Advanced composite materials (science & engineering)
Encyclopedia
Advanced composite materials (ACMs) are also known as Advanced polymer matrix composites. These are generally characterized or determined by unusually high strength fibres with unusually high stiffness
, or modulus of elasticity characteristics, compared to other material
s, while bound together by weaker matrices. These are termed advanced composite materials (ACM) in comparison to the composite material
s commonly in use such as reinforced concrete
, or even concrete itself. The high strength fibers
are also low density
while occupying a large fraction of the volume
.
Advanced composites exhibit desirable physical and chemical properties that include light weight coupled with high stiffness (elasticity
), and strength along the direction of the reinforcing fiber, dimensional stability, temperature and chemical resistance, flex performance
, and relatively easy processing. Advanced composites are replacing metal components in many uses, particularly in the aerospace industry.
Composites are classified according to their matrix phase. There are
Materials within these categories are often called "advanced" if they combine the properties of high (axial, longitudinal
) strength values and high (axial
, longitudinal) stiffness values, with low weight, corrosion resistance, and in some cases special electrical properties.
Advanced composite materials have broad, proven applications, in the aircraft
, aerospace, and sports equipment
sectors. Even more specifically ACMs are very attractive for aircraft and aerospace structural parts. ACMs have been developing for NASA's Advanced Space Transportation Program, armor protection for Army aviation
and the Federal Aviation Administration
of the U.S.A., and high-temperature shafting for the Comanche helicopter. Additionally, ACMs have a decades long history in military and government aerospace industries. However, much of the technology is new and not presented formally in secondary or undergraduate education
, and the technology of advanced composites manufacture is continually evolving.
s to components of the space shuttle
. The industry can be generally divided into two basic segments, industrial composites and advanced composites. Several of the composites manufacturing processes are common to both segments. The two basic segments are described below.
systems including polyester
, epoxy
, and other specialty resins. These materials, along with a catalyst or curing agent and some type of fiber reinforcement (typically glass fibers) are used in the production of a wide spectrum of industrial components and consumer goods: boats, piping, auto bodies, and a variety of other parts and components.
and commercial aircraft of all types, is the major customer for advanced composites. These materials have also been adopted for use by the sporting goods suppliers who sell high-performance equipment
to the golf
, tennis
, fishing
, and archery
markets.
While aerospace is the predominant market for advanced composites today, the industrial and automotive market
s will increasingly see the use of advanced composites toward the year 2000. At present, both manual and automated processes are employed in making advanced-composite parts. As automated processes become more predominant, the costs of advanced composites are expected to decline to the point at which these materials will be used widely in electronic, machinery, and surface transportation equipment.
Suppliers of advanced composite materials tend to be larger companies capable of doing the research and development
necessary to provide the high-performance resin systems used in this segment of the industry. End-users also tend to be large, and many are in the aircraft and aerospace businesses.
s, phenolic and amino resins, bismaleimides (BMI, polyimides), polyamide
s.
Of these, epoxies are the most commonly used in the industry. Epoxy resins have been in use in U.S. industry for over 40 years. Epoxy compounds are also referred to as glycidyl compounds. The epoxy molecule can also be expanded or cross-linked with other molecules to form a wide variety of resin products, each with distinct performance characteristics. These resins range from low-viscosity liquids to high-molecular weight solids. Typically they are high-viscosity liquids.
The second of the essential ingredients of an advanced composite system is the curing agent or hardener. These compounds are very important because they control the reaction rate and determine the performance characteristics of the finished part. Since these compounds act as catalysts for the reaction, they must contain active sites on their molecules. Some of the most commonly used curing agents in the advanced composite industry are the aromatic amines. Two of the most common are methylene-dianiline (MDA) and sulfonyldianiline (DDS).
Several other types of curing agents are also used in the advanced composite industry. These include aliphatic and cycloaliphatic amines, polyaminoamides, amides, and anhydrides. Again, the choice of curing agent depends on the cure and performance characteristics desired for the finished part. Polyurethanes are another group of resins used in advanced composite processes. These compounds are formed by reacting the polyol
component with an isocyanate compound, typically toluene diisocyanate
(TDI); methylene diisocyanate (MDI) and hexamethylene diisocyanate
(HDI) are also widely used. Phenolic and amino resins are another group of PMC resins. The bismaleimides and polyamides are relative newcomers to the advanced composite industry and have not been studied to the extent of the other resins.
s currently represent a relatively small part of the ACM industry. They are typically supplied as nonreactive solids (no chemical reaction
occurs during processing) and require only heat and pressure
to form the finished part. Unlike the thermosets, the thermoplastics can usually be reheated and reformed into another shape, if desired.
The three basic types of fiber reinforcement materials in use in the advanced composite industry are
Fibers used in advanced composite manufacture come in various forms, including yarns, rovings, chopped strands, and woven fabric
mats. Each of these has its own special application. When prepreg materials are used in parts manufacture, woven fabric or mats are required. In processes such as filament wet winding or pultrusion, yarns and rovings are used.
Aluminum is a very tolerant material and can take a great deal of punishment before it fails. It can be dented or punctured and still hold together. Composites are not like this. If they are damaged, they require immediate repair, which is difficult and expensive. An airplane made entirely from aluminum can be repaired almost anywhere. This is not the case for composite materials, particularly as they use different and more exotic materials. Because of this, composites will probably always be used more in military aircraft, which are constantly being maintained, than in commercial aircraft, which have to require less maintenance.
Aluminum still remains a remarkably useful material for aircraft structures and metallurgists have worked hard to develop better aluminum alloys (a mixture of aluminum and other materials). In particular, aluminum-lithium is the most successful of these alloys. It is approximately ten percent lighter than standard aluminum. Beginning in the later 1990s it was used for the Space Shuttle's large External Tank in order to reduce weight and enable the shuttle to carry more payload. Its adoption by commercial aircraft manufacturers has been slower, however, due to the expense of lithium and the greater difficulty of using aluminum-lithium (in particular, it requires much care during welding). But it is likely that aluminum-lithium will eventually become a widely used material for both commercial and military aircraft.
Stiffness
Stiffness is the resistance of an elastic body to deformation by an applied force along a given degree of freedom when a set of loading points and boundary conditions are prescribed on the elastic body.-Calculations:...
, or modulus of elasticity characteristics, compared to other material
Material
Material is anything made of matter, constituted of one or more substances. Wood, cement, hydrogen, air and water are all examples of materials. Sometimes the term "material" is used more narrowly to refer to substances or components with certain physical properties that are used as inputs to...
s, while bound together by weaker matrices. These are termed advanced composite materials (ACM) in comparison to the composite material
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
s commonly in use such as reinforced concrete
Reinforced concrete
Reinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...
, or even concrete itself. The high strength fibers
Composite laminates
In materials science, Composite laminates are assemblies of layers of fibrous composite materials which can be joined to provide required engineering properties, including in-plane stiffness, bending stiffness, strength, and coefficient of thermal expansion....
are also low density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
while occupying a large fraction of the volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
.
Advanced composites exhibit desirable physical and chemical properties that include light weight coupled with high stiffness (elasticity
Elasticity (physics)
In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....
), and strength along the direction of the reinforcing fiber, dimensional stability, temperature and chemical resistance, flex performance
Flexible response
Flexible response was a defense strategy implemented by John F. Kennedy in 1961 to address the Kennedy administration's skepticism of Dwight Eisenhower's New Look and its policy of Massive Retaliation...
, and relatively easy processing. Advanced composites are replacing metal components in many uses, particularly in the aerospace industry.
Composites are classified according to their matrix phase. There are
- polymer matrix composites (PMC's),
- ceramic matrix compositeCeramic Matrix CompositeCeramic matrix composites are a subgroup of composite materials as well as a subgroup of technical ceramics. They consist of ceramic fibers embedded in a ceramic matrix, thus forming a ceramic fiber reinforced ceramic material...
s (CMC's), and - metal matrix compositeMetal matrix compositeA metal matrix composite is composite material with at least two constituent parts, one being a metal. The other material may be a different metal or another material, such as a ceramic or organic compound. When at least three materials are present, it is called a hybrid composite...
s (MMC's).
Materials within these categories are often called "advanced" if they combine the properties of high (axial, longitudinal
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
) strength values and high (axial
Geometry
Geometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of pre-modern mathematics, the other being the study of numbers ....
, longitudinal) stiffness values, with low weight, corrosion resistance, and in some cases special electrical properties.
Advanced composite materials have broad, proven applications, in the aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
, aerospace, and sports equipment
Sports equipment
Sports equipment is a general term for any object used for sport or exercise. Examples of sports equipment include:-Exercise equipment:Examples for exercise include swiss balls, weights, equipment for the gym...
sectors. Even more specifically ACMs are very attractive for aircraft and aerospace structural parts. ACMs have been developing for NASA's Advanced Space Transportation Program, armor protection for Army aviation
Army aviation
Army Aviation refers to aviation-related units of a nation's army, often described as an air corps. These units are generally separate from a nation's dedicated air force, and usually comprise helicopters and light support fixed-wing aircraft....
and the Federal Aviation Administration
Federal Aviation Administration
The Federal Aviation Administration is the national aviation authority of the United States. An agency of the United States Department of Transportation, it has authority to regulate and oversee all aspects of civil aviation in the U.S...
of the U.S.A., and high-temperature shafting for the Comanche helicopter. Additionally, ACMs have a decades long history in military and government aerospace industries. However, much of the technology is new and not presented formally in secondary or undergraduate education
Undergraduate education
Undergraduate education is an education level taken prior to gaining a first degree . Hence, in many subjects in many educational systems, undergraduate education is post-secondary education up to the level of a bachelor's degree, such as in the United States, where a university entry level is...
, and the technology of advanced composites manufacture is continually evolving.
Overview and historical perspective
Manufacturing ACMs is a multibillion dollar industry in the United States. Composite products range from skateboardSkateboard
A skateboard is typically a specially designed plywood board combined with a polyurethane coating used for making smoother slides and stronger durability, used primarily for the activity of skateboarding. The first skateboards to reach public notice came out of the surfing craze of the early 1960s,...
s to components of the space shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
. The industry can be generally divided into two basic segments, industrial composites and advanced composites. Several of the composites manufacturing processes are common to both segments. The two basic segments are described below.
Industrial composites
The industrial composites industry has been in place for over 40 years in the U.S. This large industry utilizes various resinResin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...
systems including polyester
Polyester
Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...
, epoxy
Epoxy
Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....
, and other specialty resins. These materials, along with a catalyst or curing agent and some type of fiber reinforcement (typically glass fibers) are used in the production of a wide spectrum of industrial components and consumer goods: boats, piping, auto bodies, and a variety of other parts and components.
Advanced composites
The Advanced polymer matrix composites industry, or Advanced composite materials industry, is characterized by the use of expensive, high-performance resin systems and high-strength, high-stiffness fiber reinforcement. The aerospace industry, including militaryMilitary aircraft
A military aircraft is any fixed-wing or rotary-wing aircraft that is operated by a legal or insurrectionary armed service of any type. Military aircraft can be either combat or non-combat:...
and commercial aircraft of all types, is the major customer for advanced composites. These materials have also been adopted for use by the sporting goods suppliers who sell high-performance equipment
High-performance equipment
High-performance equipment describes telecommunications equipment that has the performance characteristics required for use in trunks or links, is designed primarily for use in global and tactical systems, and sufficiently withstands electromagnetic interference when operating in a variety of...
to the golf
Golf
Golf is a precision club and ball sport, in which competing players use many types of clubs to hit balls into a series of holes on a golf course using the fewest number of strokes....
, tennis
Tennis
Tennis is a sport usually played between two players or between two teams of two players each . Each player uses a racket that is strung to strike a hollow rubber ball covered with felt over a net into the opponent's court. Tennis is an Olympic sport and is played at all levels of society at all...
, fishing
Fishing
Fishing is the activity of trying to catch wild fish. Fish are normally caught in the wild. Techniques for catching fish include hand gathering, spearing, netting, angling and trapping....
, and archery
Archery
Archery is the art, practice, or skill of propelling arrows with the use of a bow, from Latin arcus. Archery has historically been used for hunting and combat; in modern times, however, its main use is that of a recreational activity...
markets.
While aerospace is the predominant market for advanced composites today, the industrial and automotive market
Automotive market
The automotive market is formed by the demand and the industry. This article is about the general, major trends in the automotive market, mainly from the demand side.The European automotive market has always boasted smaller cars than the United States...
s will increasingly see the use of advanced composites toward the year 2000. At present, both manual and automated processes are employed in making advanced-composite parts. As automated processes become more predominant, the costs of advanced composites are expected to decline to the point at which these materials will be used widely in electronic, machinery, and surface transportation equipment.
Suppliers of advanced composite materials tend to be larger companies capable of doing the research and development
Research and development
The phrase research and development , according to the Organization for Economic Co-operation and Development, refers to "creative work undertaken on a systematic basis in order to increase the stock of knowledge, including knowledge of man, culture and society, and the use of this stock of...
necessary to provide the high-performance resin systems used in this segment of the industry. End-users also tend to be large, and many are in the aircraft and aerospace businesses.
Thermosets and thermoplastics
Advanced composite systems are divided into two basic types, thermosets and thermoplastics. Thermosets are by far the predominant type in use today. Thermosets are subdivided into several resin systems including epoxies, phenolics, polyurethanes, and polyimides. Of these, epoxy systems currently dominate the advanced composite industry.Thermosets
Thermoset resins require addition of a curing agent or hardener and impregnation onto a reinforcing material, followed by a curing step to produce a cured or finished part. Once cured, the part cannot be changed or reformed, except for finishing. Some of the more common thermosets include epoxies, polyurethanePolyurethane
A polyurethane is any polymer composed of a chain of organic units joined by carbamate links. Polyurethane polymers are formed through step-growth polymerization, by reacting a monomer with another monomer in the presence of a catalyst.Polyurethanes are...
s, phenolic and amino resins, bismaleimides (BMI, polyimides), polyamide
Polyamide
A polyamide is a polymer containing monomers of amides joined by peptide bonds. They can occur both naturally and artificially, examples being proteins, such as wool and silk, and can be made artificially through step-growth polymerization or solid-phase synthesis, examples being nylons, aramids,...
s.
Of these, epoxies are the most commonly used in the industry. Epoxy resins have been in use in U.S. industry for over 40 years. Epoxy compounds are also referred to as glycidyl compounds. The epoxy molecule can also be expanded or cross-linked with other molecules to form a wide variety of resin products, each with distinct performance characteristics. These resins range from low-viscosity liquids to high-molecular weight solids. Typically they are high-viscosity liquids.
The second of the essential ingredients of an advanced composite system is the curing agent or hardener. These compounds are very important because they control the reaction rate and determine the performance characteristics of the finished part. Since these compounds act as catalysts for the reaction, they must contain active sites on their molecules. Some of the most commonly used curing agents in the advanced composite industry are the aromatic amines. Two of the most common are methylene-dianiline (MDA) and sulfonyldianiline (DDS).
Several other types of curing agents are also used in the advanced composite industry. These include aliphatic and cycloaliphatic amines, polyaminoamides, amides, and anhydrides. Again, the choice of curing agent depends on the cure and performance characteristics desired for the finished part. Polyurethanes are another group of resins used in advanced composite processes. These compounds are formed by reacting the polyol
Polyol
A polyol is an alcohol containing multiple hydroxyl groups. In two technological disciplines the term "polyol" has a special meaning: food science and polymer chemistry.- Polyols in food science :...
component with an isocyanate compound, typically toluene diisocyanate
Toluene diisocyanate
Toluene diisocyanate is an aromatic diisocyanate. It is produced for reaction with polyols to form polyurethanes. It exists in two isomers, 2,4-TDI and 2,6-TDI . 2,4-TDI is produced in the pure state, but TDI is often marketed as 80/20 and 65/35 mixtures of the 2,4 and 2,6 isomers respectively...
(TDI); methylene diisocyanate (MDI) and hexamethylene diisocyanate
Hexamethylene diisocyanate
Hexamethylene diisocyanate is an organic compound in the class known as isocyanates. More specifically, it is an aliphatic diisocyanate. It is produced in relatively small quantities, accounting for only 3.4% of the global diisocyanate market in the year 2000...
(HDI) are also widely used. Phenolic and amino resins are another group of PMC resins. The bismaleimides and polyamides are relative newcomers to the advanced composite industry and have not been studied to the extent of the other resins.
Thermoplastics
ThermoplasticThermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...
s currently represent a relatively small part of the ACM industry. They are typically supplied as nonreactive solids (no chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
occurs during processing) and require only heat and pressure
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
to form the finished part. Unlike the thermosets, the thermoplastics can usually be reheated and reformed into another shape, if desired.
Fiber reinforcements
Fiber reinforcement materials are added to the resin system to provide strength to the finished part. The selection of reinforcement material is based on the properties desired in the finished product. These materials do not react with the resin but are an integral part of the advanced composite system.The three basic types of fiber reinforcement materials in use in the advanced composite industry are
- carbon/graphite
- aramidAramidAramid fibers are a class of heat-resistant and strong synthetic fibers. They are used in aerospace and military applications, for ballistic rated body armor fabric and ballistic composites, in bicycle tires, and as an asbestos substitute. The name is a portmanteau of "aromatic polyamide"...
- glass fibers
Fibers used in advanced composite manufacture come in various forms, including yarns, rovings, chopped strands, and woven fabric
mats. Each of these has its own special application. When prepreg materials are used in parts manufacture, woven fabric or mats are required. In processes such as filament wet winding or pultrusion, yarns and rovings are used.
Prepreg
Resin-impregnated cloth, mat, or filaments in flat form that can be stored for later use. The resin is often partially cured to a tack-free state called "B-staging." Catalysts, inhibitors, flame retardants, and other additives may be included to obtain specific end-use properties and improve processing, storage, and handling characteristics.Limitations
Despite their strength and low weight, composites have not been a miracle solution for aircraft structures. Composites are hard to inspect for flaws. Some of them absorb moisture. Most importantly, they can be expensive, primarily because they are labor intensive and often require complex and expensive fabrication machines. Aluminum, by contrast, is easy to manufacture and repair. Anyone who has ever gotten into a minor car accident has learned that dented metal can be hammered back into shape, but a crunched fiberglass bumper has to be completely replaced. The same is true for many composite materials used in aviation.Aluminum is a very tolerant material and can take a great deal of punishment before it fails. It can be dented or punctured and still hold together. Composites are not like this. If they are damaged, they require immediate repair, which is difficult and expensive. An airplane made entirely from aluminum can be repaired almost anywhere. This is not the case for composite materials, particularly as they use different and more exotic materials. Because of this, composites will probably always be used more in military aircraft, which are constantly being maintained, than in commercial aircraft, which have to require less maintenance.
Aluminum still remains a remarkably useful material for aircraft structures and metallurgists have worked hard to develop better aluminum alloys (a mixture of aluminum and other materials). In particular, aluminum-lithium is the most successful of these alloys. It is approximately ten percent lighter than standard aluminum. Beginning in the later 1990s it was used for the Space Shuttle's large External Tank in order to reduce weight and enable the shuttle to carry more payload. Its adoption by commercial aircraft manufacturers has been slower, however, due to the expense of lithium and the greater difficulty of using aluminum-lithium (in particular, it requires much care during welding). But it is likely that aluminum-lithium will eventually become a widely used material for both commercial and military aircraft.
External links
- Buckland, Peter G. Advanced composite materials with application to bridges. September 30, 1991.
- Oak Ridge National Laboratory. Carbon-Fiber Composites for Cars. Vol. 33, No. 3, 2000.