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03/09/051 High Performance Composites Ray Loszewski.

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1 03/09/051 High Performance Composites Ray Loszewski

2 03/09/052 Purpose of Presentation Overview of boron, carbon, and silicon carbide fibers, prepregs and composite fabrication Differences in fiber structures, how made and used Performance characteristics; strengths/limitations Tailored coatings, surface treatments, and sizing Prepregs, preforms, and composite fabrication Hybrids; design and synergistic combinations Aging characteristics and composite repair Specialized applications; friction, re-entry, and etc. Important to understand the micromechanics

3 03/09/053 Disclaimer/Information Sources Requirement to show/discuss only information or hardware that is in the public domain All photos/illustrations are from Internet sources or current owners (Textron originally), e.g. Nat'l Academies Press, High Performance Synthetic Fibers for Composites (1992) Some information is taken directly from websites and/or edited to fit slide format, e.g. http://www.nap.edu/execsumm/0309043379.html http://www.specmaterials.com/

4 03/09/054 Methods of Reinforcing Plastics, Metals, and Ceramics Particulates Short or long fibers, flakes, fillers Continuous fibers or monofilaments Source of sketches: http://www.nd.edu/~manufact/pdfs/Ch09.pdfhttp://www.nd.edu/~manufact/pdfs/Ch09.pdf

5 03/09/055 Fiber Types Covered Herein Boron (B) and silicon carbide (SiC) fibers are relatively large diameter (typically 2 – 8 mils) monofilaments produced by chemical vapor deposition onto a core material, usually a 0.5 mil tungsten-filament or a 1.3 mil CMF (carbon monofilament). Carbon fibers are produced by the pyrolysis of an organic precursor fiber, such as PAN (polyacrylonitrile), rayon or pitch, in an inert atmosphere at temperatures above 982°C/1800°F, typically 1315°C/2400°F, and contain 93-95% carbon. Carbonized fibers can be converted to graphite fibers by graphitization at 1900°C to 2480°C (3450°F to 4500°F) to yield >99% carbon. Definitions adapted from: www.compositesworld.comwww.compositesworld.com High-Performance Composites Sourcebook 2004 Glossary

6 03/09/056 Fiber Size Comparison Chart 1.3 mil ( 33 µ ).5 Dia.47 mil ( 12 µ ).28 mil ( 7 µ ) 1.3 mil 1.0 Dia 4 mil 5.6 mil CVD Fibers Carbon Fibers Kevlar Fibers or Tungsten Filaments Carbon Monofilaments (CMF) (Scale 1000/1)

7 03/09/057 Fiber Spinning Process Steps Melt or Solution Spinneret Stretch (Orient) and Solidify Take-up or Idler V0V0 V1V1 V 1 >V 0 V2V2 V2V1V2V1 Packaging Heat or Chemical Treatment 1 st Step2 nd Step

8 03/09/058 (e.g. Nylon) (e.g. Kevlar) (e.g. Vectran) (Source: Dupont Kevlar® and Celanese Vectran ® Brochures) Orientation During Spinning

9 03/09/059 PAN Based Carbon Fiber Process Polymerization Spinning Precursor Stabilization GraphitizationCarbonization Surface Treat Sizing Carbon Fiber 1000-3000°C

10 03/09/0510 PAN/Pitch Process Comparison (Source: A. R. Bunsell, Fibre Reinforcements for Composite Materials, Amsterdam, The Netherlands: Elsevier Science Publishers B.V., 1988, p. 90.) Polyacrylonitrile (PAN) Pitch Carbon/Graphite

11 03/09/0511 Complete PAN Based Process (Source: http://www.harperintl.com/carbon2.htm)http://www.harperintl.com/carbon2.htm

12 03/09/0512 Carbon Fiber Properties (Photo Source: A. R. Bunsell, Fibre Reinforcements for Composite Materials, Amsterdam, The Netherlands: Elsevier Science Publishers B.V., 1988, p. 203.)

13 03/09/0513 Carbon Fiber Vs High Tensile Steel Carbon fibers per se are not very useful A matrix is needed to transfer load from fiber to fiber and to hold everything together to form a composite An oxidative surface treatment is often needed to provide functionality or attachment points for bonding A coating or sizing protects fiber and facilitates wetting

14 03/09/0514 Specific Property Comparison* http://www.advancedcomposites.com/technology.htm#properties *Note: composite materials at 60% fiber volume with epoxy

15 03/09/0515 (Source: Dupont Kevlar® Brochure 12/92) Kevlar® Fiber Structure

16 03/09/0516 (Internet Source – Lost Reference) Kink Bands and Fibrillation Microfibril is the fundamental building block in highly oriented, high modulus fibers. These fibers typically have ten times weaker compressive strength than tensile strength. Local high angle bending or folding causes compressive strain and results in local, microfibrillar misorientation or kink bands. Once enough microfibrils are broken within the kink band, the entire fiber will fail.

17 03/09/0517 (Internet Source – Lost Reference) Photomicrograph of Kink Band

18 03/09/0518 Why Boron or Boron Hybrids? Typically, graphite or microfibrillar unidirectional lamina are compression strength limited High tensile strength is unavailable when cyclic loads and stresses limit the strength to the compression strength allowable Graphite fiber + Boron fiber are often matched to yield improved balance between tension and compression strength and modulus Increased strength efficiency translates to weight and cost savings

19 03/09/0519 Boron Fiber Structure The fiber surface is nodular, with nodules oriented axially along the length. Fiber crystal structure is fine and complex with crystallite size on the order of 2 nanometers (amorphous). Large diameter and lack of well-defined crystalline structure leads to high compression properties.

20 03/09/0520 Boron Reactor Schematic Boron fiber is produced via CVD using the hydrogen reduction of boron trichloride on a tungsten filament in a glass tube reactor. The basic reaction, carried out at 1350°C, is as follows: 2BCl 3 (g) + 3H2 (g) = 2 B (s) + 6HCl

21 03/09/0521 Boron Filament Production

22 03/09/0522 CVD Fiber Structural Limitation CVD fibers are actually micro-composites Fiber structure depends on deposition parameters temperatures, gas composition, flow dynamics, etc. Theoretically, mechanical properties are limited by the strength of the atomic bonds that are involved Practically, strengths are limited by residual stresses and structural defects that are built in during CVD Residual stresses caused by volume differences in chemical reaction products, CTE mismatches during cool-down, etc. Structural defects caused by temperature gradients, power fluctuations, impurities/inclusions, gas flow instabilities, etc. Must maintain compressive stresses on fiber surface

23 03/09/0523 Boron Fiber Properties Tensile Strength 520 ksi (3600 MPa) Tensile Modulus 58 msi (400 GPa) Compression Strength ~1000 ksi (6900 MPa) Coefficient of Thermal Expansion 2.5 PPM/°F (4.5 PPM/°C) Density 0.093 lb/in³ (2.57 g/cm³)

24 03/09/0524 Fibers/Monofilaments/Hybrids 4 mil dia (100μ) 0.5 mil dia (12μ) Matrix Boron Tungsten Matrix Kevlar Fibers 0.5 mil dia (12 μ) Carbon Fibers 0.3 mil dia (7 μ) Conventional Boron/Graphite (Carbon) Hybrid HyBor® Versus Void Source of Top Photos: http://www.nd.edu/~manufact/pdfs/Ch09.pdfhttp://www.nd.edu/~manufact/pdfs/Ch09.pdf

25 03/09/0525 Understanding Hy-Bor® Hy-Bor® is a mixture of Boron and Graphite fibers commingled as a single ply High compression properties of Boron fiber improve Graphite fiber micro buckling stability Individually, each material is strain limited by the fiber properties Commingled, each fiber contributes and shares load according to principles of micromechanics

26 03/09/0526 Hy-Bor® Prepregging Process

27 03/09/0527 Hy-Bor® Compression Strength Compression Strength of Hy-Bor® directly relates to Shear Modulus* Increasing Boron fiber count increases compression strength towards theoretical 600 ksi limit * The Influence of Local Failure Modes on the Compressive Strength of Boron/Epoxy Composites, ASTM STP 497, J.A. Suarez, J.B. Whiteside & R.N. Hadcock, 1972 Influence of Boron Fiber Count on Compressive and Shear Properties of HyBor, Alliant Techsystems, J.W. Gillespie,1986

28 03/09/0528 Benefits of Hy-Bor® Provides the Maximum Compression Strength of any continuous filament-based composite material Tailored to meet specific materials properties and design objectives (Graphite fiber type and Boron fiber ratio) Prepregged to customer resin preferences Analytically treated as another lamina within a laminate stack per Classical Lamination Theory Can be mixed with carbon plies or it can be the total laminate (maximum fiber volume)

29 03/09/0529 Aging and Composite Repair Properties may deteriorate over time by exposure to high temperatures, moisture, UV radiation, or other hostile environments Degradation may be reversible or permanent; chemical (oxidation) or mechanical (fatigue) Cracks may be patched using doublers or adhesively bonded reinforced epoxies Aluminum structures cannot be repaired using graphite/epoxy due to galvanic corrosion issues Boron/epoxy doublers gaining acceptance

30 03/09/0530 Boron Doubler Reinforcement

31 03/09/0531 Boron Doubler Installation

32 03/09/0532 SCS Family of SiC Fibers Boron was ineffective in metal matrices CVD SiC made by similar process using less costly gases SCS offers Improved strength at higher temperatures Optimized surface for handling and bonding SCS-6 (5.6 mil) Developed for titanium and ceramics SCS-9A (3.1 mil) Developed for thin- gauge face sheets for NASP SCS-ULTRA (5.6 mil) Developed to achieve highest strength

33 03/09/0533 SCS SiC Fiber Process CMF vs. tungsten Pyrolytic graphite Complex chemistry and glassware High maintenance Multistage reactor Integral surface coating region Each run optimized

34 03/09/0534 Construction of SCS Fiber for Strength and Matrix Compatibility

35 03/09/0535 Schematic of SCS-6 CVD SiC

36 03/09/0536 Brittle Fracture Characteristics Distribution of strengths rather than single value Imperfections lead to stress concentrations Fracture initiates because material cannot deform plastically Cracks typically originate at defects on the core, at interfaces or the surface

37 03/09/0537 Comparison of SCS SiC Fibers

38 03/09/0538 Comparison of SCS SiC Fibers

39 03/09/0539 SCS-6 Strength Vs. Temperature

40 03/09/0540 Comparison of Strength Vs. Temperature for SiC Fibers

41 03/09/0541 Properties of Ti-6-4 Composites

42 03/09/0542 Transverse Optical Micrographs Source: Vassel A., Pautonnier F., Mechanical Behavior of SiC Monofilaments in Orthorhombic Titanium Aluminide Composites, ICCM, Pékin (Chine), 25-29 June 2001 SCS-6/Ti-22Al-27Nb Composite. Ultra SCS Metal Matrix Composite Source: Textron Specialty Materials

43 03/09/0543 Carbon/Carbon Composites Unimpressive properties at ambient but offers combination of high- temperature resistance to 2760°C (5000°F), light weight, and stiffness Expensive due to difficult processing, pore closure Rapid Densification (RD) Applications Rocket nozzles, Re-entry Brake linings, discs, torque converters (wet friction)

44 03/09/0544 Carbon/Carbon Process Flow High char yield polymer or pitch Carbon fiber Preform fabrication First Carbonization (~1000°C) Impregnation (CVD or RD) Intermediate Graphitization 2500-3000°C Carbonization 1000°C Curing of polymer or Carbonization of pitch under pressure Impregnation with liquid polymer or pitch Final graphitization 2500-3000°C C/C composite 1000°C C/C composite 2500-3000°C

45 03/09/0545 Ceramic-Matrix Composites Major hurdle is to overcome brittleness Traditional reinforcements are not very effective because cracks still propagate Conversely, SCS-6 fibers impart strength and toughness to ceramics because their carbonaceous surface coating layer arrests and/or deflects the energy, which allows for bridging of any cracks

46 03/09/0546 Applications Drive Technology Aerospace/Defense applications emphasize enabling technologies and performance Competition is more effective than consortia Many promising technologies languish due to funding cuts or satisfaction with status quo e.g. NASP and Superconducting Supercollider chicken/egg cost dilemma and public apathy Commercial applications emphasize availability and cost, i.e value for the dollar Competitive edge and marketability are important e.g. Sports equipment, fuel cells, solar, and etc.

47 03/09/0547 Closing Comments Composite design starts with the reinforcement Fiber choice depends upon the application; must weigh advantages/disadvantages, cost, etc. Matrix selection (polymeric, metal, carbon, ceramic) often dictates fiber type and material form, i.e. whether to use tow, fabric, tape, and etc. Key to solving most problems is knowledge of: How fibers are made; why they behave as they do Role of coatings, surface treatments, and sizing Reactions at the fiber surface during processing Focus on the micromechanics at interfaces


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