2. Mechanical Engineering Department Advanced Composites Dr. Talal Mandourah 1 Lecture 7 & 8 Comparison of Fibers

3. Mechanical Engineering Department Advanced Composites Dr. Talal Mandourah 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/EpGr(AS-6)/EpS-gl/EpB/Ep Sp. Ten Str10.1/4.09.7/5.47.3/2.75.2/3.6 (10^6 in) Sp Ten Mod3.5/2.25.8/3.51.4/0.965.8/4.0 (10^8 in) With the respect to specific modulus of fibers: c> s-gl; k> s-gl; etc.

4. Mechanical Engineering Department Advanced Composites Dr. Talal Mandourah 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.

5. Mechanical Engineering Department Advanced Composites Dr. Talal Mandourah 4 Comparison of Fibers Fiber fracture and flexibility

6. Mechanical Engineering Department Advanced Composites Dr. Talal Mandourah 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 ParameterUnits type I-HI-E Grtype II-HI-σ GrE-glK49 Dμm8.08.011.012.0 EGN/m^239025076130 Flexibility ratio-1.01.561.440.59 σf GN/m^22.22.73.5 Ρ (min rad of curvature)mm0.710.370.12 The flexibility, 1/Mρ, is dominated by d but also depends on E

7. Mechanical Engineering Department Advanced Composites Dr. Talal Mandourah 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

8. Mechanical Engineering Department Advanced Composites Dr. Talal Mandourah 7 Comparison of Fibers Kev filament shows compression deformation/micro buckling

9. Mechanical Engineering Department Advanced Composites Dr. Talal Mandourah 8 Processes - 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 Resin Product Form + Two-steps One-step 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

10. Mechanical Engineering Department Advanced Composites Dr. Talal Mandourah 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.

11. Mechanical Engineering Department Advanced Composites Dr. Talal Mandourah 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.

12. Mechanical Engineering Department Advanced Composites Dr. Talal Mandourah 11 Prepreg Process

13. Mechanical Engineering Department Advanced Composites Dr. Talal Mandourah 12 Cross plied machine