D AY 34: C OMPOSITE M ATERIALS Properties of Composite Materials Types of Fiber that are commonly used

T HE CONDITION OF I SOSTRAIN 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m ff L AfAf AmAm

M ODULUS E IN I SOSTRAIN 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. We divide by A L, or Volume. The result is

T HE CONDITION OF I SOSTRESS 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m ff A LfLf LmLm

M ODULUS IN I SOSTRESS We divide by the common stress, . Mulitply by A. Divide by the volume. The result is.

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

I SOSTRESS AND I SOSTRAIN Composite Modulus will lie somewhere between the two. Clearly Isostrain is the target condition.

H OW THE L OAD IS S HARED 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. 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.

W HAT A BOUT S TRENGTH ? 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.

T YPES OF F IBER Here are several kinds of fiber material. We will discuss the first Carbon (or Graphite) 2. Kevlar (Aramid) 3. Glass 4. Boron 5. Metal whiskers

C ARBON F IBER We start with fibers of polyacrilonitrile. PAN. We oxidize them by heating in air. We graphitize them by heating in an inert gas environment.

C ARBON (G RAPHITE ) F IBER 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.

C ARBON F IBER Advantages 1. Excellent strength / weight 2. Excellent stiffness / weight 3. Not weakened by high temperatures 4. Conduct electricity Disadvantages 1. Very expensive, although getting cheaper 2. Brittle 3. Conduct electricity

R ELATIVES OF N YLON : T HE A RAMIDS 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.

K EVLAR FIBER IS VERY STRONG ! Compare Kevlar and the related polymer Nylon. MaterialDensity (g/cc) UTS (ksi) Ductility %EL Elastic Modulus Kevlar Fiber (12 micron) Not given260,000 Nylon Fiber 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

K EVLAR F IBER Advantages 1. Very good strength / weight 2. Very good stiffness / weight 3. Excellent toughness Disadvantages 1. Expensive 2. Not so good in compression

G LASS F IBER 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.

G LASS F IBER Advantages 1. Cheapest 2. Good strength /weight 3. Chemically inert 4. Transparent to EM radiation. Used in radomes and nose cones. Disadvantages Poor stiffness / weight