Design and Fabrication of a Braided Composite ... - ACS Publications

Chapter 6

Design and Fabrication of a Braided Composite Monocoque Bicycle Frame 1,3

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Downloaded by UNIV MASSACHUSETTS AMHERST on August 6, 2012 | http://pubs.acs.org Publication Date: April 3, 1991 | doi: 10.1021/bk-1991-0457.ch006

Nicholas Casale , Daniel Bristow , and Christopher M. Pastore 1

Department of Mechanical Engineering, Drexel University, Philadelphia, PA 19104 Department of Textile Engineering, Chemistry, and Science, North Carolina State University, Raleigh, NC 27695-8301

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In the fast growing field of competitive bicycle racing, there exists an increasing demand for high performance, lightweight bicycle frames. Whereas there have been advances in the replacement of traditional steel alloy frames with lighter weight aluminum alloys, the frames available today have reached a minimum weight. By utilizing today's high-strength, lightweight composite materials, bicycle frames can now achieve weights never before possible without sacrificing strength or rigidity. Textile structural composites have been employed as the engineering material for the fabrication of a light weight road-racing bicycle frame. The use of these materials provides advantages over metallic structures in terms of high specific strength and stiffness as well as the ability to engineer the properties of the material to enhance structural design. The use of textiles as the reinforcing element of the composite provides advantages over traditional filament wound systems by increaseing the damage tolerance of the material and reducing the fabrication labor intensity.

In this paper, an integrated design methodology (1) is presented for the design and fabrication of a composite frame. The objective is to develop design procedures which go beyond the traditional "black aluminum' design approach (i.e., direct subsitution of composites for metal) and develop a technique for exploiting the unique and anisotropic properties of composite materials. The integrated design approach incorporates composite mechanics, processing of composite materials and finite element analysis in an iterative mode to identify locally optimal fiber architectures and frame geometries, as 1

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Current address: Phillips 66 Company, Advanced Composites, Bartlesville, OK 74004 Current address: G.E. Nuclear Energy, King of Prussia, PA 19406

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0097-6156/91/0457-0090$06.00/0 © 1991 American Chemical Society

In High-Tech Fibrous Materials; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

6. CASALE ET AL.

Braided Composite Monocoque Bicycle Frame 91 Define/Establish Design Criteria


Material Selection Fiber Architecture Selection l Modelling of Fabrication

i Unit Cell Development for FEA

I

Simulated Testing (FEA)

Fabricate Bicycle Frame Figure 1. Logical Flow of Integrated Design Methodology.

illustrated in Figure 1. To maximize the strength required by the frame, and without adding unnecessary weight, frame designs were evaluated using three dimensional stress analysis for the candidate frame geometries. By concentrating the frame's strength at critical stress areas, all excess material (hence weight) can be eliminated. To properly engineer the frame, each tube of the frame needs individual attention. The cross-sectional geometry, wall thickness, fiber orientation, andfibervolume fraction were designed specifically to meet each tube's individual stress requirements.


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HIGH-TECH FIBROUS MATERIALS


The result of this research is a design and subsequent fabrication of a light weight road racing frame built from textile structural composites. In addition to developing a minimal weight frame, this research also addressed the issue of manufacturing. The fabrication technique takes advantage of an automated braiding process which allows for hybrid materials to be incorporated into the frame in a single processing step. The structural geometry was designed to incorporate an attractive aesthetic design which coincides with its intended purpose. To this end, one of the main tubes that appears in a conventional frame was eliminated. Five prototype frames have been constructed to validate the design. Qualitative tests were performed on this frame to evaluate the composite processing and joining techniques used. Material Selection and Verification A material selection phase was carried out to identify the most promising fibers and matrices for the composite frame. The choice of material system was based upon four principal criteria: weight, stiffness, strength, and cost. Whereas strength and stiffness are necessary for the successful development of the frame, the issues of weight and cost provide the engineering constraints to the project. In particular, since the composite is a moving vehicle, niinimization of weight translates to reduced energy expenditures for riding. Additionally, the materials cost for the frame should not be high or the frame is unrealistic. Fibers. The selection offibrousmaterials for the reinforcement of the composite is critical to the performance and weight of the frame. An initial investigation was carried out examining the effects of combining different materials into a single fabric. Using the braiding process as the method of preform fabrication, it is possible to construct hybrid fabrics (2,2). The braiding machine consists of a set of oriented (braider) yarns, and one set of longitudinal yarns. It is possible to use a different fibrous material in each of these sets. Additionally, it is possible to subdivide the braider yarns into two different materials. Initially, analysis was carried out to investigate the effects of combining one fibrous material in the braiders and another in the longitudinals. The materials which were analytically investigated singly and in hybrid form are: E-Glass Kevlar (aramid) Silicon Carbide Graphite Spectra (polyethylene) Using a basis of 50° braider yarn orientation, and one half the number of longitudinals as braiders, predictions were carried out for these hybrid composites. A l l of the predictions are based upon 55% fiber volume fraction. Table I shows the predicted densities of the composite corresponding with the various hybrids. It can be clearly seen that the use of Spectra reduces the weight of the composite significantly in any combination. In High-Tech Fibrous Materials; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Braided Composite Monocoque Bicycle Frame 93

CASALE ET AL.

Table L

Density in g/cc of Candidate H y b r i d Systems


Braiders

Kevlar 2.04 1.61 1.75 1.95 1.42

E-Glass Kevlar Graphite SiC Spectra

1.83 1.39 1.53 1.73 1.20

Longitudinals Graphite 1.90 1.46 1.60 1.80 1.27

£i£

Spectra

2.00 1.56 1.70 1.90 1.37

1.73 1.29 1.43 1.63 1.10

Based on the weight considerations, E-glass and SiC fibers were eliminated from further evaluation. In addition to the weight considerations of the composite material, predictions of the composite properties were made using a modified laminate theory approach, accounting for the interlacing of the yarns in the fabric (4-£). The details of the analysis are not incorporated in this paper, but are as described in the references. Figure 2 illustrates a comparison of the tensile moduli of the remaining nine hybrid combinations in composite form. This shows both the predicted axial modulus (E ) and the transverse, or hoop-direction modulus (E ). x

y

It can be seen that graphite fibers greatly contribute to the tensile modulus of the composite. This stiffness is critical for the development of a rigid frame. However, since graphite is the heaviest of the fibers under consideration, it is still desirable to include one of the polymeric fibers. In addition to modulus, tensile strengths were also predicted for these material systems. A summary of the results are illustrated in Figure 3. This figure compares the predicted axial (S*) and hoop (Sy) strengths of the various composite systems. Although generally the frame performance is stiffness critical, strength is a primary concern in areas of stress concentration, such as joins and bends, and particularly in the bottom bracket. The Spectra fiber, due to its highly oriented microstructure, provides the greatest strength of the hybrids. Combining these predictions with the modulus and density predictions, a hybrid of graphite and Spectra can be identified as the most promising material system; graphite for its light weight and high stiffness properties and the Spectra for its extreme light weight and high tensile strength. However, concerns exist with the choice of Spectra as the strong, light weight hybrid. Due to its intrinsic chemistry (highly oriented polyethylene), Spectra will not bond well with resin. The design intention is to translate fiber properties to the composite through stress transfer at the interface between fiber and resin. In order to get a good interface between Spectra and a resin system, it is necessary to modify the surface of the fiber (using a process such as plasma or chemical etching) which will introduce surface microcracks to the fiber. It has been demonstrated that the surface




K = Kelvar G - Graphite S • Spectra

K/K

K/G K/S G/K G/G G/S S/K Material Hybrid (braid/longintudinal)

S/G

S/S

Figure 2. Predicted Tensile Module of the Hybrid Composites.

1200

1000

800

600

400

200

fIJffifljilll

K/K

K / G K/S G / K G / G G/S S/K Material Hybrid (braid/longintudinal)

S/G

Figure 3. Predicted Tensile Strength of Hybrid Composites.


S/S

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modification of Spectra provides good bonding with resin systems. However, the cost of surface modification is high, and is considered to be out of the scope of this research. Additionally, other concerns have arisen with regards the creep resistance of Spectra and the low glass transition temperature. Using Spectra will require constraints applied to the material system, principally that the composite cannot go through a cure cycle over 100°C without risking a recrystallization of the polyethylene, so the resin must be chosen accordingly. Substituting another highly drawn polymeric fiber, such as Kevlar, can increase the creep characteristics and Tg while maintaining toughness. Additionally, the good chemical bond between Kevlar and resin will provide additional strength, reduced cost, and reduced processing constraints. The exchange of Kevlar for Spectra results in a 15% weight increase. However, the additional strength and reduced processing complexities provided by Kevlar is viewed as a good reason to accept this extra weight. Considering the additional factor of safety associated with using Kevlar, it was decided that the weight increase was within the target of performance. Based on this and matrix system selection, an engineering decision was made to switch the material system from Spectra/graphite to Kevlar/graphite for the final frame. Matrix System. Having selected a fibrous material system, it is necessary to identify a suitable matrix material. The program calls for a low density polymeric resin, with wetting, gelling, and curing properties suitable for the frame geometry. Epon 828 (Shell Chemical Co.) was chosen as the base resin for the epoxy system. Epon 828 is very versatile due to its low reactivity with respect to the polystyrene foam core used as a mandrel. Excellent mechanical and adhesive properties can be achieved, and it is very chemically resistant. With the selection of 828 as the resin, investigations were carried out to determine the optimal curing agent for this particular project Three curing agents were identified for investigation: • V-40 • U-80 • D-230 (Jeffamine) Table II shows a brief summary of typical properties of the epoxies formed with the various curing agents. Table TL Properties of 828/Curing Agent Enoxies Vz4Q HzSQ D-230 Viscosity @25°C (Poise) 118 155 8 Pot life (hours, 400 g @ 25°C) 1.25 0.3 5.5 Cure time (days® 25° C) 7 10 10 Cure time (hours® 80° C) 26 1 2 Tensile Modulus (GPa) 2.1 3.5 3.1 Tensile Strength (MPa) 51 51 67 SOURCE: Data from ref. 7. EPON Resin 828, March 1987. In High-Tech Fibrous Materials; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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To evaluate these systems, sample preforms were fabricated and the resin was applied to make a composite. The yarns were wound onto braiding packages and then braided into elliptical cylinders with comparable dimensions to the frame tubes. The braided preforms were then made into composites by consolidating the fabric under atmospheric pressure. The composites were consolidated and sectioned. The sections were examined optically to provide a microscopic analysis of the resin penetration characteristics. Examination of the composites after consolidation were carried out both in terms of contained porosity determined through density calculations and optical methods. Density measurements showed a high density (8% void) and optical examinations indicated a good wetting between the fibers and matrix. Considering that the composite was densified without vacuum or pressure, the results from this test were considered to be satisfactory. In order to optimize wetting, the resin must be heated, providing a significantly lower viscosity, hence better flow characteristics through the preform. The need for thermal energy also provides impetus for switching the polymeric hybrid system from Spectra (which softens at 90° C) to Kevlar (service temperature above 220° C). Further experiments were carried out using the U-80 and D-230 curing agents. Through these investigations results indicated that D-230 was the best curing agent for this program. The selection was based upon the relatively short cure time and low viscosity. StrMrtiirql Analysis In order to verify the effectiveness of the bicycle frame design, the geometry of the frame was modelled using ANS YS finite element analysis. Figure 4 illustrates the geometry of the bicycle frame. The model was carried out using a space truss analysis with beam elements. The structural analysis was performed by Schwinn Bicycle Company. The structural analysis was run with five standard loading conditions as defined by Schwinn. The input data for the analysis consisted of bending and torsional stiffness as well as cross-sectional area for each of the beam elements. In this way, it is possible to modify the performance characteristics of individual tubes independently of the others. The input data was based on the predictions of the composite material properties and the geometry of the tubes. Using the input data, the F E A model was carried out and reviewed. Of principal concern was the deflection of the frame under standard loading. The data was compared to a high performance steel frame with a standard (seven tube) geometry. The results of the first test showed a relatively large deflection at the seat indicating a lack of stiffness in the seat/rear stay area of the frame. Based on these results, a redesign of some of the individual tube geometries was carried out to reduce the seat deflection.


6. CASALE ET AL.



The analysis was carried out again using the new tube geometries, and the results from the second analysis were more acceptable. The deflections decreased by two thirds, which is comparable to the deflection of a steel racing frame under the same loads. Although the seat deflection is still above standard, the value of seat deflection subject to a maximum downward load was on the order of one millimeter, which is within the performance window. Table HI details the final design of the tube geometries and moment properties. Table III. Geometry and Moments of the Frame Tubes Tube X-Sect. Geometry I (in ) K (in ) Top Elhptical 0.1747 0.1584 Down Elliptical 0.1411 0.1639 Chain stay Rectangular 0.0967 0.0781 Seat stay Rectangular 0.1818 0.1004 Seat Yoke Elliptical 0.1980 0.2276 4

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Prototype Fabrication Having identified a desirable frame and tube geometry, the manufacturing of the frame proceeded. For each tube element, the geometry of the tube and the desired fiber architecture was quantified and the manufacturing parameters determined. The fabrication was carried out by forming net shape mandrels and supplying the yarn in braided fabric form around the mandrel. A prototype frame was fabricated by braiding carbon and Kevlar fibers over a polystyrene foam core. The polystyrene core was made from blocks of foam shaped to the desired dimensions taken from the full scale engineering drawings, as shown in Figure 5. Although there is a considerable amount of manual labor associated with shaping the foam in this prototyping activity, this can be eliminated through the use of a mold for fabricating the mandrels. A method for braiding over such a complex structure with sharp angles and transitional tubes like the seat yoke and bottom bracket where one tube breaks off into two is needed. This problem was resolved by breaking the frame into four individual tubes, the top tube/head tube, down tube/bottom bracket, and two rear/chain stays. At this point each section was treated as an individual mandrel. These mandrels were then inserted into the braiding machine and the braided preform was laid on top of the mandrel, as shown in Figure 6. The first steps in braiding these mandrels were to braid four layers of 12k carbon fiber and Kevlar fibers at ±50° (braiders) with 12k carbon fiber longitudinals. This braid configuration results in the proper tube thickness for the frame. After braiding the tubes individually, a joining method was required. This is a very complex problem since the fibers at these joints are discontinuous and must be joined (mechanically) to their mating tubes without affecting the structural integrity of the frame. For the prototype frame, the In High-Tech Fibrous Materials; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.



Figure 4. Frame Geometry for Finite Element Analysis.

Figure 5. Shaped Foam Mandrels for Preform Processing.



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Figure 6. Braiding Over a Mandrel.

fiber ends at the joints were lapped into the braided fiber structure of the adjoining tubes and then braided over once again. The fibers must be at least a critical length (lc) to assure sufficient stress transfer with the resin and other fibers. It was calculated that these joints will be stronger than if they had been braided alone. This is due to the increase in fibrous material and specific areal coverage at these critical regions. The braid was formed around the foam mandrel (shaped from styrofoam blocks) and the assembled preform was consolidated, shrink wrapped (to simulate hydrostatic consolidation pressure), and cured at 80°C for 2 hours in a convection oven.

Conclusions This project demonstrated the ability to design and fabricate a composite structure using an integrated design approach. In particular, the research took advantage of the composite material properties rather than using a "black aliiminum" design approach. The results of analytical work were realized as a physical object in the form of a composite bicycle frame.



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Figure 7. Completed Bicycle Frame.

Through the use of the integrated design, it was possible to produce a light weight frame by taking advantage of the specific material properties available from composite materials. This approach is somewhat generic to composite design, and is currently employed in a number of various structural composite design programs (&,2). The application of this technique to a bicycle frame is a demonstration of the fundamental concept as well as an enjoyable research project Five prototype frames were completed during this project. A completed frame is shown in Figure 7. The completed frames are currendy being ridden by various bicyclists to evaluate long term effects. Early responses from the riders indicate that the frames feel as stiff as a good steel racing frame, but the benefits of reduced weight are evident. The authors would like to acknowledge the assistance of Campagnolo Corporation, for the support of this project and the donation of components; Mr. Jerry Casale for his support, and assistance in frame specifications and fabrication; Mr. John Kutz for composite processing advice; and Schwinn Bicycle Company for finite element analysis.


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Braided Composite Monocoque Bicycle Frame

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Literature Cited 1.

Tan, T . M . , Pastore, C . M . , and Ko, F. "Engineering Design of Tough Ceramic Matrix Composites for Turbine Components", Journal of Engineering for Gas Turbines and Power, in press.

2.

Douglass, W.A., "Braiding and Braiding Machinery", Centrex Publishing Company, Eindhoven, 1964.

3.

Goseberg, Fiedhelm, "The Construction of Braided Goods", Band and Flechtindustrie, No. 2, pp. 65-72, 1969.

4.

Ransom, J.B., and Knight, N.F., "Global/Local Stress Analysis of Composite Panels", NASA Technical Memorandum 101622, June 1989.

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Tarnopol'ckii, Zhigun, and Polyakov, Prostranstvenno-Armirovannyye Komposzitsionnyye Materialy: Spravochnik (Spatially Reinforced Composite Materials: A Handbook), Mashinostroyeniyie Publishing House, Moscow, USSR, 1987.

6.

Pastore, C., and Foye, R., "Computer Aided Modeling of Fabric Reinforced Composite Microstructures", Procedings of Fiber-Tex '89, Greenville, SC, Oct. 30-Nov. 1, 1989.

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Technical Bulletin Shell Chemical Company, SC:235-85.828

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Ko, F.K., and Pastore, C . M . , "Fabric Geometry and Finite Cell Models for Three Dimensional Composites" F. Ko and C. Pastore, 1st US/USSR Conference on Composite Materials, Riga, USSR, May 2025, 1989.

9.

T . M . Tan, C. Pastore, and F. Ko, "Engineering Design of Tough Ceramic Matrix Composites for Turbine Components", presented at Gas Turbine and Aeroengine Congress and Exposition, June 4-8, 1989, Toronto, Ontario, Canada, accepted for publication in Transactions of the ASME.

RECEIVED July 31, 1990


Design and Fabrication of a Braided Composite ... - ACS Publications

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