3
Evaluation of Materials Fire Performance

The development of improved fire-resistant materials for aircraft interiors requires an understanding of aircraft fire scenarios, the factors that influence the initiation and propagation of a fire, and testing and modeling methods to evaluate and predict materials performance. This chapter provides an overview of the current state of fire-safety science as it pertains to aircraft fires. The fire scenarios that are most likely to occur are described along with the parameters that are critical in each scenario. Predictive models for simulating fires and assessing fire hazards are summarized. Finally, testing methods useful in the characterization of materials performance in a fire as well as in the evaluation and ranking of new materials are described.

FIRE SAFETY

When evaluating a material for use in a given situation, it is important to assess the total fire hazard. Fire hazard is ''the potential for harm associated with fire'' (ASTM, 1994). Fire hazards are associated with the environment and with a number of characteristics of materials, products, or assemblies; these characteristics include ease of ignition, flame spread, rate of heat release, smoke generation and obscuration, toxicity of combustion products, and ease of extinguishment.

Fire risk is "an estimation of expected fire loss that combines the potential for harm in various fire scenarios that can occur with the probabilities of occurrence of those scenarios .... Risk may be defined as the probability of having a certain type of fire, where the type of fire may be defined in whole or in part by the degree of potential harm associated with it, or as potential for harm weighted by associated probabilities. However it is defined, no risk scale implies a single value of acceptable risk. Different individuals presented with the same risk situation may have different criteria for determining its acceptability" (ASTM, 1994).

The factors that need to be considered in assessing fire risk for aircraft interiors are the quantity of material present; component configuration; proximity of other combustibles; volume of the compartments to which the combustion products may spread; ventilation conditions; ignition, combustion, and toxic potency properties of the materials present; presence of ignition sources; presence of fire protection systems; number of occupants; and the time available to escape (Levin et al., 1982).

Flammability

Flammability describes the ability of a material to burn. Flammability characteristics are those properties that define, describe, or measure the behavior of a material when it is exposed to heat or fire. The flammability characteristics that are most important in measuring the material burning process are ignitability, flame spread, rate of heat release, and smoke and fire-gas production. Each characteristic is described in the following paragraphs.

Ignitability, or ease of ignition, measures the time it takes for a material to ignite once heat is applied. Ignition is the initiation of combustion as shown by glow, flame, detonation, or explosion. There are two different types of ignition processes: piloted ignition and nonpiloted ignition. Piloted ignition is the initiation of combustion as a result of contact of a material or its vapors with an external, high-energy source such as a flame, spark, electrical arc, or glowing wire (ASTM, 1994). In piloted ignition the surface temperature must exceed a critical minimum value (the firepoint) at which the rate of pyrolysis in the surface layer produces a flow of flammable vapors sufficient to support a flame (Drysdale, 1995). In nonpiloted ignition , no localized, sufficiently high-temperature region occurs in the gas phase to cause ignition, thus the surface temperature of a material must become high enough to act as an induced pilot to initiate gas-phase oxidation reactions and attain ignition.

Flame spread is a factor that signifies the rate of burning and is derived from the rate of progress of the flame front. Flame spread is affected by many parameters including surface orientation, material thickness, surface roughness, ambient pressure, humidity, size, initial fuel temperature, incident radiation, and material chemical composition.

Rate of heat release measures the rate of heat energy produced by a given amount of burning material. Heat release rate is a measure of the contribution of a material to a fire. The rate of heat release is considered the single most important measure of the fire hazard of a material. It is a quantity that can be used in predicting the rate of fire growth and its effects.



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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft 3 Evaluation of Materials Fire Performance The development of improved fire-resistant materials for aircraft interiors requires an understanding of aircraft fire scenarios, the factors that influence the initiation and propagation of a fire, and testing and modeling methods to evaluate and predict materials performance. This chapter provides an overview of the current state of fire-safety science as it pertains to aircraft fires. The fire scenarios that are most likely to occur are described along with the parameters that are critical in each scenario. Predictive models for simulating fires and assessing fire hazards are summarized. Finally, testing methods useful in the characterization of materials performance in a fire as well as in the evaluation and ranking of new materials are described. FIRE SAFETY When evaluating a material for use in a given situation, it is important to assess the total fire hazard. Fire hazard is ''the potential for harm associated with fire'' (ASTM, 1994). Fire hazards are associated with the environment and with a number of characteristics of materials, products, or assemblies; these characteristics include ease of ignition, flame spread, rate of heat release, smoke generation and obscuration, toxicity of combustion products, and ease of extinguishment. Fire risk is "an estimation of expected fire loss that combines the potential for harm in various fire scenarios that can occur with the probabilities of occurrence of those scenarios .... Risk may be defined as the probability of having a certain type of fire, where the type of fire may be defined in whole or in part by the degree of potential harm associated with it, or as potential for harm weighted by associated probabilities. However it is defined, no risk scale implies a single value of acceptable risk. Different individuals presented with the same risk situation may have different criteria for determining its acceptability" (ASTM, 1994). The factors that need to be considered in assessing fire risk for aircraft interiors are the quantity of material present; component configuration; proximity of other combustibles; volume of the compartments to which the combustion products may spread; ventilation conditions; ignition, combustion, and toxic potency properties of the materials present; presence of ignition sources; presence of fire protection systems; number of occupants; and the time available to escape (Levin et al., 1982). Flammability Flammability describes the ability of a material to burn. Flammability characteristics are those properties that define, describe, or measure the behavior of a material when it is exposed to heat or fire. The flammability characteristics that are most important in measuring the material burning process are ignitability, flame spread, rate of heat release, and smoke and fire-gas production. Each characteristic is described in the following paragraphs. Ignitability, or ease of ignition, measures the time it takes for a material to ignite once heat is applied. Ignition is the initiation of combustion as shown by glow, flame, detonation, or explosion. There are two different types of ignition processes: piloted ignition and nonpiloted ignition. Piloted ignition is the initiation of combustion as a result of contact of a material or its vapors with an external, high-energy source such as a flame, spark, electrical arc, or glowing wire (ASTM, 1994). In piloted ignition the surface temperature must exceed a critical minimum value (the firepoint) at which the rate of pyrolysis in the surface layer produces a flow of flammable vapors sufficient to support a flame (Drysdale, 1995). In nonpiloted ignition , no localized, sufficiently high-temperature region occurs in the gas phase to cause ignition, thus the surface temperature of a material must become high enough to act as an induced pilot to initiate gas-phase oxidation reactions and attain ignition. Flame spread is a factor that signifies the rate of burning and is derived from the rate of progress of the flame front. Flame spread is affected by many parameters including surface orientation, material thickness, surface roughness, ambient pressure, humidity, size, initial fuel temperature, incident radiation, and material chemical composition. Rate of heat release measures the rate of heat energy produced by a given amount of burning material. Heat release rate is a measure of the contribution of a material to a fire. The rate of heat release is considered the single most important measure of the fire hazard of a material. It is a quantity that can be used in predicting the rate of fire growth and its effects.

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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft Smoke and fire-gas production are determined by the chemical composition of the material, the fire environment, and in particular the available oxygen. Smoke is defined as the airborne solid and liquid particulates and gases evolved when a material undergoes pyrolysis or combustion (ASTM, 1994). Fire gases, according to ASTM (1994), are airborne products emitted by a material undergoing pyrolysis or combustion that exist in the gas phase at the relevant temperature. Smoke density is influenced by composition, the rate of burning or intensity of the fire and the degree of ventilation. Fundamental flammability principles are as follows (NFPA, 1988): An oxidizing agent, a combustible material, and an ignition source are essential for combustion. The combustible material must be heated to its piloted ignition temperature before it will ignite or support flame spread. Subsequent burning of a combustible material is governed by the heat feedback from the flames to the pyrolyzing or vaporizing combustible. The burning will continue until the combustible material is consumed, or the oxidizing agent concentration is lowered to below the concentration necessary to support combustion, or sufficient heat is removed or prevented from reaching the combustible material to prevent further fuel pyrolysis, or the flames are chemically inhibited or sufficiently cooled to prevent further reaction. Toxicity The toxic gases and irritants that are present in all smoke should be considered potential dangers. Toxic products can cause both acute and delayed toxicological effects. It is the acute and extremely short-term effects that prevent escape from an aircraft fire by causing faulty judgment, incapacitation, and death. The irritants in the smoke can also interfere with the ability of passengers to escape by causing severe coughing and choking and by preventing them from keeping their eyes open long enough to find the exits. In addition, delayed effects, such as tissue or organ injury, mutagenicity, carcinogenicity, and teratogenicity, may ultimately lead to permanent disability and post-exposure deaths among accident survivors. Toxic potency is "a quantitative expression relating concentration (of smoke or combustion gases) and exposure time to a particular degree of adverse physiological response (e.g., death on exposure of humans or animals).... The toxic potency of smoke from any material, product or assembly is related to the composition of that smoke which in turn is dependent upon the conditions under which the smoke is generated" (ASTM, 1994). The LC50 (the concentration that causes death in 50 percent of the test organisms in a specified time) is a common endpoint used to assess toxic potency. In the comparison of the toxic potencies of different compounds or materials, the lower the LC50 (i.e., the smaller the amount of material necessary to reach this endpoint), the more toxic the material is. FIRE SCENARIOS For fire-resistant materials, self-sustaining interactions of individual materials are of minimal importance to the real fire scenario, even in terms of local ignition and sustained smoldering.1 Fire resistance is generally sufficiently effective to inhibit flammability under all but extreme heat loads resulting from either external source coupling or large-scale involvement. Thus, important issues are dominated by systems interactions to heat exposure from surroundings rather than the fire characteristics of individual materials themselves. However, the nonthermal hazards (visibility, production of irritant gases, and toxic product generation) are more strongly connected to the characteristics of individual materials in response to these surrounding interactions. In general, the dominating initial heat source is that radiated or convected from surrounding fuel fires. In large-scale fires such as fuel pool fires, radiative heat transfer is expected to dominate the convective component (Hottel, 1959). In the case of aircraft interior fire characteristics, pool fire plume impingement on the inside of the cabin through open egresses or structural failures can also be important. Once the fire spreads to the inside of the aircraft interior. the heat release from the burning of local materials can contribute to further evolution of the overall fire scenario. eventually leading to flashover. Understanding space flashover and predicting the likelihood of its occurrence is Critical in assessing materials response in aircraft fires. Flashover is the point at which most of the combustible 1   Smoldering is the combustion of a solid without flame (ASTM, 1994) and is characterized by a glowing combustion supported by strong exothermic char oxidation reactions. Smoldering usually occurs in well-thermally-insulated, natural polymers such as cellulosic materials and wood products and rarely occurs in synthetic polymers. The cables used in transport aircraft are generally made of Kapton2® which generates a lot of char but it is not known to smolder. Phenolic composite and Tedlar (polyvinyl fluoride) constructions used commonly in interiors do not smolder nor do the fiberglass or polyimide foam materials used in cabin insulation. Smoldering is more often observed in buildings where cellulosic insulation or cotton fabrics are used in upholstered furniture.

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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft materials in an enclosed space reach their ignition temperatures at essentially the same time, so that the materials seem to burst into flame simultaneously. Flashover is a significant concern because the space becomes untenable for occupants, and the fire hazard to adjacent spaces increases significantly. Flashover conditions may, however, be preceded by such severe aircraft interior temperatures and nonthermal hazards as to preclude survivability. The FAA determined in its work during the development of the heat release regulation that the toxicity hazard from burning standard cabin furnishings did not become significant until flashover occurred (Sarkos and Hill, 1989). Catastrophic fire events can be the result of two basic aircraft fire scenarios—post-crash external fuel fires and in-flight fires (Sarkos and Hill, 1989). These two scenarios provide the basis for establishing baseline fire performance behavior and criteria for new materials development. Post-Crash External Fuel Fires The FAA Technical Center has developed criteria for the current generation of improved fire-resistant cabin materials based on the characterization of the fire environment through a series of full-scale tests (Sarkos, 1995). The scenario that has been emphasized in the FAA tests have been the post-crash fuel-fed fires with the fuselage largely intact. Post-crash scenarios have been the focus of FAA work because all accidental fire-related fatalities in the United States in the past 30 years have been due to post-crash fires (Sarkos, 1995; Murray, 1995). Although other scenarios must be considered, a largely intact fuselage (with openings for fire or smoke to enter) has been emphasized by the FAA because (1) the intact fuselage would be more likely to be an impact survivable crash, and (2) direct entry of flames provides the quickest ignition for interior furnishings (Sarkos and Hill, 1989). The following conditions are considered in the post-crash external fuel-fire scenario: One or more holes in the fuselage; only flame radiation enters. In this scenario, ignition of interior components is by radiation (no flames), and fire growth is typical of an enclosure fire. The important characteristics include piloted ignition, fire plumes, radiation interactions, and flashover. Fuels for the enclosure fire include interior components and passenger personal items such as clothing and carry-on items. One or more holes (door or rupture) in the fuselage; flames and smoke enter. In this case, the upper layer of the cabin is quickly vitiated, and thus the thermal decomposition of the exposed panels is different, internal ignition is by direct time contact near fuselage openings. If an escape door (down wind) is opened, the external times may extend along the fuselage ceiling and seriously disrupt escape. If external flames do not extend inside, the fire will develop as an enclosure fire. No holes in fuselage. The heating of the airplane skin will eventually heat the back side of the interior wall panels, resulting in some degree of aerobic pyrolysis or burnthrough of the skin and insulation systems. Heating of the airplane skin, insulation, and interior is by conduction and high-temperature radiation. The first impediment to escape is toxic pyrolysis products. Once internal ignition occurs by high temperature or a melted hole, the further fire spread is a typical enclosure fire. Effective hull protection that stays in place could provide ample egress time, and a comparison of the time to appreciable heating and degradation of the wall panels with the time for evacuation is important. In-Flight Fires Although there have been in-flight fires, in-flight scenarios have caused only a small fraction of fire deaths (Sarkos and Hill, 1989; Sarkos, 1995). For example, during the period from January, 1974, through September, 1989, 892 persistent fire or smoke events were reported for either on-ground or in-flight conditions, with 558 of these occurring in flight (Reynolds et al., 1991) of all of the reported persistent fire and smoke events, 20 progressed to the level of an accident with 9 occurring in flight. Fatalities resulted from 6 of the accidents. In-flight fire statistics are summarized in Figure 3-1. In general, in-flight fires that have resulted in fatalities have started in inaccessible areas of the interior. In response, Figure 3-1 Summary of in-flight and on-ground smoke and fire accidents (numbers in parentheses indicate number of deaths in fatal accidents). Source: Reynolds et al, (1991).

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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft the FAA has pursued research in hidden fire protection, enhanced emergency smoke venting, fire detection and advisory systems, and electrical arc tracking (Sarkos and Hill, 1989). The following conditions are considered in in-flight fire scenarios: Fire starts within the passenger compartment or lavatory. This is generally not considered serious since the fire is usually detected and extinguished before it can spread. Fire starts in inaccessible area. In-flight fires in inaccessible areas can be a serious concern because of the potentially long periods before passengers can be evacuated. For such a fire, the tenability of the cabin and the airworthiness of the aircraft could have to be maintained for up to three hours (for long overseas flights). Inaccessible areas include baggage compartments, concealed spaces behind the cabin liner, avionics compartment, electrical power center, battery compartment, cable trays, cable bundles, air-conditioning components, decompression system, and secondary or attachment structures. There is a potential for ignition in concealed spaces from an electrical fault or an overheated wire. The nearby materials would be exposed to a sustained but small hot spot or flame. The fire resistance of current materials is adequate to survive this threat. A common cause of in-flight fires is the ignition of materials that are not part of the aircraft design. Examples include trash, grease, oil, dust, and other accumulated matter left during maintenance operations. However, a catastrophic fire cannot generally be produced by a small amount of burning mass unless ignition and flame spread occur on the interior materials near the contaminants. Therefore, ignition, flame spread, and heat release rate of the interior materials are important fire properties. Since the atmosphere in interior areas outside of the passenger cabin is combined with a significant amount of cabin atmosphere and ventilated to the outside of the aircraft, a small amount of pyrolized gaseous products should not reach toxic level in the passenger cabin. Cargo (or baggage) compartment fires are complex compartment fires. Preventing a fire that may start in a cargo compartment from igniting other areas is very important. In the current jet airplane fleet almost all cargo compartments in passenger airplanes require compartment liners that are airtight and very effective fire barriers. Compartments less than 2,000 cubic feet do not have detectors or fire-suppression agents, and rely on their airtightness and small size to use up the available oxygen in the compartment to suppress a fire before it grows. Compartments larger than 2,000 cubic feet have smoke detectors and use fire-suppression agents and their airtightness to suppress fires. For larger cargo compartments such as those in freighters, one of the procedures used to fight fire is to release the pressure inside the pressure shell to that of the high-altitude atmosphere so that the oxygen content of the compartment is greatly reduced, thus slowing or extinguishing the fire. Fire stops and compartmentalization are important in containing hidden fires. Fire Dynamics and Fire Load Fire dynamics is the study of the interrelationships of the basic components in a particular environment in a fire situation. Some of the obvious components affecting fire dynamics are the materials, their characteristics and geometry, and the dynamics of the environment—ambient conditions, environment boundary conditions, occupancy status, ignition sources, fuel loads, and oxidizer quantity and sources. Important characteristics that influence fire dynamics are ignition temperature and flammability of the component and of the constituent materials. Component geometry can greatly affect ignition and flame spread. Thicker materials take longer to reach their ignition point than thinner materials. The configuration in which the material will be used (e.g., vertical or horizontal, or in a wall or comer) will influence the fire performance. The amount of material exposed is also significant in that a flammable material may be less prone to ignition if is surrounded by nonflammable or less-flammable materials. For example, materials that are located near fuel loads or ignition sources will be more likely to receive direct flame impingement if those fuels are the source of the fire. Other factors that affect the fire resistance of a material are surface roughness and material porosity or susceptibility to wicking. Generally, fire resistance of a component is improved with: increased material thickness (improves resistance to ignition and flame spread), decreased exposure of surface area to the environment and increased mass to surface area, lower physical location (closer to the floor) in a compartment, smoother surface and higher density, and higher resistance to ignition and lower flame-spread characteristics. Based on the considerations described above, it is possible to add up the fire load if the heat release on complete (or partial) combustion of the interior materials is computable (Hill, 1993). The estimation of expected toxic content is less accurate because their production depends on rate of heating, vitiation, and perhaps other conditions which are largely unknown (Levin et al., 1985). A potentially large, and difficult to define, fire load includes those items that are not considered a permanent part of an aircraft interior, including passenger clothing, carry-on items, and baggage. The passenger materials (papers and

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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft clothing) in the seats readily ignite and enhance the ignition of otherwise relatively safe seating. Once the fire has grown to produce a significant ignition source at the ceiling, stowage bin doors that may have been opened on impact or post-crash expose the interior materials which begin to pyrolyze and greatly enhance the ceiling flame extension. Estimates of fire loads are not adequate unless passenger clothing and baggage are included. FIRE MODELS An integrated computer modeling capability is necessary to evaluate and compare materials and predict their response in fires. Materials ignition and flame-spread models can be used to predict a material performance. Ignition and flame-spread models along with materials combustion considerations are described in Chapter 4. Global fire models can be used to improve the understanding of fire scenarios, to evaluate small-scale material test results and provide an estimation of full-scale performance, and to aid the development of fire performance goals. It is necessary to better understand the basic phenomena of fire and smoke propagation within enclosed spaces to provide the needed tools to establish performance goals and evaluate new fire-resistant materials. This requires the development of ignition and flame-spread models for use in a fire-growth model to understand combustion, fluid mechanics, and heat and mass transfer. Many fire protection computer models are currently available. A recent literature survey identified 74 models from 13 countries (Miller and Friedman, 1992). The two general types of models that are zone models and field models. They each have advantages and disadvantages, and hence are used for different applications. A description of the status of currently available fire models follows. A more detailed review is included in Appendix C. In zone models, compartments are subdivided into control volumes or zones. All quantities of interest are uniform within each zone; conservation of mass, energy, and momentum is applied to each zone using algebraic representations or ordinary differential equations. The advantages of zone models are the low memory requirements, speed, ability to represent large structures, structured output, and ease of use. The need for a priori assumptions and the inaccuracies that may result can also be viewed as a disadvantage. To date, zone models have considered relatively simple, frequently encountered fires that occur in rectangular rooms consisting of flat horizontal fuel beds, vertical walls, perhaps a ceiling, and furniture. Although the required modifications would not be difficult, they have not been equipped for irregular shape enclosures, nonflat ceilings, special fire-geometry interactions, or door-influenced internal air movements. Field models are based on the numerical solution to a set of partial differential equations representing conservation of mass, energy, and momentum for each chemical species. Field models are usually based on the division of a volume into a large number of computational cells and the application of finite difference techniques to solve for the appropriate quantities in each cell. These models can be used to provide very complex solutions and are often based on first principles, therefore requiring fewer assumptions than zone models. Field models are often used for ab initio calculations, can provide a more complete representation of the fire than zone models, and can make new phenomena easier to add. The disadvantages of field models are the requirements for large memory and set-up time, extensive interpretation, and the need for subgrid models (such as turbulence models) if the grid resolution is not sufficiently small. This type of model is computationally intensive and is generally only used when details of the fluid flow are needed. Toxicity models describe the toxic potency of fire atmospheres based on the toxicological interactions of the main combustion gases present (Hartzell, 1994; Levin et al., 1995; Purser, 1995). Rather than designate a specific combustion system, investigators have the flexibility of designing or choosing a system that will simulate conditions relevant to their fire scenario. One of the models, the N-gas model, is an empirical mathematical relationship containing six gases—carbon monoxide (CO2), carbon dioxide (CO), low oxygen (O2), hydrogen cyanide (HCN), hydrogen chloride (HCI), and hydrogen bromide (HBr) (Levin et al., 1987a, b, 1995). Results using the N-gas method have shown the good predictability of this approach (Braun et al., 1988, 1990, 1991; Babrauskas et al., 1990, 1991a, b). Considerations in modeling and testing of toxicity are included in Appendix D. Fire-hazard assessment models combine zone and field models with submodels for fire endurance, activation of thermal detectors or sprinkler systems, generation of toxic gases, evacuation, and survival models. However, current hazard models were not designed to be applicable to the cylindrical geometry of aircraft interiors or the particular fire scenarios described earlier in this chapter. Thus, their use to model an interior fire on an aircraft will take them outside their domain of applicability. Today's supercomputers, with their extremely high computational speed and massive storage capability, offer greater opportunity for computer modeling of fires. The arrays of differential equations (either ordinary or partial) that govern the fire phenomena can now be solved numerically. The first models were simple, but current models are building on the older models, incorporating more phenomena and producing more accurate results. As each new submodel (such as a combustion or gas radiation model) is added, the quality of the numerical solutions improves. To apply modeling tools to aircraft fires, modifications need to be made to account for the geometry and fire scenarios involved.

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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft As in all fire models, the definition of the fire will depend on the materials that are burning. New aircraft interior candidate materials will need to be extensively characterized to obtain the data necessary to input into these models. TESTING AND FIRE-SAFETY ASSESSMENT Several important properties should be considered when analyzing the burning process. The behavior of a given material is dependent not only on the properties of the fuel, but also on the fire environment in which the material is exposed. For a fire to occur, a fuel source, sufficient oxygen, and sufficient heat are all necessary. The material properties can be determined through small-scale testing in a laboratory environment. Since small-scale fire tests do not reproduce a fire, most tests do not reflect the hazard a material presents in an actual fire, but provide an indication of the behavior of the materials in an actual fire and a common point for comparison of materials. No single metric, and hence no one test method, is adequate to completely evaluate the fire hazard of a particular material system. For example, the testing procedure for evaluating composite material systems for naval submarine interiors (DOD, 1991) includes oxygen-temperature index, flame spread (ASTM E-162), ignitability (ASTM E-1354), heat release (ASTM E-1354), smoke obscuration (ASTM E-662), combustion gas generation (ASTM E-1354), and toxicity (N-gas method). Tewarson (1995) and Quintiere (1995) present typical overall characteristics of combustible materials that can presently be used to empirically rank the fire resistance of materials. Advances in the state-of-the-art testing for fire characteristics of materials and the associated developments in the field of mathematical fire modeling make quantitative evaluation of fire hazard feasible. Development of test procedures that allow scaling (e.g., calorimeters and lateral flame-spread methods from which combustion properties can be measured) enable prediction of the behavior of a material under many fire scenario conditions. For simple fire scenarios, hazard variables such as temperature, visibility, toxicity, and corrosiveness of smoke can be related to material properties, such as heat of combustion, heat release rate, smoke particulate yield, smoke extinction coefficient, and yields of combustion gases (e.g., CO2, CO, low O2, HBr, NO2, HCN, and HCl). Determining the properties of these materials can help to establish the principle flammability characteristics—ignitability, flame spread, rate of heat release, and smoke and fire-gas production—discussed earlier in this chapter. Two additional material parameters that are useful for material fire-hazard comparison are mass-loss rate and oxygen-temperature index. Mass-loss rate does not directly affect fire growth, but the measurement can be used to calculate the effective heat of combustion and, along with smoke obscuration data, is important for fire-growth-model calculations. Oxygen-temperature index is a measure of the percentage of oxygen required for a material to continue to burn at specific temperatures. As the temperature of a material increases, combustion of the material requires less oxygen. An oxygen-temperature profile can be obtained for the material. Toxicity screening tests for both acute and delayed effects are needed to evaluate the combustion products, including irritant gases of any newly proposed aircraft interior materials and products. Tests should be simple, rapid, inexpensive, use the least amount of sample possible (since, in many cases, only small amounts of the developmental material may be available), use a minimum number of test animals, and have a definitive toxicological endpoint for comparison with other material candidates. While faulty judgment and incapacitation of passengers in an aircraft fire are significant causes of worry since these conditions can prevent escape and cause death, they are complex endpoints that cannot be directly measured. Death of experimental animals (e.g., rats) is a more definitive and easily determined endpoint and can be used to compare the relative toxicities of alternative materials. Using lethality as the sole endpoint assumes that materials with greater toxicity based on a lethality endpoint will also cause more severe incapacitation and impairment. The number of experimental animals needed for such tests can be significantly reduced by utilizing one of the predictive mathematical models developed for combustion toxicology such as the N-gas model (Levin et al., 1995). Appendix D includes a detailed discussion of toxicity testing and modeling. Small-scale fire tests have historically been used as a means to screen materials and rank them on a relative basis. Current small-scale tests used for regulatory pass/fail criteria leave much to be desired in terms of being practical, rigorous, well-defined, and repeatable by interlaboratory and intralaboratory equipment or procedures. Progress is being made within the fire research community, including a substantial amount of activity sponsored and encouraged by the FAA (e.g., the International Fire Test Working Group), but additional effort is needed to continue to improve test and analysis methods. The results of any small-scale flammability test method for a material must be directly relevant to the fire scenario. For example, one possible aircraft fire scenario (described earlier in this chapter) is a fire caused by crash landing in which a large fuel fire occurs outside of an aircraft and penetrates through an opening (or openings) to an aircraft cabin. A critical factor in the escape of passengers is whether (or when) flashover occurs. As described earlier in this chapter, ignition, flame spread, and burning of interior materials might occur. Therefore, the small-scale test method should measure properties of the materials such as piloted ignition characteristics, burning rate, heat release rate, yields of soot particles and

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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft major toxic gases, burn-out time, flame-spread characteristics, and pyrolysis temperature and provide data to deduce global heat of vaporization properties. Much research has been aimed at developing means to predict the likelihood of flashover from laboratory-scale fire measurements of flammability characteristics and reduced-scale physical modeling (Pitts, 1994). Rather than being used exclusively as pass/fail screening tests, small-scale tests should be used to measure flammability properties of the materials that can be used as an input to theoretical models to predict fire hazard, described in this chapter and in Appendix C. Since the amount of sample may be limited, especially when testing new experimental materials, the small-scale tests should be designed for as small a sample as possible. However, sample size must be adequate to generate a turbulent flame, maintain a small-edge surface area to the surface area ratio, and allow measurement of flame-spread properties. Validation with Full-Scale Testing Aircraft fires are extremely complex for a number of reasons. Aircraft materials are complex mixtures of various polymers, and aircraft components are combinations of a number of materials. The chemical reactions that occur in the solid, liquid, and gaseous state of these materials are imperfectly known, and the gaseous species after pyrolysis and their subsequent reactions have not been fully clarified even in the simplest cases. In a real fire, the turbulent fluid motions, including circulation and mixing with air, further complicates the chemistry and resultant radiation production. Fire phenomena need to be better understood and characterized before computer models can be substantially improved. Systematic full-scale fire tests are needed to understand important physical processes such as the flow pattern, smoke movement, and fire growth in an aircraft cabin under the expected fire scenarios and to validate small-scale and theoretical models. Detailed measurements of temperature and concentration distribution of chemical species, flow velocity, radiant flux, and records of fire growth using video cameras are needed. The aircraft test configuration should be as realistic as possible, including as many of the interior components described in Chapter 2 as possible (e.g., ducts, wiring, seats, carpets, dividers, windows, and doors). Opening of doors should be considered as one of the parameters. Since the size of airplanes varies significantly, the effect of cabin size on fire-growth rate should be characterized so that optimum test facilities can be developed. Theoretical modeling of these full-scale fires (all types of models, such as zone and field models) should be conducted in close collaboration with the full-scale tests. The comparison of the experimental results with the predicted data would provide not only model validation but also guidelines regarding what experimental measurements are required and where they should be measured.