Composite Materials- Krishan K. Chawla

< Fibers>

1. introduction

• Reinforcement 로써 여러가지 형태가 존재하지만 가장우수한 형태는 fiber form 이다 .

▶ The use of fibers form is based on three important characteristic

• A high aspect ratio ( length / diameter, l/d)

• A very high degree of flexibility

• A smaller diameter with respect to its grain size or other microstructural unit

▶ fiber 종류 - Glass fiber, Boron fiber, Carbon fiber, Organic fiber, ceramic fiber Metallic fiber…

matrix

Reinforcement

→ Size effect 즉 작은단면적일수록 재료가 가질수있는 결함의 확률은 작아진다 .

→ elastic modulus 는 재료자체의 물성에 의존하지만 fiber 의 지름은 flexibility 에 민감함 .

→ large fraction 은 매트릭스 걸리는 하중을 fiber 로 이동시킬수 있음 .

2. Glass Fibers▶ Common glass fibers are silica based (~50%-60%)

• E Glass- electrical insulator and good strength, reasonable Young’s modulus • C Glass- stands for corrossion

• S Glass- stand for the higher silica content and glass is able to withstand higher temperatures than other glasses.

3. Boron Fibers

w

Reaction zone

B

Sic

• fabrication

1. Thermal decomposition of a CVD on substrate

2. Reduction of boron halide.

4. Carbon Fibers

• carbon fiber 는 graphite 의 a 축의 장점을 이용하기 위해 유기전구체를 고온에서 graphitization 하여 만듬 .

Precursorfiber

OxidationUp to 250℃

Carbonization250~1500℃

Graphitization1500~2500℃

• Schematic of carbon fiber production

4. Organic Fibers

• kevlar- meant mainly for use as rubber reinforcement fot tires • kevlar 29 – meant for ropes, cables, coated fabrics for inflatables, architectural… • kevlar 49 – meant for reinforcement of epoxy, polyester, and other resins for use in

aerospace, marine……

▶ organic fiber 는 이방성을 가지는 재료로써 압축응력 과 인장응력의 큰 차이를 보임 .

H H

H H

• Tensile direction- covalent bonding

• compressive direction- hydrogen , van der Waals

5. Ceramic Fibers

▶ ceramic fiber is high strength and elastic modulus with high-temperature capability and a general freedom from environmental attack

• fabrication- CVD, polymer Pyrolysis , sol-gel • silicon carbide fibers- CVD deposited on a W (sometimes carbon) substrate heated to 1300℃

CARBON MONOFILAMENT

Sic1

2 3Depth of surface

Si %

Zone 1 : carbon rich: to promote bonding with the matrix

This fiber gives a buffer layer at the surface that allow fiber strength to be maintaintedEven during the high-temperature incorporation into a metal matrix.

7. Comparison of fibers

• 진보된 우수한 fiber 의 특징은 density 가 낮다 .

Young’s modulus

Ten

sile s

tren

gth

B

Sic

• 몇몇 fiber 들은 이방성을 지니므로 응력방향을 선택받게 된다 .. • fiber 의 brittle 함을 matrix 와의 조합으로 극복한다 ..