Composite Material Fires

Composite Material Fires
Eric Stetz 3/27/2013 Aviation Fire Dynamics

Overview What are Composite Materials?
Definition Examples How are Composite Materials used in Aviation? How Composite Materials Burn Pyrolysis Structural Behavior of Burned Composites Experimental Buckling of Heated Laminate Modeling Charred and Heated Laminates Techniques to Mitigate the Burning of Composites

What are Composite Materials?
Composite Materials are generally defined as materials that are made up of some combination of two or more dissimilar components. However, this definition is somewhat inadequate, as composite materials differ from simple mixtures of materials such as some plastic compounds, and metallic alloys. Therefore, some qualifications for composite materials should be defined.

Some qualifications include: The material is manufactured, and/or the constituents are combined in some fashion that is designed or engineered. The material consists of two or more physically and/or chemically distinct phases with an interface separating and connecting them. The material has characteristics that are not depicted by any of the components in isolation. The material must contain a sufficient amount of each phase/constituent, at least 5%. [1][2]

Most composite are composed of predominantly two separate materials, one of which can be described as the matrix, and the other the reinforcement. Brauer et al., Journal of Materials Science: Materials in Medicine 19 (2008)

Although usually considered high tech, composite materials have actually been around for a long time. Many naturally occurring materials are composite in nature, such as wood (cellulose fibers in a lignin matrix) and bone (collagen fibers in a mineral matrix) Early examples of composites include bricks made from mud and straw, concrete, and plywood. Modern composite materials are made from wide ranges of different constituents. Common examples include fiberglass and carbon fiber (carbon reinforced polymer). [1]

Some Modern Constituent materials used for Composite Construction include: Matrix Materials Metal Matrix Composites Ceramic Matrix Composites Polymer Matrix Composites Reinforcement Materials Glass Fibers Boron Fibers Carbon Fibers Organic Fibers Ceramic Fibers Whiskers [1]

Metal Matrix Composites Advantages of Metals as a matrix material Strong and Tough Ductile Resistant to fire Good electrical and thermal conductivity Do not outgas Disadvantages of Metals as a matrix material Expensive Difficult to fabricate

Metal Matrix materials are usually light metals including: Aluminum Magnesium Titanium Cobalt Cobalt-Nickel alloy

Ceramic Matrix Composites. Advantages of Ceramics as a matrix material High Hardness High Elastic Modulus Low Density High Temperature Resistance Disadvantages of Ceramics as a matrix material Brittle Low Failure Strain Low Thermal Shock Resistance Low Tensile Strength

Some examples of ceramic materials used as matrix materials include: Borosilicate Glass Soda Glass Mullite (Porcelain) Magnesium Oxide Silicon Nitride Aluminum Oxide Silicon Carbide [1]

Polymer Matrix Composites Some Advantages to using polymers as a matrix material include: Cheap Easy to Produce Some Disadvantages to using polymers as a matrix materials include: Low Strength and Modulus Lower temperature limits Easily degradable in light or solvents Higher CTE then metals and ceramics

Some examples of polymers used as matrix materials include: Thermosets Epoxy Polyester Thermoplastic Nylon Polycarbonates PET, PBT Polyetherether ketone (PEEK) [1]

Glass Fiber Reinforcement Glass fibers are silica based materials that contain mixtures of several other oxides, such as calcium, boron, sodium, aluminum, and iron. Glass fibers have low density, high strength, yet only a moderate Modulus. They are susceptible to degradation in moisture. They are also relatively cheap and come in a variety of forms. The most common form of glass fibers are called E-glass, due to its electrical insulation properties. There are also C-glass and S-glass variants corresponding to higher corrosion resistance and higher silica content. [1]

Boron Fiber Reinforcement Boron fibers are made from the chemical vapor disposition of Boron onto a substrate. Boron fibers have a higher strength and Modulus than glass fibers, but are very brittle and subject to internal stresses and defects that can greatly decrease their strength. [1]

Carbon Fiber Reinforcement Carbon fibers are simply fibers made from pure elemental carbon, usually aligned in a graphitic structure. Carbon fibers have a very low density ( g/cm3). High quality carbon fibers can be made with a very high modulus (400 Gpa) and very high tensile strength (2-4 Gpa). Carbon fiber’s good properties and ease of production have made it one of the most popular reinforcement materials for modern composite materials. [1],[Matweb.com]

Organic Fiber Reinforcement Organic fibers are fibers made from organic compounds by drawing out the polymer molecule chains to be aligned in the fiber direction. This greatly increases the strength and modulus of the polymer. Organic fibers have good density, strength and modulus. However, they are limited to low temperature applications. The most common organic fibers are polyethylene and aramid. Aramid fibers include the commercial products known as Kevlar and Nomex. [1]

Ceramic Fiber Reinforcement Ceramic Fibers are usually made from chemical vapor disposition or similar process of silicon carbide, silicon nitride, or boron carbide. Ceramic fibers have high strength (2 Gpa), high modulus (200 Gpa), high temperature resistance and good corrosive resistance. [1]

Whisker Reinforcement Whiskers are very short, strong fibers of a non-uniform dimension and properties. They are typically mixed with the matrix, rather then woven like traditional fibers. They are usually made from ceramic materials such as silicon carbide. While strong, the large variation in sizes and dimension can cause a large variation in strength. Controlling alignment and mixture of the whiskers in a composite is also a significant problem. [1]

How are Composites Used in Aviation?
Composite materials are an ideal aerospace material, because of their typically high strength to weight ratio. The most commonly used types of composites in aircraft are fiberglass and carbon fiber reinforced polymers, where the polymer is some type of epoxy resin. Therefore, the focus will be on the burning of these types of materials. The use of these composite materials in commercial aircraft components and structures has increased steadily over the last 30 years.

Composite Material Use in Commercial Transportation Aircraft Over Time [3]

Fiberglass composites are now commonly used for cabin interior components, such as separators, panel walls, overhead compartments, and cargo holders. [6] Carbon fiber reinforced polymer is increasing used for structural parts of aircraft, such as the fuselage, wings, and tail. One of the newest airliners, the Boeing 787, is comprised of 50% composite materials.[7]

How Composite Materials Burn
Understanding how composite materials respond to high temperature fires caused by aviation fuel is very important due to their increasing role in commercial aircraft. Due to the nature of composite materials, any weakness in one of the constituent materials of the composite will undermine the structural integrity of the entire material. In high temperature environments, the polymer matrix would be the first constituent to degrade/fail.

When carbon reinforced polymer composites are exposed to high heat and fire above the thermal limits of the polymer, the composite undergoes a reaction releasing toxic, volatile gases and turns into a layer of char. [8] This reaction can be modeled as pyrolysis, where high temperature causes the release of compounds from a fuel, leaving a high carbon solid called char.

The decomposition of epoxy due to pyrolysis can result in the release of phenol, 4-isopropylphenol, and bisphenol A. The approximate proportions of carbon, hydrogen and oxygen in this pyrolysis gas can be represented by the formula CH1.3O0.2. The specific heat and enthalpy of this product gas can be assumed to be similar to methane (CH4) because the specific heats are similar. [8]

The following reaction models the pyrolysis gas that leaves the composite, CH1.3O (O2+3.76N2)  CO2+0.65H2O+1.225(3.76)N2 The DesJardin paper estimates a heat of combustion of ΔhC=28.8 kJ/g. This leads to a heat of formation for the pyrolysis gas of, h°CH1.3O0.2= kJ/g and an adiabatic flame temperature of, Tad=2300 K [8]

The following image shows the effects of high heat, simulating a JP-8 fuel fire at 2500 K, on a composite panel for different short periods of time. Delamination and damage to the laminate surface can been seen to increase with time. [9] [9]

These heat tests also show that ply delamination can occur near the surface of a composite even before the resin has fully degraded and burned away. This delamination is likely caused by internal pressures generated by moisture in the composite converting to steam. [9]

Fire tests conducted on fiberglass panels used in the interiors of commercial aircraft found that the panels were consumed very quickly in the fire, and produced large amounts of obstructive smoke. [6] This is due to the fact the epoxies catch fire a very low temperatures, and when burned produce toxic and volatile compounds that can feed the fire. [5]

Structural Behavior of Burned Composites
Understanding the effects of heat and fire on a composite’s structural integrity is very important to ensure the safety of an aircraft. Unlike metallic materials such as aluminum and titanium, heat and fire can quickly compromise the strength of a composite part, leading to rapid structural failure that could be catastrophic for the aircraft. [5]

When heated to temperatures of only C, most carbon epoxies can lose up to 50% of their compression stiffness and strength. [5] Aluminum and Titanium would need to be heated to temperatures of 200°C and 500°C respectively to lose the same amount of strength. [5]

Even before pyrolysis and charring occurs, a laminate can be weakened to the point where it will buckle under compression from even a moderate force. Tests preformed with a loaded composite panel exposed to a constant heat flux found that simply heating the laminate above the viscous softening temperature of the polymer matrix was enough to cause buckling failure of the laminate. [5] [5]

The first mode of failure for the laminate is through viscous softening of the polymer matrix. This is when the temperature of the laminate reaches the point where the polymer matrix begins to melt to a degree where it loses structural strength. The second mode of failure is matrix decomposition. This occurs at a higher temperature then viscous softening, and involves pyrolysis of the polymer matrix. The last mode of failure is oxidation of the fibers. This occurs only at extreme temperatures, and involves the burning of the actual fibers in air. [5]

Heat fluxes of 10, 25, and 50 kW/m2 were applied to a carbon reinforced epoxy laminate while it was under constant compression load for a variety of load forces. [5] [5]

The laminate was found to fail quickly under any stress above 10% of the nominal buckling stress for all of the heat flux cases. The 10 kW/m2 case only heated the laminate above the epoxy’s viscous softening temperature (50-100°C), but not above the matrix decomposition temperature. The 25 kW/m2 case heated the laminate above both the epoxy viscous softening temperature and the matrix decomposition temperature ( °C), but not above the fiber oxidation temperature. The 50 kW/m2 case heated the laminate enough that it reached the viscous softening temperature, the matrix decomposition temperature and the fiber oxidation temperature (>500°C). [5]

The structural failure of a composite laminate can also be modeled to predict the method in which it will fail. The laminate is modeled as a column of material with either a char layer, if it has undergone pyrolysis, or no char layer. A heat flux is applied to the column and the temperature distribution through the laminate is determined. [4] [4]

Then, the temperature distribution of the laminate is used to find the temperature dependent modulus of the polymer. This can then be used to calculate the response of the laminate under the applied temperature and force loading. Structurally the char layer is neglected, as it can be assumed to provide almost no structural support. Interestingly, the char layer must be considered when determining the heat transfer to the laminate from the heat source, as the char layer actually provides more insulation from the heat to the deeper composite layers than a standard layer of laminate. [4]

Fire Mitigation Techniques
NASA and other composite manufacturers are currently researching alternate resins for use in composite with better flammability and safety properties. These include modified epoxies and phenolics, bismaleimides, and polyimides. [6] Using higher temperature resistant polymers with high mechanical glass transition temperatures would prolong the strength of polymer matrix composites exposed to fire and high heat.

Summary Composite materials are becoming a major part of all modern aircraft structures. Currently used carbon fiber and fiberglass reinforced polymers are much more susceptible to heat and fire then traditional metal aerospace materials. This weakness to heat and fire greatly impacts the structural integrity of an aircraft made from composites that encounters a accident involving fire. Newer constituent materials for composites need to be developed to ensure the safety of passengers and crew, and to ensure the structural integrity of aircraft continues to increase.

References [1]Chawla, K. K., Composite Materials: Science and Engineering 2nd Edition, Springer Science, New York, NY, 1998 [2] Rawlings, R. D., Matthews, F. L. , Composite Materials: Engineering and Science, CRC Press LLC, Boca Raton, FL, 2006 [3] Harris, Charles E., “Opportunities for Next Generation Aircraft Enabled by revolutionary Materials,” AIAA SDM Conference, Denver, Colorado, 2011 [4] Liu, Liu, Kardomateas, George, “Thermal Buckling of a Fire-Damaged Composite Column Exposed to Heat Flux,” AIAA Journal, Vol. 44, No. 9, 2006, pp [5] Burns, L. A., Feih, S., Mouritz, A. P., “Compression Failure of Carbon Fiber-Epoxy Laminates in Fire”, Journal of Aircraft, Vol. 47, No. 2, 2010, pp [6] Sarkos, C. P., Spurgeon, J. C., Nicholas, E. B., “Laboratory Fire Testing of Cabin Materials Used in Commercial Aircraft,” Journal of Aircraft, Vol. 16, No. 2, pp [7] Burns, Lauren, “Fire-Under-Load Testing of Carbon Epoxy Composite,” 47th AIAA Aerospace Sciences Meeting, Orlando, Florida, 2009

References [8] McGurn, Matthew, DesJardin, Paul, “Modeling of Charring and Burning Carbon-Epoxy Composites in Fire Environments,” 50th AIAA Aerospace Sciences Meeting, Nashville, Tennessee, 2012 [9] Czarnecki, G. J., Ripberger, E. R., Meilunas, R. J., Milan, W., “Thermal Degradation of Composites,” 52nd AIAA Structures, Structural Dynamics and Materials Conference, Denver, Colorado, 2011

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