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concentration of single carbon fibers. Carbon-epoxy composite parts weighing about 45 kg were burned in 10.7 m
diameter JP-4 jet fuel pool fires for 20 minutes. The fire-released fibers were collected using an array of filters
suspended inside the smoke plume at a height of 40 m. The filters consisted of stainless steel canisters with stainless
steel mesh to trap the fibers carried in the smoke plume. Collected fiber samples from these tests were analyzed
for fiber count and size distribution by optical and electron microscopy.
The results showed that fibers were released in several forms ranging from single fibers to large fiber clumps and
fragmented pieces of composite laminate [23]. Single fibers constituted less than 1 percent of the carbon fiber mass
initially present in the composite. Under certain conditions involving thin composites with turbulence (e.g., air blast
or explosion) the total number of single fibers released from burning composite parts increased significantly. There
was a threefold increase in the total mass of collected single fibers under the turbulent fire conditions [22-23].
Microscopic fiber analysis revealed that fiber diameter was significantly reduced in the fire due to fiber oxidation and
fibrillation.
Overall, the collected fibers had a mean diameter in the range of 4.2 μm versus 7μm for the virgin fibers. At extreme
flame temperatures (> 900 C) and under oxygen-rich test conditions, large amounts of fibers were completely
consumed through oxidation. The fiber diameters were reduced drastically inside the flame after the fibers were
released from the composite. Sussholz [24] determined that reduction in the fiber diameter in fires occurred due to
partial surface oxidation and fibrillation effects–splitting of fibers into smaller, finer fibrils due to surface pitting and or
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surface flaws. The main reasons for fibrillation of carbon fibers were found to be the presence of sodium impurities
and morphological flaws such as voids in the fiber structure. Elemental analysis of the carbon fibers confirmed the
presence of sodium impurities. The surface oxidation effects were significantly more pronounced in the regions of
low crystalline density.
Considering the potential health implications from inhalation of micron-sized fibers, NASA conducted a scanning
elec-tron microscopy (SEM) study for physical characterization of the fibers. Seibert [25] summarized the results of
the SEM analysis for respirable fibers (diameter <3 μm, length <80 μm) which constituted less than 24 percent of
the total fibers released from burning composites. The respirable fibers had an average diameter of 1.5 μm and
were 30 μm long. Overall, the fiber sizes spectrum ranged from ≈ 0.5 to 5 μm in diameter. To quantify the concentration
of respirable fibers and determine the potential exposure levels, Sussholz [35] estimated that an aircraft fire
involving fiber composites would release 5x10 respirable fibers per kilogram of carbon fibers released (or five
percent by weight). This quantity corresponds to an estimated peak exposure of 5 fibers/cm within the smoke
plume. This exposure is only half the permissible OSHA limit [26] for time-weighted average (TWA) over an 8-hour
period for asbestos fibers.
The fiber concentration was also measured directly via sampling of fibers from the smoke plume in large-scale tests
conducted at Dugway Proving Grounds [22]. All fibers with an L/D ratio greater than 3 were counted and fiber
concentrations were determined for the 20-minute burn time. The results indicated an average fiber concentration of
less than 0.14 fibers/cm . This is ten times lower than the OSHA mandated permissible exposure limit (PEL) of 1.0
f/cm for short term exposure, averaged over a sampling period of 30 minutes [26]. Lacking evidence of any known
pathological effects, the authors concluded that carbon fiber exposure should be treated in the same manner recommended
by NIOSH for fibrous glass [27].
The U.S. Coast Guard (USCG) conducted a series of tests to characterize the graphite fibers emitted from burning
graphite/epoxy composites [28]. The study primarily focused of on the size distribution of fibers released during
small- and large-scale burn tests with advanced composites. The laboratory-scale tests were conducted using the
Cone calorimeter at 50 and 75 kW/m 2 on composite parts from an HH-65A helicopter. A modified sampling system
was used in the cone calorimeter to maximize the collection of fibers after the epoxy resin was completely burned.
The fiber size distribution was determined through SEM analysis. The study revealed that 23 percent (by weight) of
the fibers generated were in the respirable range. Overall, the fiber diameter ranged between 0.5-9 μm and the
　
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