Lesson 9: Types and Applications of Composites
Overview
A host of high-technology applications require materials that have specific and unusual properties that cannot be met by any of the monolithic conventional metals, ceramics, and polymers. Some of these requirements have been met through the judicious combination of two or more distinct materials into composite materials that possess materials properties better than those found in the monolithic classes of materials. In this lesson, we will organize the composites into four main classifications and explore the strengths, as well as many of the current applications of these materials.
Learning Objectives
By the end of this lesson, you should be able to:
- Define and contrast the use, cost, and ease of fabrication of polymer-, ceramic-, and metal-matrix composites.
- List and define the four main classifications of composite materials.
- Note the three common fiber reinforcements used in polymer-matrix composites and, for each, cite both desirable characteristics and limitations.
- Cite the desirable features of metal-matrix composites.
- Note the primary reason for the creation of ceramic-matrix composites.
- Name and briefly describe the two sub-classifications of structural composites.
Lesson Roadmap
Lesson 9 will take us one week to complete. Please refer to Canvas for specific due dates.
| To Read | Pages 247 to 283 (Chapter 13) of Materials for Today's World, Custom Edition for Penn State University (custom e-book) |
|---|---|
| To Watch | Monuments to Man: The Impact and Influence of Concrete on Civilization |
| To Do | Lesson 9 Quiz |
Questions?
If you have general questions about the course content or structure, please post them to the General Questions and Discussion forum in Canvas. If your question is of a more personal nature, feel free to send a message to the course instructor through Canvas email. The instructor will check daily to respond.
Introduction to Composites
Although humans have used composite materials for millennia, the concept of composites as a distinct classification of materials was not recognized until the mid-20th century. Composite materials are formed from two or more distinct phases of materials. This is in contrast with metal alloys, which we studied in an earlier lesson. In metal alloys, additional atoms, compounds, or phases are dissolved into the base metal. This solid mixing does not result in distinct phases, which are present in composite materials. Possibly the earliest usage of a composite was by the ancient Mesopotamians (circa 3400 BCE) who realized that gluing wood at angles produced better properties than single-ply wood. Modern five-ply plywood has five plies arranged in steps of 45° (0, 45, 90, 135, and 180 degrees) for better strength. A photo of an unknown type of plywood is shown below.
Around 1500 BCE in the Fertile Crescent, humans began adding straw to strengthen clay bricks. Human structures were no longer limited to wood or the piling of stone. Unreinforced clay bricks, like most ceramics, are strong under compression stress, but unstable when subject to tensile stresses. So, unreinforced clay bricks carry the load but will readily fall apart. Except for its unstable nature under tensile stresses, clay is otherwise an ideal building material. As a raw material, it is available almost everywhere and, before drying, it can be easily worked into the desired shape. Strengthening clay through the addition of straw, gravel, or bitumen greatly enhances its applicability as a building material. Before moving to the next section, please watch this brief introductory video (2:07) on composites.
To Watch
Hello, YouTube! Welcome to the Composite HUB. What exactly is a composite material? Let's find out.
A composite is made by combining two or more different types of materials together. Sounds very simple right? Well, not really. There is actually a small catch to this. Materials that make up a composite do not dissolve in each other. They remain separate. They are just locked together. Most composites are made out of a matrix, which is a soft material. And a reinforcement, which is stiff and strong. Reinforcement usually comes in the form of particles or fibers. And it is completely surrounded by the matrix. Once combined, they give unique properties, which are superior to the individual materials on their own.
You might be surprised to hear that composite materials have been around for 1000s of years. Mesopotamians came up with the first man-made composite around 3400BC when they made plywood by sticking thin wood sticks at different angles. Around 1500 BC Egyptians used mud and straw to create strong buildings. Straw continued to provide reinforcement for ancient products such as pottery and boats. Later in the 1200s Mongols invented the first composite bow using a mixture of wood, bone, animal glue, and birch bark. Until the invention of gunpowder, this was considered one of the most powerful weapons on the earth.
The modern era of composites began in the 1900s when scientists invented plastics and glass fiber. Glass fiber, when combined with a plastic matrix, creates an incredibly strong and lightweight material. Unfortunately, many of the great advancements in composites were as a result of war, as alternative materials were required for lightweight applications in military aircraft.
In the 1970s the composite industry began to mature. Better plastic matrices and reinforcing fibers such as carbon fiber were developed. It has since been replacing metal as the new material of choice. Composite materials are now used in the aircraft industry, automobile industry, sports equipment, and many more.
Hope you enjoyed this video. If you want to know more about composites, subscribe to the Composite HUB.
Composite Terms and Classifications
Composite materials are materials which are a combination of two or more distinct individual materials. These combinations are formed to obtain a more desirable combination of properties. This is called the principle of combined action. One example of this principle is the use of composites for aircraft structures. These composites are designed to be lighter weight with comparable strength to metal structural elements that they are replacing. Typically, a composite is formed with a continuous phase called the matrix. As shown in the figure below, the matrix phase surrounds another phase which is discontinuous and referred to as the dispersed phase.
The purpose of the matrix phase is to keep the dispersive phase in place, transfer stress to the dispersed phase, and protect the dispersed phase from the environment. The purpose of the dispersed phase typically depends on which material type it is composed of:
- Metal dispersive phases are typically used to increase yield strength, tensile strength, and/or provide stability over the life of the product.
- Ceramic dispersive phases are typically used to produce materials which resist fracture.
- Polymer dispersive phases are typically used to increase the modulus of elasticity, yield strength, tensile strength, and/or provide stability over the life of the product.
Composites are typically classified by the type of dispersive phase used: particle reinforced, fiber reinforced, or structural. Further details on these different types of dispersive phase types will be forthcoming in the reading for this lesson, but first please watch this short four-minute video introducing composites. Note that in this video what we are calling the dispersive phase they refer to as the reinforcement phase.
To Watch
From huts of mud and grasses to monuments of stone and steel. The rise of modern civilization has been paced by our development of new materials. We began with Earth, wood, and rocks. We built shelters and tools. We harnessed fire and learned to coax metal from stone. And then one day a brickmaker added straw to his clay the result was a stronger brick and the birth of manmade composites.
So what is a composite, really? Simply it's two different materials combined. The uniform substance like portland cement is called a monolithic material. Throw in a handful of gravel another monolithic material and you have concrete. A composite. In a composite you can still see the individual monolithic materials the cement in the gravel they're just locked together.
So why make composites? We combine two similar materials to create a new material that has the characteristics we need for a particular application. Portland cement is pretty tough, but you wouldn't build a bridge out of it. It's not strong enough or durable enough. Throw in some gravel and now it's durable enough for traffic, but still not strong enough to span supports. Drop in a nice grid of steel rebar and now you've got a composite material that's strong enough for bridge decking. In addition to increased strength and durability, composites also allow us to customize materials with the weight flexibility conductivity and stability we need. Although composites can have several different components they all have two things in common a matrix and reinforcement.
In our concrete bridge, the cement is the matrix and the gravel and rebar are both reinforcements. Many modern composites use resins as a matrix. Add wood or wood fibers and you have a broad family of products from plywood and particle board to high-density fiberboard and composite decking panels. Adding glass fiber or fabric as reinforcement creates fiberglass widely used everything from auto body parts and both holes to tennis rackets and swimming pool liners. Many of the most recent advances in composites have been in the field of aerospace where highly specialized fibers such as graphite or on and Kevlar are used to create incredibly strong yet amazingly lightweight materials.
Composites, they're everywhere. Building materials, furniture, toys, sports, equipment, the games we play, the roads we walk, the cars we drive, the planes we fly. Composites - they make so much of what we do every day possible.
Now that you have watched this video, please proceed to the next section.
Reading Assignment
Things to consider...
When you're reading the text for this lesson, use the following questions to guide your learning. Remember to keep the learning objectives listed on the overview page for this lesson in mind as you learn from this text.
- What are the four main classifications of composite materials?
- What are the common fiber reinforcements used in polymer-matrix composites, and their desirable characteristics and limitations?
- What are metal-, ceramic-, and polymer-matrix composites?
- What are the desirable features of metal-, ceramic-, and polymer-matrix composites, in terms of use, cost and ease of fabrication?
- What is the primary reason for the creation of ceramic-matrix composites?
- What are laminates and sandwich panels, and what are their typical uses?
Reading Assignment
Pages 247 to 283 (Chapter 13) of Materials for Today's World, Custom Edition for Penn State University. (custom e-book)
Video Assignment: Monuments to Man: The Impact and Influence of Concrete on Civilization
Now that you have read the text and thought about the questions I posed, go to Lesson 9 of Canvas and watch this 45-minute video about the most effective of all building materials, composite concrete, and how humankind has discovered, developed, and utilized it throughout history. In "Monuments to Man: The Impact and Influence of Concrete on Civilization," we see how concrete creates our modern cities and how it affects how humankind works and lives in these concrete jungles.
Video Assignment
Go to Lesson 9 in Canvas and watch the Monuments to Man: The Impact and Influence of Concrete on Civilization Video. You will be quizzed on the content of this video.
Summary and Final Tasks
Summary
Composite materials give us the opportunity to combine two (or more) materials to gain the best of both materials. Many composite materials are composed of a dispersed phase which is embedded into a second phase called the matrix. The matrix completely surrounds the dispersed phase and holds them together. Most composites in use today have been created to have improved stiffness, toughness, and ambient and high-temperature strength. In this lesson, a simple scheme for the classification of composite materials which consists of four main divisions: particle-reinforced, fiber-reinforced, structural, and nanocomposites, was presented and defined. Particle reinforced composites have a dispersed phase which consists of particles whose dimensions are approximately the same in all directions. Fiber-reinforced composites have large length-to-diameter ratio particles (fibers) as the dispersive phase. Structural composites are multi-layered and designed to have low densities and high degrees of structural integrity. For nanocomposites, the dimensions of the dispersed phase particles are on the order of nanometers.
Reminder - Complete all of the Lesson 9 tasks!
You have reached the end of Lesson 9! Double-check the to-do list on the Overview page to make sure you have completed all of the activities listed there before you begin Lesson 10.