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.
By the end of this lesson, you should be able to:
Lesson 9 will take us one week to complete. Please refer to Canvas for specific due dates.
To Read | Read pp 246-282 (Ch. 13) in Introduction to Materials ebook |
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To Watch | Monuments to Man: The Impact and Influence of Concrete on Civilization |
To Do | Lesson 9 Quiz |
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.
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.
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:
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.
Now that you have watched this video, please proceed to the next section.
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.
Read pp 246-282 (Ch. 13) in Introduction to Materials ebook
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.
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.
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.
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.