high resistance to strain due to after-impact compression (a little over 7000m). It was thus concluded that use of 5-axis 3-D composites can improve residual strength and allow for high-strain durability.

Fig. 7   Relationship between the area of the damage and enery level.

         (*1Expected from reference⁵⁾ )

           (*2 Z-yarn Vf effective in the Z-direction)

           (*3 Normalized with the 1200 in-lbs/in value with stitched 2-D)

Fig. 8   Result of CAI test.

7. Conclusions

    A basic 5-axis 3-D composite fabrication system that employs a 5-axis 3-D loom and an RFI process has been presented. The disclosed fabrication system includes fully automated weaving of 5-axis 3-D fabric, and eliminates 35% of the production process required by conventional production systems by obviating the cutting and laying-up of each laminar weave. Furthermore, the 5-axis 3-D woven composites were shown to have superior open-hole strength and very high damage-resistance qualities.

    Trial applications of the 5-axis 3-D composite are shown in Figure 9. The picture shows a B767-type fuselage side panel with stringers and window. Stringers are also fabricated using folded 5-axis 3-D fabric. The bottom picture shows the rear fender of a Honda NSX, showing that complex quadratic surfaces can be molded. The 5-axis 3-D fabric fits the surface with very little disintegration of the 3-D woven structure. Future application to curved-surface structures are expected.

References

1)    Dexter,H.B.; Innovate Textile Reinforced Composite Materials for Aircraft Structures, Proceedings of 28^th International SAMPE Technical Conference, 1996, p.404-416.

2)    Deaton,J.W., Dexter,H.B., Marks,A. And Rohwer,K.; Evaluation Braided Stiffener Concepts for Transport Aircraft Wing Structure Applications, NASA CP-3311, 1995, part 1, p.61-97

3)    Poe,C.C.,Jr., Dexter,H.B. and Raju,I.S., A Review of the NASA Textile Composite Research, AIAA Paper No.97-1321, p.1126-1238.

4)    Enomoto, K.; Study on Low-Cost Damage-resistant Reinforced Outer Shell Structures, Society of Japan Aerospace Companies Report, 1999

5)    Nohmal,Y.; Study on Low-Cost Reinforced Composite Shell Structures with Improved Inter-laminar strength, Society of Japan Aerospace Companies Report, 1997

A fuselage side panel with stringer and window

The rear fender of a Honda NSX

Fig.9   Trial applications of the 5-axis 3-D composite