- Email: [email protected]
Cost effective manufacturing process of thermoplastic matrix composites for the traditional industry: the example of a carbon-fiber reinforced thermoplastic flywheel
Composite Structures 52 (2001) 517±521
www.elsevier.com/locate/compstruct
Cost eective manufacturing process of thermoplastic matrix composites for the traditional industry: the example of a carbon-®ber reinforced thermoplastic ¯ywheel C.A. Mahieux * Advanced Mechanics, ABB Corporate Research Ltd, 5405 Baden, Dattwil, Switzerland
Abstract Composite materials were successfully introduced and are now widely used for aerospace applications. Due to their high speci®c strength and stiness, polymer-based composite materials should also be attractive candidates for many products of the traditional industries such as gas turbines, oil industry, or water and gas piping. The introduction of composite materials in the traditional industry is however a very slow process. Many factors can be identi®ed as possible reasons such as the lack of previous examples on which to assess the durability of such composite products or reparability issues. However, the major factor hindering a broader use of composite materials for traditional products remains cost. Unlike the case of the aerospace industry, the use of composite materials is often not an enabling technology for traditional products: steel designs can be modi®ed in order to increase the current product limitations. Therefore, the price of the composite system should be competitive when compared to the price of the equivalent system based on traditional materials such as steel or aluminum. In order to illustrate this concept, the case of steel risers for deepwater oil production is shortly discussed in the introduction of the present paper. When trying to reduce the price of composite products, the challenge often lies in lowering the manufacturing cost. The present paper focuses on applied manufacturing methods for various parts and products aiming to reduce cost. The associated performance of hot pressing and winding of short ®ber and continuous ®ber reinforced thermoplastic (AS4/PEEK) are compared for a high-speed ¯ywheel type of application. Based on the mechanical performance and ease of fabrication, conclusions are drawn on a promising area of further investigation. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Flywheel; Tape ®ber placement; Thermoplastic composites
1. Introduction The very high speci®c strength and stiness of ®ber reinforced polymer matrix composite materials make them very attractive candidates for many industrial applications. However, the use of polymer matrix composite materials is often restricted to ``high tech'' industries (airplane and aerospace industry) or the performance demanding leisure industry (tennis rackets, golf clubs, mountain bikes). The timid development of the use of polymer matrix composites for other applications has several causes, such as the diculties in developing life prediction tools, unsolved reparability or joining issues. Cost is a main driver in the material selection for most industrial applications.
*
Fax: +41-056-486-7315. E-mail address: [email protected] (C.A. Mahieux).
From a business perspective, it is important to divide the use of composite materials in two categories. The ®rst category includes all products that require the use of innovative materials. The requirements in term of weight, strength, stiness, environmental resistance (e.g., corrosion) are very strict and the designs cannot be adapted to use more conventional materials (such as metals). In this case, composite materials represent an enabling technology. This is the case for example of the aerospace applications, where the performance is a higher driver than cost. For more traditional industries (were composites are not an enabling technology), high speci®c strength and stiness are still very attractive properties. In the ®eld of power generation for example, carbon-®ber reinforced polymers can be used in the ®rst compression stage of gas turbines (low temperature). Using composites in this case increases the price of a single blade. However, the weight saving enables the use of much smaller bearings and supporting structure, resulting in a lower system cost.
0263-8223/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 8 2 2 3 ( 0 1 ) 0 0 0 4 1 - 1
518
C.A. Mahieux / Composite Structures 52 (2001) 517±521
Cost remains the main driver, the price of the composite technology should be competitive with respect to the materials being traditionally used (i.e., metals). The cost of the use of composite materials involves dierent aspects: · The cost of the raw material. It is reasonable to assume that this cost (today 2±5 times higher for carbon-®ber reinforced polymers than for steel [1]) will remain higher than the cost of aluminum or steel. · The cost of the manufacturing process. · The system cost. For many applications, the use of composite materials will induce a direct increase in price for the product itself. However, the price of the global system will often decrease by a signi®cant amount: the high speci®c strength of high performance ®ber reinforced polymer composites results in light parts requiring less structural support. The functions can also be integrated and the number of parts decreased impacting the production and assembly costs. This situation can perfectly be illustrated by the case of composite ¯exible risers for oshore applications. Risers are used to transport oil from the seabed to the surface [2,3]. Risers (rigid or ¯exible) are traditionally made of steel with inner or outer liners. Flexible (spoolable) risers present many advantages, among those the fact that the full-length pipe can be spooled on a reel and transported on a ship to the oshore well. The riser only requires two connections (one at the reel and one at the water surface). Flexible risers can be used in conjunction with ¯oating production, storage and ooading [4] (FPSO) that are the most cost ecient surface system (simply made of two boats). When going to greater depth traditional unbonded ¯exible risers encounter serious problems such as collapse under own weight or need for very large, expensive, impractical buoyancy devices. Therefore composite ¯exible risers become ideal materials for this application. Lightweight very long risers can directly be applied to the FPSO. However, even for large depths, a rigid riser could still be used in conjunction with a larger, more sophisticated and more costly surface platforms (e.g., SPAR). In other words, the ¯exible composite riser/¯exible production system cost must be lower than the steel rigid riser/large platform system. This puts great constraint on the cost of the composite product. We have little in¯uence over the cost of the raw material. Therefore, the greatest improvement can today be obtained by lowering the manufacturing cost. Not only in the ®eld of oshore technology has the industry been extending the performance of its various products by adapting the designs to enable the use of the traditional materials (i.e., steel). This case also applies to rotating machinery, such as turbine or air handing equipment. The expected performances are high including good fatigue resistance, good temperature and environmental integrity. These requirements often guide
the choice of the composite constituents. For high speed rotating machinery operating at temperatures below 200°C (e.g., fans, thrust bearings, ¯ywheels), high performance carbon-®ber reinforced thermoplastics are often ideal candidates. This paper illustrates current issues in achieving high performance inexpensive manufacturing of rotating wheels (¯ywheel type). The material selection for this application is brie¯y discussed and led to the selection of a thermoplastic matrix. Two one-step-manufacturing processes are described: tape ®ber placement and hot pressing. The mechanical performance of the obtained prototypes are discussed and conclusions are drawn on the feasibility of such processes for ¯y wheels. 2. Application to ¯ywheels Flywheels are used in energy storage systems. They can be viewed as electromechanical batteries: ¯ywheels store kinetic energy in a rotating mass. The main goal for ¯ywheels is to achieve highest rotational speed possible, therefore the highest strength/density ratio. A good toughness is necessary to avoid catastrophic failure. Good dimensional integrity during rotation is also required to avoid unbalanced and subsequent vibrations. Ideal materials for ¯ywheels exhibit high tensile strength and low density. Selection of the reinforcing ®bers for ¯ywheels is discussed in detail in the literature [5]. DeTeresa indicates that high strength carbon-®bers are good candidates for this application due to their high intrinsic speci®c strength and good stress rupture behavior. With carbon-®ber-based composites, the problem mainly lies in the statistical variation of the strengths. However, models have been proposed to predict the fracture of composite ¯ywheels considering the inherent strength scatter of carbon-®ber reinforced materials [6]. The choice of the matrix material is not as straight forward. The literature focusing on thermosetbased ¯ywheels [7±10] is extensive but is more restricted on thermoplastic wheels [11]. An interesting idea brought up by Gabrys and Bakis [12] using an elastomeric matrix for the ¯ywheel. The enhanced ductility of the elastomeric matrix limits the radial stresses [5] and allows thick-walled composite rings [7]. However, the manufacturing process of such elastomer-based structures present challenges [7]. Thermoplastic matrices such as PEEK exhibit at room temperature higher ductility as standard epoxy [13] (but lower than elastomers). PEEK was selected in the present study as a compromise between epoxy and elastomer. The use of a thermoplastic-based composite with an appropriate manufacturing process enables signi®cant pre-tension during winding, generating bene®cial compressive radial stresses. The maximum tension to a standard thermoset
C.A. Mahieux / Composite Structures 52 (2001) 517±521
519
matrix composite tow does not exceed 70 MPa [5]. The selected material was a standard carbon reinforced PEEK matrix, AS4/APC2 from ICI Fiberite supplied as a 10 mm tape. The innovative part of the present study is the focus ``¯exible'' processing enabling the evolution from a simple ring towards more complex conical shapes. Flywheels can have various shapes, some of which are given in Fig. 1. The present study focuses on a constant thickness disk (type (a)) and a half conical wheel. Several processes could be used to manufacture composite wheels such as autoclave processing, ®lament winding, thermoforming or pressing. A promising candidate for the manufacturing of thermoplastic complex shapes is tape ®ber placement. Tape ®ber placement when adapted to a multi-axis robot is very ¯exible and enable shape evolution without requiring major hardware investment. Tape ®ber placement [14] consists in a simultaneous lay-up-heating action as shown in Figs. 3 and 4. This on-line welding process is illustrated by Fig. 2. The quality of the part will depend on various parameters such as winding speed, roll pressure and
temperature, heat source temperature, and nature of the heat source. The in¯uence of these parameters have been widely discussed in the literature [15±18]. However, these studies were performed with various set-ups (single or multiple heating head, various pre-heating systems, single or multiple rolls. . .) leading to divergent conclusions. It is therefore dicult to generalize the conclusions from these studies, in order to optimize the tape ®ber placement process. The advantages of tape ®ber placement using cost ecient heat sources (such as hot gas or infrared) include a limited need for manual labor, an enhanced versatility (the robot can be reprogrammed for the manufacturing of various shapes) and a limited energy need (the heat source is focused on the welding point and pre-heating is not required). All these advantages reduce the manufacturing cost. Two disks of constant thicknesses (Fig. 1, type (a)) were wound [19] using tape ®ber placement. The winding speed was 30 mm/s (standard speed for our process) for the ®rst wheel and 40±81 mm/s for the second wheel. The heat source was hot gas, enabling a nip point temperature of 800°C. The inner and outer diameters of the
Fig. 1. Example of ¯ywheel types: (a) constant thickness disk; (b) disk with rim; (c) conical wheel.
Fig. 3. Tape ®ber placement apparatus.
Fig. 2. Tape ®ber placement process ± schematic.
Fig. 4. Tape ®ber placement ± head close up.
520
C.A. Mahieux / Composite Structures 52 (2001) 517±521
two ®nal wheels were of 50 and 300 mm, respectively, with a thickness of 30 mm. The two wheels were tested [19] in a spin chamber. The maximum tip speeds obtained were of 550 m/s for the ®rst test and 597 m/s for the second test. Beyond these speeds the wheel started vibrating and the experiments were stopped. The stored energies were respectively 64 and 75 W h. The wheels did not burst but exhibited the presence of cracks on the circumference. The design could be further optimized by varying the pre-tension with radius. The tension generated by the tape decreases with an increasing part diameter [5]. Therefore, increasing gradually the level of pre-tension applied to the tape could lead to homogeneous residual stresses. The next step was the manufacturing a more complex shape, a half conical-wheel, following the same procedure. The winding angle is small and around 1±2°. The half conical wheel shown in Fig. 5 broke at 650 m/s. A schematic of the failure mechanism location is shown in Fig. 6. The failure can easily be explained by the lack of strength along the axial direction (parallel to the rotation axis), the axial strength being mainly de®ned by the matrix strength. This problem could be solved by adding ®bers with various orientations (thus adding strength in the vertical direction z). Hand laminating the structure would probably lead to a high quality product but would require an expensive manufacturing process involving manual labor, not adapted to mass production. Reinforcement using short carbon-®ber could also enhance the axial strength (z direction) of the wheel. In order to investigate the feasibility of this second option a prototype of a short ®ber reinforced (AS4/PEEK) half conical wheel was manufactured according to the following steps [20]: · Pre-consolidation procedure by pressing of random carbon-®ber plates following standard procedures
(2 h at 400°C in 10 tons press followed by cool down without pressure for 6 h). · Cutting of rings using water jet cutting. · Hot pressing of the assembled rings (Fig. 7): the rings were kept at 120°C for 16 h to dry. The temperature was raised to 350°C for 2 h 15 min under 70 tons. The set up was then allowed to cool down slowly for 6 h down to room temperature without applied pressure. The resulting wheel is shown in Fig. 8. Cracks and macro-voids are present all over the surface and revealed a poor consolidation. Similar experiments were performed with increased pressure and with AS4/PEEK fabric material 0; 45; 90; 45; 0s but did not lead to better consolidation. Due to the extensive presence of cracks and defects, these wheels were not subjected to spin tests. The next step, much more expensive, would be to laminate the wheel. To reduce the cost, the tapes could be placed using a robot then pressed. However, it is obvious that this two step method would require a longer time and larger expense than a single step method, such as tape ®ber placement.
Fig. 5. Half conical wheel obtained via tape ®ber placement.
Fig. 7. Random ®ber rings in press [20].
Fig. 6. Schematic of the failure location for the conical wheel.
C.A. Mahieux / Composite Structures 52 (2001) 517±521
521
References
Fig. 8. Resulting pressed wheel [20].
3. Summary and conclusions A reduced manufacturing cost is required to extend the use of high performance polymer matrix composites to the traditional industry. Among the various manufacturing methods, tape ®ber placement appears as one of the cheapest and most systematic (automated) method for mass manufacturing of high performance thermoplastic based composite complex-shaped products. This method was applied to the manufacturing of carbon-®ber reinforced ¯ywheels. The AS4/PEEK material system was chosen for its relative ductility and the possibility to apply pre-tension to minimize tensile radial stresses during rotation. For simple and conical wheels, tape ®ber placement of AS4/PEEK showed reasonable performance, well superior to pressed random ®ber composite. More energy should be invested today on automated processing in order to lower the cost and increase the reliability of such manufacturing processes. Manufacturing of rings and pipes via tape ®ber placement or ®lament winding leads today to high quality products. However, manufacturing laminated structures and open shapes remains a challenge for this technology. Focusing subsequent studies on cost-eective automated manufacturing processes stands today as a key to a broader use of thermoplastic composites for high performance traditional industry products.
[1] ABB Corporate Research, private communication. [2] Kalman M, Belcher J. Flexible risers with composite armor for deep water oil and gas production. Composite for oshore operations ± 2. American Bureau of Shipping; 1999. p. 151±65. [3] Narzul P. The ¯exible riser Option, ONS 94, Stavanger, Norway, 1994. [4] Wang SW, Williams JG, Lo KH, editors. Composite for oshore operations ± 2. American Bureau of Shipping; 1999. [5] DeTeresa SJ. Materials for advanced ¯ywheel energy-storage devices. MRS Bull 1999;November:51±6. [6] Ichikawa M, Tanaka S. A probabilistic approach to fracture of composite ¯ywheels. Int J Fract 1980;16:R63±6. [7] Gabrys CW, Bakis CE. Simpli®ed analysis of residual stresses in in-situ cured hoop-wound rings. J Compos Mater 1998;32(13):729±37. [8] Chiu IL, et al. Epoxy resin system for composite ¯ywheels. Compos Technol Rev 1983;5(1):18±20. [9] Grudowsky TW, et al. Flywheels for energy storage. In: Proceedings of the 27th International SAMPE Technical Conference, October 9±12, 1995. p. 760±68. [10] Wells SP, et al. The design, manufacture, and testing of a composite ¯ywheel for energy storage. AMD-Vol. 194, Mechanics in materials processing and manufacturing. ASME; 1994. p. 397± 404. [11] Callagher P, et al. In-situ consolidation of thermoplastic thick rings. In: International SAMPE Symposium and Exhibition (Proceedings), May 31±June 4, 1998;43(2):1943±54. [12] Gabrys CW, Bakis CE. Filament would elastomeric matrix composites for ¯ywheel energy storage systems. In: Proceedings of the American Society for Composites, October 7±9, 1996. p. 729±37. [13] Cambridge Engineering Software, selector database. [14] Mallick V, et al. Robot based thermoplastic ®ber placement process. ABB Rev 1998;2. [15] Ghasemi MN, et al. Thermal analysis of in-situ thermoplastic composite tape laying. J Thermoplastic Compos Mater 1991;4:20± 45. [16] Pitchumani R, Don RC, Gillespie Jr JW, Ranganathan S. Analysis of on-line consolidation during a thermoplastic towplacement process. ASME Publication HTD-289, Heat and mass transfer in composites processing; 1994. p. 223±34. [17] James DL. Experimental analysis and process window development for continuous ®lament wound APC-2. ASME Publication HTD-289, Thermal Processing of Materials: Thermo-Mechanics, Controls and Composites, 1994, p. 203±12. [18] Sommez FO, Hahn HT. Modeling of heat transfer and crystallization in thermoplastic composite tape placement process. J Thermoplastic Compos Mater 1997;10:198±240. [19] Wilson P. Flywheel energy storage: overview and examination of the application of composite technology to advanced ¯ywheels. ABB Corporate Research, TN 96-001, 1996. [20] Landert M, Meier G. ABB private communication.
www.elsevier.com/locate/compstruct
Cost eective manufacturing process of thermoplastic matrix composites for the traditional industry: the example of a carbon-®ber reinforced thermoplastic ¯ywheel C.A. Mahieux * Advanced Mechanics, ABB Corporate Research Ltd, 5405 Baden, Dattwil, Switzerland
Abstract Composite materials were successfully introduced and are now widely used for aerospace applications. Due to their high speci®c strength and stiness, polymer-based composite materials should also be attractive candidates for many products of the traditional industries such as gas turbines, oil industry, or water and gas piping. The introduction of composite materials in the traditional industry is however a very slow process. Many factors can be identi®ed as possible reasons such as the lack of previous examples on which to assess the durability of such composite products or reparability issues. However, the major factor hindering a broader use of composite materials for traditional products remains cost. Unlike the case of the aerospace industry, the use of composite materials is often not an enabling technology for traditional products: steel designs can be modi®ed in order to increase the current product limitations. Therefore, the price of the composite system should be competitive when compared to the price of the equivalent system based on traditional materials such as steel or aluminum. In order to illustrate this concept, the case of steel risers for deepwater oil production is shortly discussed in the introduction of the present paper. When trying to reduce the price of composite products, the challenge often lies in lowering the manufacturing cost. The present paper focuses on applied manufacturing methods for various parts and products aiming to reduce cost. The associated performance of hot pressing and winding of short ®ber and continuous ®ber reinforced thermoplastic (AS4/PEEK) are compared for a high-speed ¯ywheel type of application. Based on the mechanical performance and ease of fabrication, conclusions are drawn on a promising area of further investigation. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Flywheel; Tape ®ber placement; Thermoplastic composites
1. Introduction The very high speci®c strength and stiness of ®ber reinforced polymer matrix composite materials make them very attractive candidates for many industrial applications. However, the use of polymer matrix composite materials is often restricted to ``high tech'' industries (airplane and aerospace industry) or the performance demanding leisure industry (tennis rackets, golf clubs, mountain bikes). The timid development of the use of polymer matrix composites for other applications has several causes, such as the diculties in developing life prediction tools, unsolved reparability or joining issues. Cost is a main driver in the material selection for most industrial applications.
*
Fax: +41-056-486-7315. E-mail address: [email protected] (C.A. Mahieux).
From a business perspective, it is important to divide the use of composite materials in two categories. The ®rst category includes all products that require the use of innovative materials. The requirements in term of weight, strength, stiness, environmental resistance (e.g., corrosion) are very strict and the designs cannot be adapted to use more conventional materials (such as metals). In this case, composite materials represent an enabling technology. This is the case for example of the aerospace applications, where the performance is a higher driver than cost. For more traditional industries (were composites are not an enabling technology), high speci®c strength and stiness are still very attractive properties. In the ®eld of power generation for example, carbon-®ber reinforced polymers can be used in the ®rst compression stage of gas turbines (low temperature). Using composites in this case increases the price of a single blade. However, the weight saving enables the use of much smaller bearings and supporting structure, resulting in a lower system cost.
0263-8223/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 8 2 2 3 ( 0 1 ) 0 0 0 4 1 - 1
518
C.A. Mahieux / Composite Structures 52 (2001) 517±521
Cost remains the main driver, the price of the composite technology should be competitive with respect to the materials being traditionally used (i.e., metals). The cost of the use of composite materials involves dierent aspects: · The cost of the raw material. It is reasonable to assume that this cost (today 2±5 times higher for carbon-®ber reinforced polymers than for steel [1]) will remain higher than the cost of aluminum or steel. · The cost of the manufacturing process. · The system cost. For many applications, the use of composite materials will induce a direct increase in price for the product itself. However, the price of the global system will often decrease by a signi®cant amount: the high speci®c strength of high performance ®ber reinforced polymer composites results in light parts requiring less structural support. The functions can also be integrated and the number of parts decreased impacting the production and assembly costs. This situation can perfectly be illustrated by the case of composite ¯exible risers for oshore applications. Risers are used to transport oil from the seabed to the surface [2,3]. Risers (rigid or ¯exible) are traditionally made of steel with inner or outer liners. Flexible (spoolable) risers present many advantages, among those the fact that the full-length pipe can be spooled on a reel and transported on a ship to the oshore well. The riser only requires two connections (one at the reel and one at the water surface). Flexible risers can be used in conjunction with ¯oating production, storage and ooading [4] (FPSO) that are the most cost ecient surface system (simply made of two boats). When going to greater depth traditional unbonded ¯exible risers encounter serious problems such as collapse under own weight or need for very large, expensive, impractical buoyancy devices. Therefore composite ¯exible risers become ideal materials for this application. Lightweight very long risers can directly be applied to the FPSO. However, even for large depths, a rigid riser could still be used in conjunction with a larger, more sophisticated and more costly surface platforms (e.g., SPAR). In other words, the ¯exible composite riser/¯exible production system cost must be lower than the steel rigid riser/large platform system. This puts great constraint on the cost of the composite product. We have little in¯uence over the cost of the raw material. Therefore, the greatest improvement can today be obtained by lowering the manufacturing cost. Not only in the ®eld of oshore technology has the industry been extending the performance of its various products by adapting the designs to enable the use of the traditional materials (i.e., steel). This case also applies to rotating machinery, such as turbine or air handing equipment. The expected performances are high including good fatigue resistance, good temperature and environmental integrity. These requirements often guide
the choice of the composite constituents. For high speed rotating machinery operating at temperatures below 200°C (e.g., fans, thrust bearings, ¯ywheels), high performance carbon-®ber reinforced thermoplastics are often ideal candidates. This paper illustrates current issues in achieving high performance inexpensive manufacturing of rotating wheels (¯ywheel type). The material selection for this application is brie¯y discussed and led to the selection of a thermoplastic matrix. Two one-step-manufacturing processes are described: tape ®ber placement and hot pressing. The mechanical performance of the obtained prototypes are discussed and conclusions are drawn on the feasibility of such processes for ¯y wheels. 2. Application to ¯ywheels Flywheels are used in energy storage systems. They can be viewed as electromechanical batteries: ¯ywheels store kinetic energy in a rotating mass. The main goal for ¯ywheels is to achieve highest rotational speed possible, therefore the highest strength/density ratio. A good toughness is necessary to avoid catastrophic failure. Good dimensional integrity during rotation is also required to avoid unbalanced and subsequent vibrations. Ideal materials for ¯ywheels exhibit high tensile strength and low density. Selection of the reinforcing ®bers for ¯ywheels is discussed in detail in the literature [5]. DeTeresa indicates that high strength carbon-®bers are good candidates for this application due to their high intrinsic speci®c strength and good stress rupture behavior. With carbon-®ber-based composites, the problem mainly lies in the statistical variation of the strengths. However, models have been proposed to predict the fracture of composite ¯ywheels considering the inherent strength scatter of carbon-®ber reinforced materials [6]. The choice of the matrix material is not as straight forward. The literature focusing on thermosetbased ¯ywheels [7±10] is extensive but is more restricted on thermoplastic wheels [11]. An interesting idea brought up by Gabrys and Bakis [12] using an elastomeric matrix for the ¯ywheel. The enhanced ductility of the elastomeric matrix limits the radial stresses [5] and allows thick-walled composite rings [7]. However, the manufacturing process of such elastomer-based structures present challenges [7]. Thermoplastic matrices such as PEEK exhibit at room temperature higher ductility as standard epoxy [13] (but lower than elastomers). PEEK was selected in the present study as a compromise between epoxy and elastomer. The use of a thermoplastic-based composite with an appropriate manufacturing process enables signi®cant pre-tension during winding, generating bene®cial compressive radial stresses. The maximum tension to a standard thermoset
C.A. Mahieux / Composite Structures 52 (2001) 517±521
519
matrix composite tow does not exceed 70 MPa [5]. The selected material was a standard carbon reinforced PEEK matrix, AS4/APC2 from ICI Fiberite supplied as a 10 mm tape. The innovative part of the present study is the focus ``¯exible'' processing enabling the evolution from a simple ring towards more complex conical shapes. Flywheels can have various shapes, some of which are given in Fig. 1. The present study focuses on a constant thickness disk (type (a)) and a half conical wheel. Several processes could be used to manufacture composite wheels such as autoclave processing, ®lament winding, thermoforming or pressing. A promising candidate for the manufacturing of thermoplastic complex shapes is tape ®ber placement. Tape ®ber placement when adapted to a multi-axis robot is very ¯exible and enable shape evolution without requiring major hardware investment. Tape ®ber placement [14] consists in a simultaneous lay-up-heating action as shown in Figs. 3 and 4. This on-line welding process is illustrated by Fig. 2. The quality of the part will depend on various parameters such as winding speed, roll pressure and
temperature, heat source temperature, and nature of the heat source. The in¯uence of these parameters have been widely discussed in the literature [15±18]. However, these studies were performed with various set-ups (single or multiple heating head, various pre-heating systems, single or multiple rolls. . .) leading to divergent conclusions. It is therefore dicult to generalize the conclusions from these studies, in order to optimize the tape ®ber placement process. The advantages of tape ®ber placement using cost ecient heat sources (such as hot gas or infrared) include a limited need for manual labor, an enhanced versatility (the robot can be reprogrammed for the manufacturing of various shapes) and a limited energy need (the heat source is focused on the welding point and pre-heating is not required). All these advantages reduce the manufacturing cost. Two disks of constant thicknesses (Fig. 1, type (a)) were wound [19] using tape ®ber placement. The winding speed was 30 mm/s (standard speed for our process) for the ®rst wheel and 40±81 mm/s for the second wheel. The heat source was hot gas, enabling a nip point temperature of 800°C. The inner and outer diameters of the
Fig. 1. Example of ¯ywheel types: (a) constant thickness disk; (b) disk with rim; (c) conical wheel.
Fig. 3. Tape ®ber placement apparatus.
Fig. 2. Tape ®ber placement process ± schematic.
Fig. 4. Tape ®ber placement ± head close up.
520
C.A. Mahieux / Composite Structures 52 (2001) 517±521
two ®nal wheels were of 50 and 300 mm, respectively, with a thickness of 30 mm. The two wheels were tested [19] in a spin chamber. The maximum tip speeds obtained were of 550 m/s for the ®rst test and 597 m/s for the second test. Beyond these speeds the wheel started vibrating and the experiments were stopped. The stored energies were respectively 64 and 75 W h. The wheels did not burst but exhibited the presence of cracks on the circumference. The design could be further optimized by varying the pre-tension with radius. The tension generated by the tape decreases with an increasing part diameter [5]. Therefore, increasing gradually the level of pre-tension applied to the tape could lead to homogeneous residual stresses. The next step was the manufacturing a more complex shape, a half conical-wheel, following the same procedure. The winding angle is small and around 1±2°. The half conical wheel shown in Fig. 5 broke at 650 m/s. A schematic of the failure mechanism location is shown in Fig. 6. The failure can easily be explained by the lack of strength along the axial direction (parallel to the rotation axis), the axial strength being mainly de®ned by the matrix strength. This problem could be solved by adding ®bers with various orientations (thus adding strength in the vertical direction z). Hand laminating the structure would probably lead to a high quality product but would require an expensive manufacturing process involving manual labor, not adapted to mass production. Reinforcement using short carbon-®ber could also enhance the axial strength (z direction) of the wheel. In order to investigate the feasibility of this second option a prototype of a short ®ber reinforced (AS4/PEEK) half conical wheel was manufactured according to the following steps [20]: · Pre-consolidation procedure by pressing of random carbon-®ber plates following standard procedures
(2 h at 400°C in 10 tons press followed by cool down without pressure for 6 h). · Cutting of rings using water jet cutting. · Hot pressing of the assembled rings (Fig. 7): the rings were kept at 120°C for 16 h to dry. The temperature was raised to 350°C for 2 h 15 min under 70 tons. The set up was then allowed to cool down slowly for 6 h down to room temperature without applied pressure. The resulting wheel is shown in Fig. 8. Cracks and macro-voids are present all over the surface and revealed a poor consolidation. Similar experiments were performed with increased pressure and with AS4/PEEK fabric material 0; 45; 90; 45; 0s but did not lead to better consolidation. Due to the extensive presence of cracks and defects, these wheels were not subjected to spin tests. The next step, much more expensive, would be to laminate the wheel. To reduce the cost, the tapes could be placed using a robot then pressed. However, it is obvious that this two step method would require a longer time and larger expense than a single step method, such as tape ®ber placement.
Fig. 5. Half conical wheel obtained via tape ®ber placement.
Fig. 7. Random ®ber rings in press [20].
Fig. 6. Schematic of the failure location for the conical wheel.
C.A. Mahieux / Composite Structures 52 (2001) 517±521
521
References
Fig. 8. Resulting pressed wheel [20].
3. Summary and conclusions A reduced manufacturing cost is required to extend the use of high performance polymer matrix composites to the traditional industry. Among the various manufacturing methods, tape ®ber placement appears as one of the cheapest and most systematic (automated) method for mass manufacturing of high performance thermoplastic based composite complex-shaped products. This method was applied to the manufacturing of carbon-®ber reinforced ¯ywheels. The AS4/PEEK material system was chosen for its relative ductility and the possibility to apply pre-tension to minimize tensile radial stresses during rotation. For simple and conical wheels, tape ®ber placement of AS4/PEEK showed reasonable performance, well superior to pressed random ®ber composite. More energy should be invested today on automated processing in order to lower the cost and increase the reliability of such manufacturing processes. Manufacturing of rings and pipes via tape ®ber placement or ®lament winding leads today to high quality products. However, manufacturing laminated structures and open shapes remains a challenge for this technology. Focusing subsequent studies on cost-eective automated manufacturing processes stands today as a key to a broader use of thermoplastic composites for high performance traditional industry products.
[1] ABB Corporate Research, private communication. [2] Kalman M, Belcher J. Flexible risers with composite armor for deep water oil and gas production. Composite for oshore operations ± 2. American Bureau of Shipping; 1999. p. 151±65. [3] Narzul P. The ¯exible riser Option, ONS 94, Stavanger, Norway, 1994. [4] Wang SW, Williams JG, Lo KH, editors. Composite for oshore operations ± 2. American Bureau of Shipping; 1999. [5] DeTeresa SJ. Materials for advanced ¯ywheel energy-storage devices. MRS Bull 1999;November:51±6. [6] Ichikawa M, Tanaka S. A probabilistic approach to fracture of composite ¯ywheels. Int J Fract 1980;16:R63±6. [7] Gabrys CW, Bakis CE. Simpli®ed analysis of residual stresses in in-situ cured hoop-wound rings. J Compos Mater 1998;32(13):729±37. [8] Chiu IL, et al. Epoxy resin system for composite ¯ywheels. Compos Technol Rev 1983;5(1):18±20. [9] Grudowsky TW, et al. Flywheels for energy storage. In: Proceedings of the 27th International SAMPE Technical Conference, October 9±12, 1995. p. 760±68. [10] Wells SP, et al. The design, manufacture, and testing of a composite ¯ywheel for energy storage. AMD-Vol. 194, Mechanics in materials processing and manufacturing. ASME; 1994. p. 397± 404. [11] Callagher P, et al. In-situ consolidation of thermoplastic thick rings. In: International SAMPE Symposium and Exhibition (Proceedings), May 31±June 4, 1998;43(2):1943±54. [12] Gabrys CW, Bakis CE. Filament would elastomeric matrix composites for ¯ywheel energy storage systems. In: Proceedings of the American Society for Composites, October 7±9, 1996. p. 729±37. [13] Cambridge Engineering Software, selector database. [14] Mallick V, et al. Robot based thermoplastic ®ber placement process. ABB Rev 1998;2. [15] Ghasemi MN, et al. Thermal analysis of in-situ thermoplastic composite tape laying. J Thermoplastic Compos Mater 1991;4:20± 45. [16] Pitchumani R, Don RC, Gillespie Jr JW, Ranganathan S. Analysis of on-line consolidation during a thermoplastic towplacement process. ASME Publication HTD-289, Heat and mass transfer in composites processing; 1994. p. 223±34. [17] James DL. Experimental analysis and process window development for continuous ®lament wound APC-2. ASME Publication HTD-289, Thermal Processing of Materials: Thermo-Mechanics, Controls and Composites, 1994, p. 203±12. [18] Sommez FO, Hahn HT. Modeling of heat transfer and crystallization in thermoplastic composite tape placement process. J Thermoplastic Compos Mater 1997;10:198±240. [19] Wilson P. Flywheel energy storage: overview and examination of the application of composite technology to advanced ¯ywheels. ABB Corporate Research, TN 96-001, 1996. [20] Landert M, Meier G. ABB private communication.