Carl Zweben, Ph. D.
ASME LIFE FELLOW
Edited by Jean M. Hoffman
Most engineers likely know the new Boeing 787 Dreamliner is made primarily of carbon-fiber-reinforced epoxy composites. Considering that carbon fibers were first developed in the late 1960s, this is remarkable progress. Carbon-epoxy composites have outstanding properties so it is not surprising that they have become the baseline materials in aerospace and sporting goods. The fact that composites don’t have the production constraints of metal let Boeing engineers optimize 787 aerodynamics. The composite airframes also weigh less but are stronger than conventional airframes, with a commensurate impact on operating efficiency and performance. They don’t corrode and resist fatigue that eventually makes metal structures weaken and crack.
It is not widely known, however, that aerospace and sporting goods aren’t the largest users of these materials. In fact, most polymermatrix composites (PMCs) go into industrial and commercial uses.
A COMPOSITE ISN’T WHAT YOU THINK
A composite material is two or more materials bonded together. This distinguishes composites from metallic alloys, in which one constituent is dissolved in another. Biological structural materials in nature including wood, bamboo, bone, teeth, and shell are all composites. Use of synthetic composite materials is not new. Bricks made from straw-reinforced mud are mentioned in the Old Testament. This material also has been widely used in the American Southwest for centuries, where it is known as adobe. In current terminology, these types of composites are called organic fiber-reinforced ceramic-matrix composites.
Composites have historically been used for their outstanding mechanical properties and low densities. Increasingly, though, what’s driving their use is physical properties. These include thermal conductivities higher than that of copper and low coefficients of thermal expansion and densities. When it comes to typical mechanicalengineering applications, advanced composites offer significant improvements over such traditional materials as steel, aluminum, cast iron, and granite.
Mention composites and engineers tend to think of carbon-reinforced epoxy and glassfiber- reinforced polymers (polyester, vinyl ester, and nylon). However, composites fall into four key classes: PMCs, metal-matrix composites (MMCs), carbon matrix composites (CAMCs) and ceramic-matrix composites (CMCs). Each category includes a wide range of materials.
By a wide margin, PMCs are the most important class. They consist of polymer matrices like an epoxy reinforced with fibers. In a few cases, whisker (elongated single-crystal) reinforcements are used. The material is called a hybrid composite when it combines different types of reinforcements.
Although there are numerous composite materials having a wide range of properties, it’s possible to make some generalizations about their properties: They all have high strength and high stiffness. They have low density and strongly resist fatigue and creep. They have low coefficients of thermal expansion (CTE) and basically don’t corrode. Some composites also have extremely high thermal conductivity, high temperature capability, or both.
E-glass is the most widely used reinforcement, primarily because it has been around the longest, and is the least expensive. Its main drawback is low modulus. This has led to use of carbon fibers, which are much stiffer and stronger. Carbon fibers, of which there are many, have become the workhorse reinforcements for PMCs. They are made from three key precursor materials: polyacrylonitrile (PAN), petroleum, and coal tar pitch. Fiber elastic moduli range from 235 to 895 GPa (34 to 130 Msi). Tensile strengths range from 3,200 to 7,000 MPa (450 to 1,000 kpsi). Fiber densities are quite low, 1.7 to 1.9 gm/cm3.
There are many other synthetic fibers used in structural composites, including various types of ceramic, such as silicon carbide, boron, and aluminum oxides. There’s also highmodulus polymerics including aramids (e.g., “Kevlar” 49) and ultrahigh-molecular-weight polyethylene (UHMWPE). These types of fibers are also candidates for a special class of composites used in ballistic protection. These so-called armor-grade composites are constructed with low (less than 20% by weight) resin content to maximize the inherently high resistance of their fibers to transverse impacts. There is growing use of renewable natural fibers, such as bast and kennaf, although these are not high-performance materials.
For applications in which both mechanical properties and low weight are important, useful figures of merit are specific strength (strength divided by density) and specific stiffness (stiffness divided by density). The accompanying figure presents specific stiffness and specific tensile strength of conventional structural metals (steel, titanium, aluminum, magnesium, and beryllium) and selected composite materials. The composites are epoxy-matrix PMCs reinforced with a variety of fibers and one MMC, aluminum containing silicon-carbide particles (AlSiC). PMC reinforcements include boron and E-glass fibers and a variety of carbon fibers: standard modulus (SM), ultrahigh strength (UHS) and ultrahigh modulus (UHM) made from polyacrylonitrile (PAN), and pitch precursors. With the exception of beryllium, all of the monolithic metals fall in a small box.
The properties of composites reinforced with fibers depend strongly on their orientation. The upper end of the bar for each type of composite represents a unidirectional material, in which fibers are all aligned in one direction. The lower end represents quasi-isotropic composites, which have the same elastic and thermal properties in every direction in the plane containing them. Strength properties are roughly the same. However, through-thickness moduli and strengths are much lower. Because unidirectional composites are weak in directions perpendicular to the fibers, they are rarely used in practice. In general, the design engineer selects laminates somewhere between unidirectional and quasi-isotropic.
Heat dissipation is a critical problem in both electronic and optoelectronic semiconductors, such as diode lasers and light-emitting diodes (LEDs). Copper and aluminum can cause high thermal stresses when they are attached to the semiconductors and ceramics used in electronic and optoelectronic applications. That’s because these metals have high CTEs, but semiconductors have CTEs in the range of about 2 to 7 ppm/K. An increasing number of low-density PMCs and MMCs have been developed with higher thermal conductivities and lower CTEs than copper. They can also reduce weight by as much as 85%, and size by up 65%. And they are candidates for low-cost, net-shape-fabrication processes.
AlSiC composites are the most important of the new-generation thermal-management materials replacing copper, aluminum, and alloys of copper-tungsten alloy, nickelcobalt- iron (Kovar), and a copper- Invar-copper. Kovar has a CTE similar to that of hard (borosilicate) glass and is a candidate for glass-to-metal seals. Invar is a nickel-steel alloy noted for an extremely low CTE.
SiC content in AlSiC composites can be adjusted to match the CTE to that of ceramics (aluminum oxide and aluminum nitride) used in packaging, with particle volume fractions of 0.7. Composites with particle loadings of 0.2 have CTEs resembling those of the glass-reinforced epoxy composites of printedcircuit boards, leading to their use in laptop computers.
Carbon-epoxy composites are being used to reduce the CTE of E-glass-reinforced printed-circuit board (PCB) materials such as FR-4. Their use reduces thermal stresses and warping, which are key modes of failure. In addition, use of thermally conductive carbon fibers lets the PCB be a path for heat dissipation.
Where to use it
Industrial and commercial composite applications include automobile structural parts, engines, drive shafts, gear cases, clutches, and brakes. Metal-matrix composites, for example, go into automobile engines, robot end effectors, helicopter rotor systems, high-speed machinery, and electronics thermal management.
E-glass fiber-reinforced PMCs are commodity materials used in a large number of applications. Everything from boats to bathtubs to chemical-industry tanks and piping. However, as discussed earlier, these materials have low stiffness. As a result, the greatest potential for high-performance applications lies with PMCs reinforced with carbon fibers, and with MMCs, CCCs, and CMCs. So that’s where we’ll focus our discussion.
Carbon-fiber-reinforced polymer (CFRP) aerospace/defense applications include aircraft, spacecraft, and launch-vehicle structures; aircraft engines; helicopter rotor blades; weapon systems; optomechanical equipment; and ships. In addition, CFRPs widely serve in sports and leisure gear such as golf clubs, skis, tennis rackets, and fishing rods, and are the baseline material in America’s Cup sailboat hulls and masts.
Key commercial and industrial PMC applications include machine components, robots, energy-storage flywheels, coordinate-measuring machines and other precision equipment, compressed natural gas and hydrogen-vehicle fuel tanks, windturbine blades, high-speed trains, infrastructure, biomedical equipment, electronic and optoelectronic thermal management, and countless others. There are numerous developmental applications, such as oil and natural-gas exploration and production, process industries equipment and electrical power lines.
Many widely used materials are actually MMCs but are not recognized as such. For example, the material used in cutting tools and drills commonly called “tungsten carbide” consists of tungsten-carbide particles embedded in a cobalt matrix, making it a metal-matrix composite. This MMC has much greater fracture toughness than monolithic tungsten carbide, which is a brittle ceramic. Tungsten-carbide particlereinforced silver has been used for commercial circuit-breaker contact pads for many years. Ferrous alloys reinforced with titanium-carbide particles, marketed under the trade name “FerroTic,” worked for many years in industrial applications needing hardness, high stiffness and lower density than monolithic ferrous materials.
Aluminum MMCs reinforced with discontinuous ceramic fibers go into automobile engine blocks and pistons to boost wear resistance, allowing elimination of cast iron inserts and sleeves. Other MMC applications include robot and high-speed machine parts, power-transmission lines, helicopter rotor-blade sleeves, fighter-aircraft ventral fins, and jet-engine fan exit-guide vanes. They have also served in a number of military optomechanical- system parts.
The most important structural MMCs today consist of various AlSiC materials. They have a wide range of properties and are made by a variety of processes. In general, they have much higher specific stiffness and lower CTE than monolithic aluminum.
Carbon/carbon composites are widely used in high-temperature aerospace applications such as rocket nozzles and aircraft brakes. Commercial and industrial applications include glassmaking equipment, heat-treat furnace furniture, X-ray targets, and racing-car brakes and clutches. Silicon-carbide fiberreinforced- carbon composites have been used in aircraft engines.
CMCs are the least developed of the four classes of composites. Applications include aircraft engine parts, missile and spacecraft thruster nozzles, high-end automobile brake rotors and cutting tools.
Why doesn’t everybody use Composites?
Though composite use is on the rise there remain significant barriers that keep them out of some areas. Cost, both real and perceived, is certainly one of the key issues.
The acquisition cost of composites is often higher than that of incumbent materials. However, this is not always true. Some composite manufacturing processes allow parts consolidation that can reduce machining and assembly costs. In addition, many fabrication processes are highly automated, and this impacts overall costs. Even if acquisition cost is higher, there can be significant life cycle benefits that can make the outlay worthwhile, including reduced fuel and energy consumption, higher throughput, longer life, less down time, and so forth.
Another major barrier is that many engineering schools don’t offer courses on composites. So there is a general unawareness of these materials, as well as how to design and analyze components using them. On this topic the American Society of Mechanical Engineers (ASME International) offers short courses specifically tailored to the needs of mechanical engineers. They can be found at: http://catalog.asme.org/Education/ShortCourse/Advanced_Composite.cfm.