Composite Reinforcement Fibers

• Composite materials reinforcements are the focus.
• The presentation is from the University of Samsun, Faculty of Aeronautics and Astronautics, Aerospace Engineering.

Fiber Introduction

• Fiber use relies on three key characteristics.
• A smaller diameter relative to its grain size helps, because smaller fibers are more resistant and near-perfect.
• High aspect ratio (length/diameter) contributes to a large fraction.
• Fibers require a high degree of flexibility. Elastic modulus relates to fiber flexibility.
• Common fiber types include glass, boron, carbon, organic, ceramic, and metallic.
• Smaller fiber size reduces the chance of imperfections in the material. This relationship isn't consistently linear.
• Flexibility is shown by 1/MR.

Fiber Spinning & Orientation

• Fiber spinning is the process of extruding a liquid through small holes in a spinneret to create solid filaments.
• Spiders and silkworms naturally produce continuous filaments this way.
• Wet spinning, dry spinning, melt spinning, and dry jet-wet spinning are important fiber spinning techniques.
• Extrusion through a spinneret results in chain orientation.
• Surface molecules undergo more orientation than interior molecules because the spinneret edge affects the near-surface molecules; known as the skin effect.
• This effect alters all fiber properties.

Glass Fibers

• Glass fiber structure is non-crystalline and short-range network-based.
• Common glass fibers are silica-based (~50-60%) with added oxides.
• E Glass is an electrical insulator with good strength and a reasonable Young's modulus.
• C Glass is corrosion-resistant and has improved chemical resistance.
• S Glass (R Glass in EU) can withstand higher temperatures due to higher silica content and has superior mechanical properties, strong, for aerospace applications.
• Glass fiber production involves a sol-gel technique, using metal alkoxides to form a sol which is gelled and converted to glass by heating.
• All glass fibers are coated for protection due to being easily damaged, for post-processing steps, as surface damages are not good.
• Too much sodium oxide impairs glass formation tendency.
• A 3-D glass network yields isotropic behavior and similar E and CTE, Coefficient of thermal expansion.
• Density is low, and strength is relatively high.
• GFs are used generally for epoxy, phenolic, and polyester resins.
• Moisture reduces GF strength.
• GFs are susceptible to static fatigue, which causes sub-critical crack growth and failure over time at relatively low loads.
• Moderate modulus-to-weight ratio directs the aerospace industry to use other fibers (boron, carbon, and ceramic) for vital structural parts, but GF are still widely used in building, construction, automotive, marine, rail, and road applications.

Boron Fibers

• Boron is brittle.
• Boron fibers (BF) are made using Chemical Vapor Deposition (CVD).
• BF is a composite fiber with a substrate and boron surface.
• Tungsten wires or carbon are generally used as substrate material.
• Boron in BF is nearly amorphous, giving high strength.
• They have high strength and low density.
• Thermal decomposition of CVD on a substrate is used to manufacture Boron Fibers which are relatively weak due to lack of adherence between core and boron.
• Reduction of boron halide on refractory substrate is also used. A tungsten wire is used for the above process.
• Tungsten substrate usage, increases the fiber's total density while improving quality and uniformity.
• Extreme temperatures can cause recrystallization weakening the fiber.
• Interaction of boron with a tungsten core generally yields tungsten borides.
• Residual stresses may form during production.
• Stresses affect the physical properties of BF.
• BFs generally go through heat treatment, in order to get rid of residual stresses.

Carbon Fibers

• Carbon fiber is made of Graphite structure, produced through graphitization.
• Carbon fiber fabrication includes fiberization, stabilization, carbonization, and graphitization, as optional.
• To obtain high-modulus fibers, coordinate the orientation of Graphite crystals via heat and mechanical (stretch) treatments.
• Ex-PAN Carbon Fiber (CF) fabrication stabilizes under tension at medium T (250 °C) to prevent melting during high-T treatments and is then slowly heated at 1000-1500 °C for a highly ordered structure.
• For better texture, an optional 3rd heat treatment is applied at 3000 °C.
• Cellulose, for example cotton and ex-pitch based carbon fibers also exist.

Organic Fibers

• Organic Aramid fibers are synthetic
• Kevlar is mainly used as rubber reinforcement:
• Kevlar 29 is for ropes, cables, and coated fabrics for inflatables and architecture.
• Kevlar 49 reinforces epoxy, polyester, and other resins for aerospace and marine applications.
• These have a tensile direction-covalent bonding, compressive direction-hydrogen.

Ceramic Fibers

• Ceramic fibers provide high strength and elastic modulus, can withstand high temperatures and are environmentally resistant.
• Silicon Carbide Fibers are fabricated using CVD, polymer Pyrolysis, and sol-gel.
• Silicon carbide fibers are CVD deposited on a tungsten substrate heated to 1300°C.
• Carbon rich zones within silicon carbide fibers promote bonding, the broad zone promotes the ability to be shaped, and inner gradient is necessary for maintaining filament strength.
• A buffer layer on the surface allows fiber strength to be maintained even during high-temperature incorporation into a metal matrix

Whiskers

• Whiskers are monocrystalline but not G.B (Grain Boundary)
• They are short fibers, achieving a very high aspect ratio.
• This results in extremely high strength, close to theoretical value.
• Absence of imperfections exist (dislocation etc.)
• Whiskers lack uniform dimensions or properties.
• Are difficult to handle in composite production.
• Production processes include Chemical Vapor Deposition (CVD, coking and VLS process

Fiber Type Comparison

• A comparison of Young's modulus and tensile strength by fiber type:
• Aramid, Glass, C-pitch isotropic, SHT, HT, C-PAN, B, SiC, HM, SHM, C-pitch mesophase , Al2O3 all have varying differences in their properties.