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Comparison of Fibers

Comparison of Fibers. Lecture 7 & 8. Comparison of Fibers. The most important factors: 1- Weight saving 2- Low density 3- High modulus 4- High strength. Specific 0 ° Tensile properties of Fibers/Unidirectional Laminates Kev49/Ep Gr(AS-6)/Ep S-gl/Ep B/Ep

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Comparison of Fibers

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  1. Comparison of Fibers Lecture 7 & 8

  2. Comparison of Fibers The most important factors: 1- Weight saving 2- Low density 3- High modulus 4- High strength Specific 0° Tensile properties of Fibers/Unidirectional Laminates Kev49/Ep Gr(AS-6)/Ep S-gl/Ep B/Ep Sp. Ten Str 10.1/4.0 9.7/5.4 7.3/2.7 5.2/3.6 (10^6 in) Sp Ten Mod 3.5/2.2 5.8/3.5 1.4/0.96 5.8/4.0 (10^8 in) With the respect to specific modulus of fibers: c> s-gl; k> s-gl; etc.

  3. Comparison of Fibers • Thermal Stability • C= B > Gl > Kev (not in air) • Compressive properties: • Gl?B/C have compressive strength “similar” to their tensile strengths • Kevlar’s σc = 20 % σt (not good in compression) • due to anisotropy and low shear modulus • in tension, loads carried by strong covalent bonds. • In compression, weak hydrogen bonding and Van Der Waals forces lead in buckling in fibers.

  4. Comparison of Fibers Fiber fracture and flexibility

  5. Comparison of Fibers • The s-s curves suggest that in tension all fibers fracture in a brittle manner without yielding. • C and gl do fracture in a brittle manner without a reduction in cross sectional area. • Kevlar fractures in a ductile manner • get necking, local drawing • Bending • diameters of fibers have large affect on their degree of bending. • Important to operations such as weaving, filament winding, injection molding, etc. • Flexibility of fibers can be expressed in terms of moment, M required to bend a fiber with a circular cross-section to radius of curvature, M=πEd^4/64ρ, where d is the diameter of fiber • Parameter Units type I-HI-E Gr type II-HI-σ Gr E-gl K49 • D μm 8.0 8.0 11.0 12.0 • E GN/m^2 390 250 76 130 • Flexibility ratio - 1.0 1.56 1.44 0.59 • σf GN/m^2 2.2 2.7 3.5 • Ρ (min rad of curvature)mm 0.71 0.37 0.12 • The flexibility, 1/Mρ, is dominated by d but also depends on E

  6. Comparison of Fibers • Observations from the table: • flexibility of kev dominated by d • flexibility of type II > E-gl >type I due to modulus • Bending of fibers results in high surface tensile stress leading to brittle fracture • For given fracture strength, σf, there will be a minimum ρ which fibers can sustain before fracture occurs; ρmin= Ed/2 σf • Resistance to breakage • E-gl (0.12) > type II C (0.37) > type I Gr (0.71) • Kev doesn’t fit pattern due to low compression strength-bending produce high surfce compression as well as tensile stresses: • Low compressive strength causes fiber to buckle • Gl fibers can have abrasive damage during handling, leads to reduction of σf and increase in ρmin

  7. Comparison of Fibers Kev filament shows compression deformation/micro buckling

  8. Processes One-step • Autoclave/vacuum bag • Compression molding • Filament winding • Pultrusion • Injection molding • Resin transfer molding RTM • Continuous laminating • Thermoplastic specific • - one step process known as “wet Fiber Product + Resin Product Form Two-steps • process, e.g. filament winding. • Fundamental objective is to get resin into the fiber bundle (wetting) process to a composite having suitable: • fiber volume • Void content • Degree of cure

  9. Processes • In the bottom rout, R & F are combined b a prepregger (material supplier) to yield a product form- starting material composed of (F) impregnated with resin (R) • Preimpregnated Reinforcements (Prepregs) • are ready to mold (or process) combinations of F&R • Reinforcement include: • Roving, yarn, filaments, twos • Available individually or as collimated tapes: 3”, 6”, 12” to 60” wide • Woven fabrics • Fiber types including gl, gr, kev, (B is available in continuous tape only) • Prepreg preparation • 1- Solution Process: • principally applicable to fabrics. Solvent lower the viscosity for ease of impregnation. Also useful for tape when resin viscosity is too high for the hot-melt. • fabric/cloth gets dipped in solvent of formulated resin in suitable solvent. • Pass impregnated materials through drying tower to remove substantial amount of solvent.

  10. Processes • end up with big rolls of prepreg (up to 60” wide and 100” long • 2- Hot-melt process • major applicability to continuous, unidirectional tape. Used for solvenless resin systems that achieve a wetting (low) viscosity by heat application • Final product is a sheet of material rolled up on a spool, ready to ship to a user.

  11. Prepreg Process

  12. Cross plied machine

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