NIST GCR 04-863
Composites Manufacturing Technologies: Applications in Automotive, Petroleum, and Civil Infrastructure Industries
Economic Study of a Cluster of ATP-Funded Projects

Appendix 3: Composite Materials for Engineered Products

Engineers can utilize over 50,000 materials for the design and manufacture of engineered products. These materials, depending on their mechanical and physical characteristics such as strength, stiffness, density, melting temperature, and conductivity, can be divided into four main categories: metals, plastics, ceramics, and composites.

While metals and plastics are currently the dominant materials for engineering applications, there has been an increasing utilization of composite materials, reflecting their superior performance characteristics. The key factor holding back additional increases in composite utilization has been the high cost of composites.

Composites are systems composed of at least two distinctly dissimilar materials, acting in concert. The properties of the polymeric composite system are not attainable by individual components acting alone. Engineered composites can be defined as systems of reinforcing fibrous materials in a polymer matrix binder.

The reinforcing fiber provides strength and stiffness to the composite. The matrix material binds the fibers together, provides form and rigidity, transfers the load to the fibers, and protects the load-bearing fiber from corrosion and wear. For composites in structural applications, continuous or long-fiber configurations are typical, whereas for nonstructural applications, short fibers can be used (Figure A3-1).

Figure A3-1: Continuous-Fiber versus Short-Fiber Composites

Common composite reinforcing fibers include glass fibers and carbon/graphite fibers. For specialized applications, boron and aramid (Kevlar) fibers can also be used. General fiber properties are described below:

• Kevlar fibers are organic fibers which have high strength and stiffness which is possible in fully aligned polymers. Kevlar fibers have a very low resistance to failure under axial compression.
• Glass fibers (fiberglass) have excellent electrical insulating properties, good chemical and moisture resistance, and low cost. Glass fibers come in several grades (higher performance S-glass with greater tensile strength and higher use temperatures versus lower performance E-glass) and are widely used in industrial composites.
• Carbon/graphite fibers are high-strength, high-modulus, lightweight fibers, used as reinforcement for high-performance applications.
• Mixed fiber structures use a combination of Kevlar, glass, carbon, or other fibers to fabricate hybrid composites to optimize composite properties and cost.

The matrix material (polymer resin and elastomer) surrounds and binds the fiber in the composite structure and has a lower modulus and greater elongation than the fiber reinforcement. The matrix determines the service operating temperature of a composite as well as processing parameters for parts manufacturing. Polymer matrix materials may come as thermoset and thermoplastic resins:

• During curing, thermoset resins form three-dimensional molecular chains (crosslinking chains) and, once cured, cannot be remelted and reformed. The higher degree of the cross-linking chains, the more rigid and thermally stable the composite material will be. The most common resin materials are epoxy, polyester, vinyl ester, phenolics, and cyanate esters.
• Thermoplastic resin molecules do not cross-link and thus remain flexible and reformable. They are, in general, more ductile and tougher than thermosets and are used in a wide variety of industrial (nonstructural) applications. At the same time, they have lower stiffness and strength values, which can be selectively improved by the appropriate choice of fiber reinforcement. Typical thermoplastic resins include nylon, polypropylene, polyetheretherketone (PEEK), polyester, and teflon.

Table A3-1 indicates representative physical properties of various composites, compared to common engineering materials. Composite materials have high specific strength (strength-to-density ratio) and high specific stiffness (modulus-to-density ratio).

Table A3-1: Typical Properties of Some Engineering Materials

Source: Mazumdar 2002.

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Date created: July 14, 2004
Last updated: August 3, 2005

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