DAY 34: COMPOSITE MATERIALS Properties of Composite Materials Types of Fiber that are commonly used

THE CONDITION OF ISOSTRAIN

This applies to long fiber composites. Compressed or pulled along fiber direction.

The fibers and matrix material have the same strain. The load is shared. is the average normal stress.

m

f

LAf

Am

mmff AAA

MODULUS E IN ISOSTRAIN

We assume Hookean Behavior. (Think carefully about this assumption!) We can divide by the constant strain shared by all components. Oh yes, we also multiply by L.

mmff AELAELAEL We divide by A L, or Volume. The result is

mmff VEVEE

THE CONDITION OF ISOSTRESS

This applies to long fiber composites. Compressed or pulled across the fiber direction.

The fibers and matrix material have the same stress. The displacement is shared. is the average normal strain.

m

f

ALf

Lm

mf mmff LLL

MODULUS IN ISOSTRESS

We divide by the common stress, . Mulitply by A.

m

m

f

f

E

AL

E

AL

E

AL

Divide by the volume. The result is.

m

m

f

f

E

V

E

V

E

1

mffm

mf

EVEV

EEE

EXAMPLE

Suppose we had 62% fiber content by volume. We are dealing with a Carbon Fiber – epoxy composite.

50.106.138.0)78.1(62.0 mmff VV

ksiEVEVE mmffstrainiso 25560200038.04000062.0

ksi

EVEV

EEE

mffm

mfstressiso 4870

200062.04000038.0

200040000

ISOSTRESS AND ISOSTRAIN

Composite Modulus will lie somewhere between the two. Clearly Isostrain is the target condition.

HOW THE LOAD IS SHARED

We pause to think about how the load is shared. Suppose we have 60% fiber by volume. Suppose the modulus of the fiber is 20 times that of the matrix.

30

14.0

206.0

mm

ff

mmm

fff

mm

ff

mm

ff

m

f

VE

VE

VE

VE

V

V

A

A

P

P

So we find that the load carried through the fibers is many, many times more than the load carried by the matrix. So each material is doing what it does best.

WHAT ABOUT STRENGTH?

Strength does not follow the following scheme exactly. It is because of the separate failure of the two materials. They don’t fail at the same level – there is a progressive failure in the composite.

TYPES OF FIBER

Here are several kinds of fiber material. We will discuss the first 3.

1. Carbon (or Graphite)2. Kevlar (Aramid)3. Glass4. Boron5. Metal whiskers

CARBON FIBER

We start with fibers of polyacrilonitrile. PAN. We oxidize them by heating in air. We graphitize them by heating in an inert

gas environment.

CARBON (GRAPHITE) FIBER The fiber is now made up of sheets of

graphite, which will be very strong in the direction of the fibers orientation.

Sizing is added at the surface of the fibers to make them bond more securely to the matrix material.

CARBON FIBER

Advantages1. Excellent strength / weight2. Excellent stiffness / weight3. Not weakened by high temperatures4. Conduct electricity Disadvantages1. Very expensive, although getting cheaper2. Brittle3. Conduct electricity

RELATIVES OF NYLON: THE ARAMIDS

Here is a truly wonderful engineering polymer.

There is high crystallinity and alignment in the fiber.

Strength comes from the alignment of crystalline and non-crystalline regions. Pulling on primary bonds.

KEVLAR FIBER IS VERY STRONG!

Compare Kevlar and the related polymer Nylon.

Material

Density (g/cc)

UTS (ksi)

Ductility%EL

Elastic Modulus

Kevlar Fiber(12 micron)

1.47 500 Not given 260,000

NylonFiber

1.22 76 30

This is showing off the polymer in its strongest possible form, a thin fiber. Uses: Armor, belts, hoses, reinforcing fiber in a composite.

Kevlar:Very high

mp—500C

KEVLAR FIBER

Advantages1. Very good strength / weight2. Very good stiffness / weight3. Excellent toughness Disadvantages1. Expensive2. Not so good in compression

GLASS FIBER

Glass fiber is amorphous. No slip systems. Without surface flaws, it can be very strong.

The glass used in fiberglass is practically free from surface flaws. It comes in very small diameter.

This fiber is very easy and cheap to make Sizing is key to protecting the surface of the

fibers and transferring the load.

GLASS FIBER

Advantages1. Cheapest2. Good strength /weight3. Chemically inert4. Transparent to EM radiation. Used in

radomes and nose cones. Disadvantages Poor stiffness / weight