Composites

Take a piece of Styrofoam and have a student bend it. What happens? The board will break!

Repeat the activity but place a piece of tape on both sides of the Styrofoam Once again bend the board after the tape is applied. What observations can be made?

What are composite materials? Engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties Two components remain separate and distinct at the macroscopic or microscopic scale Two materials are not soluble in each other Composite material usage is continually growing

Composites: Composition Contain two distinct materials Multiphase material “Binder material” – matrix material that surrounds other material “Reinforcement” – material that is providing the extra support (fibers, or fragments) that are bound together

Material properties that can be improved by forming a composite Naturally not all of these improvements occur at the same time!

Composites in Nature Nature is a great composite manufacturer: Wood Lignin matrix reinforced with cellulose fibers Bone Bone plates reinforced with collagen Sea shells Abalone shell: calcium carbonate held together by proteins

Early Examples of Composites Israel mud bricks + straw Straw strengthened the mud bricks in building materials Ancient Egyptians (1500 BC) Use of glue laminated wood (plywood) Use of straw to reinforce clay walls Medieval swords and armor (1800 AD) constructed with layers of metals

Modern Composites 1950’s -1960’s Demand for materials with low weight and high strength, rigidity for applications such as aerospace, electronics, etc. Post-war economy: companies invested in large-scale laboratories Cold War: drive for aerospace technology Example: Fiberglass – glass fibers are surrounded by a plastic matrix

Composite Example: Fiberglass Glass fibers are surrounded by a plastic matrix Glass fibers are brittle on their own Plastic matrix holds the fibers together and protects from damage and outside forces

Modern Composites 1970’s+ Development of new types of fibers (carbon, boron, and more) Carbon Fibers – stronger than fiberglass, but more expensive Used in aerospace and military applications Fibers consist mostly of the atom carbon Fibers have similar structure to graphite

Types of Composites Commonly types of composites: Combinations of some or all of the above are also used

Fiber-reinforced Composites Long fibers placed into materials material is stronger and stiffer Example…..Ordinary glass plate fractures with little stress of a few thousand pounds per square inch. With a fibrous composite added to the plate, fractures need 400,000 pounds per square inch to as much as 1 million pounds per square inch.

Properties of Matrix Materials Fibers and whiskers are of little use unless they are bonded together to take the form of a structural element that can carry loads The binder material is known as the Matrix The purpose of the matrix is to –Support of the fibers or whiskers –Protection of the fibers and whiskers –Matrix is of considerably lower density, stiffness or strength than a fiber or whisker –The combination of fiber or whiskers with a matrix can have very high stiffness and strength and a very low density

Matrix Materials Matrix materials can be: Polymers – polymer resins Metals – Metal matrix composites Ceramics Carbons

Common Fiber Materials carbon or graphite (C) glass (Si02) organic fibers silicon carbide fibers (SiC) boron fibers (B) metallic fibers (Mg, Ti)

Common Fiber – reinforced composites Fiberglass Carbon Fiber Reinforced Polymer (CFRP) Fiber reinforced concrete Bicycles – carbon fiber reinforced plastic Automobile materials – metal matrix composites

Characteristics of Fiber Composites Fiber Length Amount/Concentration of Fiber Volume fraction of fiber can increase strength and stiffness Max volume fraction with better properties: 80% Orientation of Fiber Shape of Fiber

Fiber Length Critical Length needed for strength improvement This length is dependent on diameter and tensile strength of fiber Longer fibers with small diameters are preferred Large aspect ratio (length/diameter of fibers) leads to a high strength

Fiber Orientation How does the orientation of the fibers affect the composite properties? What other factors might influence the properties?

Fiber Orientation Orientation can affect the properties Can make the properties anisotropic

Fiber Shapes Fiber can be different shapes Cylindrical fibers are most common

Carbon fiber reinforced polymers (CFRP) Carbon Fibers – 5-10 um in diameter Polymer is often an epoxy (thermosetting polymer) Applications: Aerospace, transportation Boeing 787 is composed of CFRP Sports equipment – hockey sticks, racing bicycles

Carbon Fibers Composed of >90% carbon high flexibility high tensile strength low weight low thermal expansion High strength-to-volume ratio

Particle-Reinforced Composites Often Isotropic Examples: Reinforced rubber Carbon black particles help strengthen auto tires Concrete Gravel and sand help to reinforce cement

Structural Composites Properties depend on the geometric design Laminar Composites: Structural Sandwich

Laminar Composites Panels of materials are combined to make a stronger material Panels are arranged in different orientations to maximize strength overall Examples: plywood, skis

Structural Sandwich Sandwich Panels – consists of two outer sheets, separated by an inner core of less dense material Outer sheets are stronger material Bear most of in-plane loading Core material separates outer sheets and provides shear rigidity Applications: building materials, aerospace

Limitations of Composites High cost of fabrication Mechanical characterization of structure is complicated Repair of composites is not simple due to complicated structures Flaws/cracks can go undetected

Applications Aerospace Airbus 380 superjet is made of 20% composites Boeing 787 has 50% composite materials

Why composites? Offer unique properties unlike individual materials Often lightweight, but strong materials Offer flexibility in design of materials

Supplementary Slides

Websites http://www.tms.org/pubs/journals/JOM/9602/Scala-9602.html History of composites http://matse1.matse.illinois.edu/comp/comp.html Composites videos http://users.ox.ac.uk/~smit0008/Publications_files/ORI-Aviation-Materials-2009.pdf Article on materials in aerospace