composite.docx

7/27/2019 composite.docx

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[FISIKA] bahan komposit (2)Ihsan Hariadi Thu, 08 Jun 2000 11:30:21 -0700

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MILIS FISIKA INDONESIA (MFI)

INDONESIAN FORUM FOR PHYSICS AND PHYSICS MANAGEMENT

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Dari guntingan tulisan dari sebuah diktat kuliah di bawah ini, info

mungkin bisa kita peroleh, antara lain:

-> "filosofi" bahan komposit, termasuk fakta bahwa di sekitar kita

sebetulnya sudah tersedia bahan-bahan di alam yang bisa

dikategorikan sebagai bahan komposit (bahan komposit alam).

-> pentingnya struktur "serat (fiber)" sebagai bentuk yang

banyak digunakan untuk komponen bahan "rangka"/ reinforcement

bahan komposit

-> beberapa jenis serat yang penting : glass fiber, carbon fiber,

dan KEVLAR: serat polimer yang kuat sekali, merupakan bahan utama

untuk baju / rompi anti peluru. Di samping tahan peluru, dikatakan

juga anti tusukan. Kalau Kevlar nanti sudah begitu murah, mungkin

bahan ban luar mobil juga bisa dilapisi kevlar, sehingga tahan

tusukan paku ... (tapi kalau tukang tambal nanti jadi ndak lakugimana ya ... ;-) )

-> contoh penggunaan bahan komposit sebagai bahan biomedik, misalnya

untuk gigi, atau prothesis (anggota tubuh buatan).

hope you'll enjoy reading ... ;-)

xoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxo< ihsan>

xoxoxoxoxoxo

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Introduction to Composite Materials

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http://www.mail-archive.com/[email protected]&q=subject:%22%5BFISIKA%5D+bahan+komposit+%282%29%22http://www.mail-archive.com/[email protected]&q=subject:%22%5BFISIKA%5D+bahan+komposit+%282%29%22http://www.mail-archive.com/[email protected]&q=from:%22Ihsan+Hariadi%22http://www.mail-archive.com/[email protected]&q=date:20000608http://www.courses.ahc.umn.edu/medical-school/BMEn/5001/notes/composites.htmhttp://www.courses.ahc.umn.edu/medical-school/BMEn/5001/notes/composites.htmhttp://www.courses.ahc.umn.edu/medical-school/BMEn/5001/notes/composites.htmhttp://www.courses.ahc.umn.edu/medical-school/BMEn/5001/notes/composites.htmhttp://www.mail-archive.com/[email protected]&q=date:20000608http://www.mail-archive.com/[email protected]&q=from:%22Ihsan+Hariadi%22http://www.mail-archive.com/[email protected]&q=subject:%22%5BFISIKA%5D+bahan+komposit+%282%29%22


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M. Arif Iftekhar

BMEn 5001, 10/26/98

Note : The figures will be provided in a hand-out.

-----

Objectives:

----------

(o) What are composite materials?

(o) Why are composite materials significant?

(o) What are the common structural components?

(o) What are the various kinds of composite materials?

(o) How are composite materials of various kinds processed?

(o) What are some important structure property relationships?

(o) What are the concerns for medical applications?

Definition of composite materials:

---------------------------------

Practically everything is a composite material in some sense. For

example, a common piece of metal is a composite (polycrystal) of

many grains (or single crystals). The following is an operational

definition for the purpose of this lecture:

A composite material:

--------------------

-> Consists of two or more physically and/or chemically

distinct suitably arranged or distributed phases, with an

an interface separating them.

-> It has characteristics that are not depicted by any of the

components in isolation

Most commonly, composite materials have a bulk phase, which is

continuous, called the matrix, and one dispersed, non-continuous,

phase called the reinforcement.

The concept of composite materials is ancient: to combinedifferent materials to produce a new material with performance

unattainable by the individual constituents. An example is adding

straw to mud for building stronger mud walls. Some more recent

examples, but before engineered materials became prominent, are

carbon black in rubber, steel rods in concrete, cement/asphalt

mixed with sand, fiberglass in resin etc.


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In nature, examples abound:

----------------------------

-> a coconut palm leaf,

-> cellulose fibers in a lignin matrix (wood)

-> collagen fibers in an apatite matrix (bone) etc.

The essence of the concept of composites is this: the bulk phase

accepts the load over a large surface area, and transfers it to

the reinforcement, which being stiffer, increases the strength of

the composite. (fig.1) The significance here lies in that there

are numerous matrix materials and as many fiber types, which can

be combined in countless ways to produce just the desired

properties. (fig. 11)

Most research in engineered composite materials has been done

since 1965. Today, given the most efficient design, of say an

aerospace structure, a boat or a motor, we can make a composite

material that meets or exceeds the performance requirements. Most

of the savings are in weight and cost. These are measured in

terms of ratios such as stiffness/weight, strength/weight, etc.

Components of Composite Materials

---------------------------------

* Bulk phase: matrix materials

Polymers

Metals

Ceramics

* Reinforcement: fibers and particulate

Glass

Carbon

Organic

Boron

Ceramic

Metallic

* Interface

--------------------------------------------------------------

Fibers

------


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Reinforcements are not necessarily in the form of long fibers.

They can be particles, whiskers, discontinuous fibers, sheets

etc. A great majority of materials is stronger and stiffer in the

fibrous form than in any other form. This explains the emphasis

on using fibers in composite materials design.

There are many naturally occurring fibers: cotton, flax, jute,

hemp, ramie, wood, straw, hair, wool, silk etc., but these have

varying properties, and present many processing challenges.

Fibers used in advanced composites have very high strength and

stiffness but low density.

They also should be very flexible (to allow a variety of methods

for processing) and have high aspect ratio (length/diameter),

that allows a large fraction of the applied to be transferred via

the matrix to the fiber.

Fibers are added to a ductile matrix (like polymers and metals)

usually to make it stiffer, while fibers are added to a brittle

matrix (like ceramics) to increase toughness.

Glass fibers

------------

1) The most common and inexpensive fiber used is glass fiber,

usually for the reinforcement of polymer matrices.

2) Typical composition is 50-60% SiO2, and other oxides of Al,

Ca, Mg, Na, etc.

3) There are mainly three types of glass fibers: E, C, S

a) E offers good electrical insulation

b) C is resistant to chemical corrosion

c) S has high silica content, and can withstand greater

temperatures

4) 90% of all continuous glass fibers is of the E type.

5) "Sizing" is a treatment applied to glass fibers to protect the

strands from surface defects, bind the filaments into a strand,

and reinforce the interface bond to the matrix. It is usually

based on PVAc and silane coupling agents.

6) Glass fibers are available as:


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a) Chopped strands

b) Continuous yarn

c) Roving

d) Fabric sheets

7) Moisture decreases glass fiber strength.

8) Glass fibers are susceptible to static fatigue, i.e. they

cannot withstand static loads for long periods of time.

9) Properties:

a) Density is quite low (~2.55 g/cc)

b) Tensile strength is quite high (~1.8 GPa)

c) Stiffness is, however, low (70 GPa)

d) Therefore, whereas strength/density is high, stiffness

/density is low.

Carbon fibers

-------------

1) Carbon is a very light element, with density about 2.3 g/cc

2) The graphitic structure is preferred over the diamond-like

crystalline forms for making fibers. (fig7)

3) Fibers are made by carbonization of precursor fibers, followed

by graphitization at high temperature. The precursors are usually

PAN (polyacrylonotrile) and Rayon, which are both textile

polymers. (fig8)

4) Since the graphitic structure is made of densely packed

hexagonal layers, stacked in a lamellar style, the mechanical and

thermal properties are highly anisotropic. Controlling the

orientation of the crystalline layers is a crucial issue.

a) Young's modulus in the layer plane can be 1000 GPa, while

that along the c-axis is equal to about 35 GPa. Carbon hasexcellent compression properties.

b) Transverse CTE is 5.5 to 8.4E-06/K, while parallel CTE is

-0.5 to -1.3E-06/K

5) Carbon fiber composites find wide applications in the

aerospace and sporting goods industries. The ability to tailor


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the required stiffness and strength properties of internal bone

plates gives them a definite advantage over metallic parts

6) Carbon fiber adds electrical conductive properties to

composites. While this may be preferred where static charges need

to be dissipated, it can cause corrosion problems, especially inproximity to metallic parts.

Oriented organic fibers

-----------------------

1) Strong covalent bonds in polymers, if aligned long the fiber

axis of high molecular weight chains, can lead to impressive

properties. (fig 10)

2) Two examples are UHMPE (ultra high molecular weight

polethylene) called Spectra, made by Allied Corp., and Kevlar

made by DuPont.

3) Spectra is a very light fiber (denisty ~0.97 g/cc) made from

gels and solution. It has a stiffness of about 200 GPa. Its

primary disadvantage is its low melting point (around 150

Celsius), but this may not be an issue in biomedical

applications.

4) Kevlar is an aramid (aromatic polyamide) composed of oriented

aromatic chains, which makes them rigid rod-like polymers. (fig

9) It has a very high Tg and poor solubility. Since very

concentrated acids are used in its processing, this can be an

issue in biomedical applications if all acid residues are not

extracted. Although very strong in tension, Kevlar has very poor

compression properties. Its stiffness can be as high as 125 GPa.

The fibers are mostly used to increase toughness in otherwise

brittle matrices.

Other fibers

------------

1) Boron fibers are very brittle, very stiff and quite expensive.They are used in very high-end applications, and find little use

in medical applications, where carbon fibers can provide the

needed performance.

2) Ceramic fibers, such as Alumina and SiC (Silicon carbide) are

advantageous in very high temperature applications, and also

where environmental attack is an issue.


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3) Metallic fibers, such as steel and tungsten, have high

strengths and show very consistent properties, unlike ceramic

fibers. Since density is very high for these fibers, they are

rarely used to reduce weight in a composite. Drawing very thin

metallic fibers (less than 100 micron) is also very expensive.

---------------------------------------------------------------

Matrix materials

----------------

Polymers, metals and ceramic material structure and properties

have been covered in previous lectures and will not be repeated

here.

---------------------------------------------------------------

Interface

---------

1) The interface is a bounding surface or zone where a

discontinuity occurs, whether physical, mechanical, chemical etc.

2) More often than not, the interface between fiber and matrix is

rather rough, instead of ideal planar. (fig. 14)

3) The matrix material must "wet" the fiber. Coupling agents are

frequently used to improve wettability. Well "wetted" fibers

increase the interface surface area.

4) To obtain desirable properties in a composite, the applied

load should be effectively transferred from the matrix to the

fibers via the interface. This means that the interface must be

large and exhibit strong adhesion between fibers and matrix.

Failure at the interface (called debonding) may or may not be

desirable. This will be explained later in fracture propagation

modes.

5) Bonding with the matrix can be either weak van der Walls

forces or strong covalent bonds.

6) The internal surface area of the interface can go as high as

3000 cm2/cm3.

7) Interfacial strength is measured by simple tests that induce


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adhesive failure between the fibers and the matrix. The most

common is the Three-point bend test or ILSS (interlaminar shear

stress test)

--------------------------------------------------------------

Fabrication of Composites

-------------------------

PMC (Polymer Matrix Composites)

1) Filament winding (fig.17)

2) Pultrusion (fig.18)

3) Compression molding (fig.21)

4) Laminate stacking (fig.22)

5) Wet flow methods

a) Injection molding (fig. 20)

b) Resin transfer molding (fig.19)

CMC (Ceramic Matrix Composites)

1) Usually a two-step process

a) Incorporation of a reinforcing phase into an

unconsolidated matrix

b) Matrix consolidation

2) Several methods are common:

c) Melt infiltration (fig. 23)

d) Chemical vapor deposition/infiltration (CVD)(fig. 24)

--------------------------------------------------------------

Properties of Composites

------------------------

We will consider the results of incorporating fibers in a matrix.The matrix, besides holding the fibers together, has the

important function of transferring the applied load to the

fibers. It is of great importance to be able to predict the

properties of a composite, given the component properties and

their geometric arrangement.

1) Isotropic vs. anisotropic


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a) Fiber reinforced composite materials typically exhibit

anisotropy. That is, some properties vary depending upon

which geometric axis or plane they are measured along.

b) For a composite to be isotropic in a specific property,such as CTE or Young's modulus, all reinforcing elements,

wether fibers or particles, have to be randomly oriented.

This is not easily achieved for discontinuous fibers,

since most processing methods tend to impart a certain

orientation to the fibers. One example is the classic

Skin-Core-Skin pattern seen for injection molded short

fiber composites.

c) Continuous fibers in the form of sheets are usually used

to deliberately make the composite anisotropic in a

particular

direction that is known to be the principally loaded axis or

plane.

2) Rule of Mixtures

The Rule of Mixtures is a rough tool that considers the composite

properties as volume weighted averages of the component

properties. It is important to realize that this rule works

accurately only in certain simple situations, such as determining

composite density and elastic modulus (fig 6). For most other

properties, this provides only a rough estimate for initial

design purposes. Below are some equations derived for

unidirectional continuous fiber composites. The derivations can

be found in any introductory book on composite mechanics. The

principle used is that in longitudinal direction, both fibers and

matrix have the same strain (isostrain) and in transverse

direction, both fibers and matrix have the same stress

(isostress).

The subscripts f, m, v and c refer to fiber, matrix, voids and

composite respectively. The subscripts l and t refer tolongitudinal and transverse respectively. E is the Young's

Modulus, , and r (rho) is density. a (alpha) is the coefficient

of thermal expansion (CTE). Lowercase v refers to volume,

whereas uppercase V refers to volume fraction (volume of a

component divided by total volume).

a. Density : ... (figure)


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b. Young's Modulus

-> Longitudinal : ... (figure ..)

-> Transverse : ... (figure ..)

c. Coefficient of Thermal Expansion

-> Longitudinal : ... (figure ..)

-> Transverse : ... (figure ..)

3) CTE mismatch

Because of a difference in the thermal expansion properties of

fibers and matrix, the composite is not allowed to deform

uniformly under thermal stress, and this can lead to

microcracking of the matrix and debonding at the interface. This

is a particularly important concern in dental composite materials

where thermal stresses are significant.

4) Interface, fracture propagation and toughness

a) In a ductile matrix, like most polymers and metals, a

strong interfacial bond is important, since the fibers

carry most of the load in such matrices. Fibers tend to fail

first, usually by cohesive failure through the fiber cross-

section. This is because the fibers cannot strain as much as

the matrix (e.g. carbon in epoxy). Cracks are few, and tend

to propagate slowly. When the cracks hit the interface,

strong interfacial bonds stop them (fig. 16).

b) In a brittle matrix, like ceramics, the matrix carries

most of the load, which is usually compressive (like in

teeth or bone), and fibers are added only to increase

toughness. That us, to increase the time to catastrophic

failure by holeding the matrix together after cracking.

Fibers here are more ductile than the matrix (e.g. glassin alumina) and the matrix fails first. As the cracks

propagate and reach the interface, a weak interfacial bond

is desired. This enhances debonding, and the cracks are

not stopped, but deflected along the length of the fibers.

This effectively delays the time it takes the cracks to

propagate through the entire matrix, and thus increases

toughness. (fig.15)


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--------------------------------------------------------------

Biomedical Issues

-----------------

1) A composite is made of two or more materials, and eachindividual component can elicit a different biological response.

This complicates testing for biocompatibility since every

additional material increases the requirements for approval under

federal regulations.

2) Composite material processing for non-medical applications

tends to involve strong solvents and reagents. Particularly in

thermosetting matrices like epoxy and phenolics, considerable

solvent residues are present. Modifying these processes for

medical-grade implantable products is challenging.

3) The history of advanced composites is not very long, such that

long-term concerns related to fatigue and corrosion are difficult

to address.

Biomedical Applications

-----------------------

The most successful applications of composite materials have been

in the area of dental materials. This will be discussed further

in the lecture on Orthopedic and Dental Applications. Common

composites in this area are polyurethanes and ceramics

impregnated with borosilicate glass fibers or particles.

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