Structural Composites

Summary

A composite material is formed when two or more separate materials are combined to make a new material that has its own characteristic properties. Structural composites are used to fabricate structures and other objects for which that material's properties are advantageous. Examples of structural composites range from simple or low-tech materials such as adobe brick, plywood, and reinforced concrete to more developed or high-tech materials such as the advanced composite materials used in modern aircraft and spacecraft. The range and variety of structural composites and their applications, even within a single type, are nearly limitless, with each new material resulting in the development of even newer materials and applications.

Definition and Basic Principles

Structural composites, or composite materials, are specialized materials made by combining two or more simpler materials. In the resulting composite material, the properties of the component materials combine to produce new and unique properties. In composite materials, the whole is truly greater than the sum of the parts. The vast majority of structural composite materials can be described as being made from two types of materialsa matrix or binding material and a structural reinforcement material. The matrix material serves to contain and bind together the reinforcement material and to prevent it from bending, deforming, or breaking under load. The reinforcement material provides the load-bearing strength and other properties that are the foundation of the composite material's intrinsic value.

Structural composites are designed to respond to forces only in the direction of the forces placed on them. They may be constructed to have strength on one axis or in one direction only, or to have strength in several directions. A structural composite such as concrete exhibits the same strength in all directions and is described as being isotropic. Structural composites such as fiberglass-reinforced polymer (FRP) sheets have varying strengths in different directions and are described as anisotropic. If the layers of reinforcement material in a laminated structural composite are designed to complement each other, the resulting material can be made to exhibit essentially equal strengths in all directions. This condition is called quasi-isotropic and is an extremely important consideration in designing and constructing structural composites. The essential differences between anisotropic, isotropic, and quasi-isotropic composite materials are best appreciated by examining the properties of some exemplary structural composites.

Basic concrete consists of a mixture of sand, gravel, Portland cement, and water. Individually, these materials have practically no structural value, but when mixed together, they form the composite material known as concrete. A chemical reaction between the water and the anhydrous silicate compounds of the Portland cement results in the formation of mass quantities of hydrated silicate crystals. These crystal blooms grow into and mesh with one another to bind the sand and gravel components into a solid mass of stonelike consistency. Because its components have no directionality of their own and are uniformly blended, concrete displays identical properties in all directions and is thus isotropic.

In a fiberglass-reinforced polymer structure, fibers of glass, carbon, or other materials are employed as the reinforcement material. Each fiber has compressive or tensile strength only in the direction of its long axis. Lateral pressure can break and destroy the fibers. Stacking numerous fibers together and enveloping them in a polymer resin matrix material can greatly increase the force required to break them, but their strength nevertheless remains primarily along their main axes. The resulting structure thus displays varying strength properties in different directions, making it anisotropic.

If the reinforcement fibers are woven together like cloth, a significant strength component is added orthogonally (at right angles) to the length of the structure as the fiber weaves back and forth across the material. Arranging successive layers of woven fibers—so that their main axes point in different directions—results in the formation of a structure whose major strengths are almost uniformly distributed in many different directions. Because this is only an apparent distribution and not an actual distribution of behavior, the material is called quasi-isotropic.

Background and History

Structural composites and the principles of their construction have been known and used for literally thousands of years. Adobe brick, concrete, and laminated wood are the earliest known examples. The Assyrians and Babylonians are known to have used a type of concrete based on clay. A more robust type of concrete made from heated powdered clam and oyster shells that used lime (calcium oxide) as the binding agent was developed in ancient Egypt. Laminated structures such as wood pieces that have been mechanically fastened together or bonded together with an adhesive material of some kind have a similarly long history.

The development and growth of chemical knowledge, especially since the Industrial Revolution of the eighteenth and nineteenth centuries, brought about tremendous growth in the variety and quality of composite materials that could be made. Portland cement was developed (or perhaps redeveloped) by Joseph Aspdin in 1824, and it has been the cornerstone of the structural composites industry ever since. All other types of concrete used in the twenty-first century are derived from the basic Portland cement variety.

Modern chemistry and materials, especially since the development of polymers, have resulted in a veritable explosion of the structural composites field that continues unabated. The vast majority of composite materials in use or being developed are based on polymeric matrices such as epoxies and resins and on mineral or metal fibers such as glass, aluminum, steel, basalt, graphite, and boron. The use of low-density, high-strength metals as the matrix material is also being explored. One example of this approach is the structural composite known as glass-reinforced fiber metal laminate (GLARE), a composite of glass fibers in a matrix of aluminum metal that is used in the construction of the Airbus A-380 aircraft.

The field of structural composites, or composite materials, continues to grow, especially as materials developed for one purpose find other uses, usually in applications in which high strength and low weight, as well as unique shapes and formability, are desirable properties.

How It Works

The basic principle behind structural composites is that of additive strength. A composite material is made by combining a material with a particular set of properties with another material with a different set of properties. Ideally, the properties of the composite material surpass or greatly differ from those of the individual materials used to create it. The principle is the same regardless of whether it is applied at the atomic or very large scale. The results and usefulness of the particular application are generally seen in the bulk properties of the composite material.

The additive strength principle can be demonstrated using sticks. First, take a single stick that has the thickness of a pencil and bend and break it. Then, tie several pencil-size sticks together and attempt to bend and break them as a single unit. Logically, it should require only a force equal to that required to break one stick, multiplied by the number of sticks. In practice, however, a great deal more force is required. This is because the individual units support one another against an applied force. A single unit is easily deformed to the point of failure, but the addition of units makes it much more difficult to achieve the amount of deformation needed to break any of them. This classic demonstration shows that the strength of composite structures is enhanced beyond the sum of the parts.

Individual long structures such as fibers, sticks, pipes, and rods are anisotropic, meaning they have varying strengths in different directions because of their three-dimensional shapes, even though the actual materials from which they are made may be completely isotropic. Iron atoms are isotropic, and a steel sphere, for example, is equally isotropic. If a steel ball is manipulated into a long, thin rod, the shape confers vastly different properties from those that the steel had as a sphere. Laterally, the rod will have an equally high compressive strength as the sphere because of its circular cross-sectional shape. Longitudinally, however, the rod will not bear compressive loads as well and will easily undergo lateral deformation such as bending. Theoretically, because of the isotropic nature of the iron atoms in the steel rod, it must be equally resistant to compression as it is to tension, and if prevented from deforming or bending, the rod should bear a tremendous load. This is the function of the matrix material in a structural composite. Matrix materials are invariably isotropic in nature. They must bind the other materials equally well in all directions. A matrix material is also required to be chemically and physically inert in regard to the nonmatrix materials that it contains.

The principle of additive strength can be applied equally, regardless of the type of composite material. Composite materials can be broadly classified as laminates, reinforced plastics, cermets, fabrics, and filled composites. Each class of composite materials has its own particular strengths and weaknesses and its own methods of production, although there is some crossover in the areas of application.

Laminates. Laminates include plywood, corrugated cardboard, and similar products. In these structural composites, an adhesive binder is used to unite layers of material. In plywood, sheets of wood veneer—oriented at right angles to each other according to the direction of the wood grain—are alternately stacked with an interposing layer of a strong adhesive, and the assembly is compressed by high-pressure rollers. The alternating orientation of the wood fibers gives a product many of the qualities of a regular board cut in the traditional manner but provides much higher resistance to inherent problems such as splitting or warping and enhances flexibility. The manufacture of plywood also allows a much higher percentage of a harvested log to be used instead of being turned into sawdust.

Corrugated cardboard is constructed in a similar manner, using sheets of heavy manila paper. Typically, a sheet of the same material is formed into a corrugated pattern instead of a flat sheet. The corrugated sheet is then glued and sandwiched between the two flat sheets. The corrugations have strength properties that mimic those of an array of separate tubes, and they impart that strength to the composite structure. Additional layers of corrugated and plain sheet material can be applied either in parallel or orthogonal orientations to produce larger cardboard structures with strength rivaling that of wooden boards. In addition, larger laminated structures of alternating layers of wood or other anisotropic material and adhesive are commonly used. The most apparent of these is the laminated beam composed of strips of wood and adhesive. Such beams provide a strength and versatility that cannot be achieved by traditional one-piece beams.

Reinforced Plastics. Reinforced plastics represent the most technologically advanced field of structural composites. They also have the broadest and most diverse range of applications. Fiber-reinforced plastics (FRPs) are divided into normal and advanced composites. The methods of working with each are very similar in many respects. The designation relates to the nature of the matrix and reinforcement materials and the tolerances required of the finished products. To create a fiber-reinforced plastic, an array of selected fiber reinforcement material is impregnated with a polymer resin and formed into a desired shape. The mass is then compacted to consolidate the fibers together as tightly as possible, and the matrix material polymerizes to form a solid mass about the reinforcement fibers.

An extensive variety of matrix materials, such as polymer resins, are available for use with a similarly broad assortment of reinforcement materials. The fiber materials come in many forms, from single strands to thick woven sheets. Fiber materials typically include various grades of glass; natural fibers like silk and hemp; metals such as steel, tungsten, aluminum, and boron; mineral fibers such as basalt; high-strength polymers such as Kevlar and other polyamides; and carbon fibers that provide a versatile combination of high strength and low weight.

Cermets. Cermets are ceramic and metal powders consolidated with heat and pressure into specific shapes. They are then processed into a final form. The resulting forms can have machinability like ordinary metal parts but with reduced mass and improved wear resistance or superior hardness and strength properties for machining tools.

Fabrics. Fabrics qualify as structural composites when they are produced as a combination of different fiber materials, such as cotton and polyester. The particular value of fabrics as structural composites is in their ability to drape to a given shape or to provide a protective covering with specific properties. The apparent simplicity of fabric structure belies the amount and complexity of research undertaken in the fabric industry, primarily for the development of new materials for fabric use and applications for those fabrics.

Filled Composites. Filled composites are the single largest area of the structural composites field, primarily because of the volume of concrete and highway paving blends that are used around the world. A filled composite consists of a mass of fine particulate inert matter blended into a matrix material. Filled composites include concrete, asphalt, and various materials such as linoleum sheets and floor tiles, particle board, and composite flooring.

Applications and Products

On the Atomic Scale. The most fundamental of structural composites are metal alloys. These materials do not appear to have the material interfaces that normally characterize structural composites. One could argue that these interfaces are provided by the atomic boundaries themselves, but the point is academic. The important feature is that the principle of additive strength begins at the atomic and molecular level and carries over to the bulk properties of the composite material. In alloys, the combination of two or more distinctly different materials at the atomic level produces a new material with distinctive properties of its own.

Although alloys do not typify the concept of structural composites or composite materials, they do demonstrate that the principle of their functionality applies on both very small and very large scales. The application of those principles on that small scale is increasingly relevant in the development of new and useful materials and methods and is intimately bound to the manipulation of atoms and molecules to produce specific atomic and molecular structures. Two examples illustrate the formation of structural composites from the atomic scaleboron fiber and the combination of materials to produce specific electronic properties or physical structures.

Boron fiber was first produced for commercial testing and use in 1964. The process by which it is produced makes boron fiber a structural composite in its own right. To make boron fiber, a very fine filament of the element tungsten is passed slowly through a sealed reaction chamber that contains an atmosphere of vaporized boron. Boron atoms deposit on the tungsten filament and build up in metallic crystal form to produce an enhanced fiber of desired dimensions. The filament then passes out of the reaction chamber to be wound on a spool. The boron fibers are very stiff compared to other fibers typically used in structural composites, and this property lends itself to purposes for which other fibers are neither adequate nor suitable. For example, boron fiber composites are extremely well suited to applications calling for extreme stiffness or rigidity coupled to very low weight, making it a major application in aircraft and spacecraft structural technology.

A more important example of atomic-scale structural composites is the combination of materials to produce specific electronic properties or physical structures, which has increasing significance in microtechnology and nanotechnology as well as in electronics. For example, silicon and germanium are combined to construct silicon-germanium (Si-Ge) diodes, transistors, and other semiconductor devices that are the basis of modern electronics. Modern integrated circuits and computer processing chips can contain hundreds of thousands of such miniature structures on a single unit. With the development of devices such as the scanning tunneling microscope (STM) and the atomic force microscope (AFM), specific composite structures can be constructed on an atomic scale, literally atom by atom. This remains a developing technology, the value of which increases as the field of nanotechnology evolves.

Structural vs. Composite Structures. The similar terms structural composites and composite structures have very different meanings. A composite structure can be defined as any construct of more than one material in which the function of each material remains separate from the functions of the other materials. A structural composite, on the other hand, is a combination of different materials. The separate materials combine in their functionality so that the structural composite behaves as though it were just a single material. It is this unity of functionality that makes structural composites so useful and valuable.

Building and Construction Materials. In terms of volume usage, filled composites comprise the single largest composite material category. Filled composites consist of a mixture of small particles in a solidified matrix. The matrix material or binder can be any mobile material into which a quantity of particulate matter can be blended and which becomes solid—or at least highly resistant to structural change—after emplacement. Examples of filled composite building materials include linoleum in sheet and tile form, roofing shingles, composite flooring (simulated hardwoods), particle board, medium-density fiberboard (MDF), Aspenite (wafer board), synthetic wood (typically, a blend of recovered thermoplastics and sawdust extruded into standard lumber shapes), sealing putties, concretes, plasters, patching compounds, paints, and paving mixtures. Other composite building materials include plywood, laminated beams, reinforced glass and mirrors, fiber-reinforced plastics, structural components, and paper or fabric surface covering materials. Of these examples, some have a limited number of forms or compositions, while others can be produced in a limitless variety.

Fiber-Reinforced Plastics. The term structural composites generally refers to materials constructed from polymeric resins and fiber reinforcements, called fiber-reinforced plastics (FRP). One significant advantage of FRP structures is that the methodology can be employed for producing a single item as well as multiple items. The product and application are often the same thing. The second major advantage of FRP structures is the complexity of the shape that can be produced in one step, with strengths and properties relative to weight that cannot be matched by other structural technologies. FRP structural composites are thus widely used in the broadest variety of applications, from the simplest fiberglass patching compounds to the most advanced structural components of existing aircraft and spacecraft.

Common uses of FRP structures include simple fabrications such as tub and shower enclosures, spas and hot tubs, automobile body and marine patching compounds, customized and regular replacement automobile body components, and low-tolerance or low-performance shrouds and housings. Production methods for such structural composites in low-tech applications are relatively simple. They do not call for critical placement of reinforcing materials or extensive structure consolidation before the curing process. Similar but far more demanding methods are used to produce advanced FRP structures that must meet close tolerances and high-performance specifications.

Molds and forms used for advanced FRP fabrications are constructed from specialized materials that closely match the thermal responses of the materials being molded and are typically custom-machined with exacting precision. Highly specialized thermosetting resins and advanced reinforcement materials such as carbon, aramid, boron, and titanium fibers are used in particular patterns. The reinforcing materials are pre-impregnated with the matrix resin on site (alternatively, commercially available prepregs are used) and laid into the mold or form by hand. The resulting stack is then consolidated or compacted under reduced pressure and heated to cure the matrix resin's solid state. This stage of the process often requires an autoclave to provide a controlled sequence of temperature and pressure changes for the consolidation and curing of the structure.

Advanced composites are used primarily in the aerospace industries. Still, advanced fiber-reinforced plastics are increasingly being applied in other areas because of their versatility, low cost, ease of preparation and repair, and the strength and durability of FRP structures. For example, FRP methods and materials have been used in the construction of sections of multilane highways that pass through problematic terrain and bridges for both pedestrian and vehicular traffic. The inherent strength of reinforcement materials such as carbon fiber has also been applied to repairing traditional structures such as masonry walls, concrete pillars, and wooden beams to extend their working lives and, in many cases, to eliminate the need for replacing those structures. Since the development of waterproof polymers and fiberglass combinations, simple FRP construction has also been used to manufacture boats. Advanced FRP composites and methods are being applied to the construction and modification of watercraft on an increasing scale, from the simplest computer designs to the most advanced military warships.

Cermets. Cermets are composite materials made from ceramic and metal powders. The material combination is formed and pressed into the desired shape and consolidated by sintering (making objects from powder). It is then processed to produce the finished product. Like FRP composites, cermets are also amenable to use in producing a single unit or multiple components. Cermets are used in two ways. In one mode of use, the final product is intended to be primarily ceramic, and the metal component is either chemically altered or removed after forming, leaving the ceramic component. This method is used mainly for the production of reduced-weight components with high wear resistance and strength properties. In the other mode of use, the presence of the metal is required to maintain and support the ceramic component. This method is used primarily to produce high-strength, high-durability metal-cutting tools. The specific properties of the cermet material, especially variations that include carbonitride ceramic materials, offer enhanced working lives and performance characteristics at a lower cost than specialty metals alloys.

Fabrics. Structural composites in fabrics are used in an entirely different manner from other structural composites because fabrics are typically used on their strengths and merit rather than in combination with a binding or matrix material. Fabric composites may employ combinations of very specialized materials, especially if they are to be used in FRP applications. Most fabric structural composites, such as rayon-cotton or cotton-polyester, are used in the garment and furniture industries for clothing and protective enclosures and coverings. Specialized fabrics combining such materials as carbon and Kevlar fibers are used in FRP structural composites.

Structural Composites for Energy Storage. With the development of technologies such as mobile electronics and electric cars that demand increasing reserves of energy storage, those in the field have identified the use of structural composites for this purpose as a promising area for growth. Using multifunctional structural composites could not only increase energy storage but also eliminate the need for a battery separate from the structural elements, possibly making devices lighter and more portable. In particular, supercapacitors made with polymers, carbon fibers, and carbon nanotube fibers are widely studied. Batteries, dielectric capacitors, and fuel cells have also been investigated.

Careers and Course Work

Because structural composites is a discipline based on engineering and materials, those who undertake a career in the field will be required to acquire mathematical skills and a basic knowledge of physics as a minimum standard body of technical knowledge. This will allow the individual to perform basic work with structural composite materials. Specific techniques and methods can be learned through subsequent hands-on training provided on the job by experienced and qualified practitioners. Completion of a secondary school technical program that includes a mathematics component of geometry and trigonometry, as well as physical sciences, is the necessary foundation for work in this field.

Individuals who wish to pursue more advanced practical training at an applied level should undertake a postsecondary program of study. Practical fields related to structural composites include the construction and machinist trades, civil engineering technology, aircraft maintenance, engineering technology, and chemical engineering technology. These areas of study will involve design and practical applications of filled composites, fiber-reinforced plastics, laminates, fabrics, and the use of cermets in machining operations.

To pursue research and development of advanced applications and materials, undergraduate and graduate training in a university or college program is typically required. The exact nature of the program will depend on the type of materials and design applications being pursued. At a minimum, one should expect to undertake training in organic and polymer chemistry, inorganic chemistry, analytical chemistry, physical chemistry, industrial chemistry, chemical engineering, mechanical engineering, strengths of materials, physics, and applied mathematics. Depending on the individual's particular area of specialization, a selection from these courses of study will provide the graduate with an understanding of the theoretical principles necessary for the design and development of new materials and the applications of structural composites.

Social Context and Future Prospects

The broad applicability of structural composites in society and the equally broad applications that already exist make it very difficult to predict any specific future developments in the field. An ongoing need certainly exists for the maintenance and repair of the present composite infrastructure. Concrete structures age and require maintenance or replacement. Similarly, FRP materials break down over time through environmental action, eventually requiring repair or replacement. In addition, each area in which structural composites are used continues to produce new products the increasing population requires. At the same time, new materials and methods are continually being developed and applied, and as technology advances, computer-aided design, manufacturing, and engineering are increasingly used to improve the quality and quantity of production.

The value and importance of structural composites will increase with the growth of nanotechnology and its applications. This will be particularly true for the design and production of nanostructures and microstructures that carry out very specific tasks. An example is the work being carried out in producing the so-called laboratory on a chip, a device designed to perform very specific analytical tests on numerous microscopic samples. Test protocols are used for atmospheric, aquatic, and biological sample analysis to detect the presence of specific compounds, environmental contaminants, and pathogens. Future developments in this area are expected to provide hundreds of analyses from nearly microscopic amounts of sample materials and emplaced reagents. This will call for the development and application of very specific materials and physical structures that will be best provided by structural composites.

As the interest in space exploration continues, the applicability of present materials must inevitably be examined. The usefulness of carbon-fiber structures in space has been well proven by applying that technology in the National Aeronautics and Space Administration's space shuttle program with the Canadarm remote manipulators. The suitability of other materials and methods has yet to be determined, and questions in this area of application are many. These questions include, for example, the feasibility of preparing concrete on the surface of the Moon or Mars using indigenous materials and the properties of this concrete compared with terrestrial concrete. Additionally, scientists are questioning whether FRP production processes can occur in an orbiting facility, with orders-of-magnitude lower pressures than can be readily produced on Earth. The answers to these questions, and others like them, will significantly affect the global economy as they are applied to nonspace applications, in much the same way that developments from the aerospace industry have historically contributed to the private sector and the well-being of society.

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