Composite Materials - Department of Aerospace Engineering ...

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Composite
Material Fires

Eric Stetz

3/27/2013

Aviation Fire Dynamics

Overview


What are Composite Materials?

o
Definition

o
Examples


How are Composite Materials used in Aviation?


How Composite Materials Burn

o
P
yrolysis


Structural Behavior of Burned Composites

o
Experimental Buckling
o
f Heated Laminate

o
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.

What are Composite
Materials?

Some qualifications include:

1.
The material is manufactured, and/or the
constituents are combined in some fashion that is
designed or engineered.

2.
The material consists of two or more physically
and/or chemically distinct phases with an interface
separating and connecting them.

3.
The material has characteristics that are not
depicted by any of the components in isolation.

4.
The material must contain a sufficient amount of
each phase/constituent, at least 5%.

[1][2]

What are Composite
Materials?


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) 121
-
127

What are Composite
Materials?


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]

What are Composite
Materials?


Some Modern Constituent materials used for
Composite Construction include:

o
Matrix Materials


Metal Matrix Composites


Ceramic Matrix Composites


Polymer Matrix Composites

o
Reinforcement Materials


Glass Fibers


Boron Fibers


Carbon Fibers


Organic Fibers


Ceramic Fibers


Whiskers

[1]

What are Composite
Materials?

Metal Matrix Composites


Advantages of Metals as a matrix material

o
Strong and Tough

o
Ductile

o
Resistant to fire

o
Good electrical and thermal conductivity

o
Do not outgas


Disadvantages of Metals as a matrix material

o
E
xpensive

o
Difficult to fabricate



What are Composite
Materials?


Metal Matrix materials are usually light metals
including:

o
Aluminum

o
Magnesium

o
Titanium

o
Cobalt

o
Cobalt
-
Nickel alloy

What are Composite
Materials?

Ceramic Matrix Composites.


Advantages of Ceramics as a matrix material

o
High Hardness

o
High Elastic Modulus

o
Low Density

o
High Temperature Resistance


Disadvantages of Ceramics as a matrix material

o
Brittle

o
Low Failure Strain

o
Low Thermal Shock Resistance

o
Low Tensile Strength

What are Composite
Materials?


Some examples of ceramic materials used as matrix
materials include:

o
Borosilicate Glass

o
Soda Glass

o
Mullite

(Porcelain)

o
Magnesium Oxide

o
Silicon Nitride

o
Aluminum Oxide

o
Silicon Carbide

[1]


What are Composite
Materials?

Polymer Matrix Composites


Some Advantages to using polymers as a matrix
material include:

o
Cheap

o
Easy to Produce


Some Disadvantages to using polymers as a matrix
materials include:

o
Low Strength and Modulus

o
Lower temperature limits

o
Easily degradable in light or solvents

o
Higher CTE then metals and ceramics

What are Composite
Materials?


Some examples of polymers used as matrix
materials include:

o
Thermosets


Epoxy


Polyester

o
Thermoplastic


Nylon


Polycarbonates


PET, PBT


Polyetherether

ketone (PEEK)

[1]

What are Composite
Materials?

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]

What are Composite
Materials?

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]

What are Composite
Materials?

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 (1.6
-
2.0
g/cm
3
). 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
]

What are Composite
Materials?

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]


What are Composite
Materials?

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]


What are Composite
Materials?

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]


http://
www.acm
-
usa.com
/silar
-
sc
-
9m/

How are Composites
Used in Aviation?


C
omposite 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.


How are Composites
Used in Aviation?

Composite Material Use in Commercial
Transportation Aircraft Over Time

[3]

How are Composites
Used in Aviation?


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.

How Composite Materials
Burn


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.


How Composite Materials
Burn


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 CH
1.3
O
0.2
.


T
he specific heat and enthalpy of this product gas
can be assumed to be similar to methane (CH
4
)
because the specific heats are similar. [8]


How Composite Materials
Burn


The following reaction models the pyrolysis gas that
leaves the composite
,


CH
1.3
O
0.2
+1.225(O
2
+3.76N
2
)


CO
2
+0.65H
2
O+1.225(3.76)
N
2



The
DesJardin

paper estimates a heat of combustion of
Δh
C
=28.8 kJ/g.


This leads to a heat of formation for the pyrolysis gas of,

h
°
C
H1.3O0.2
=
-
4.5785 kJ/g



and an adiabatic flame temperature of,

T
ad
=2300 K

[8]

How Composite Materials
Burn


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]

How Composite Materials
Burn


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]


How Composite Materials
Burn


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]

Structural Behavior of
Burned Composites


When heated to temperatures of only 80
-
150C,
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]



Structural Behavior of
Burned Composites


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]

Structural Behavior of
Burned Composites


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]

Structural Behavior of
Burned Composites


Heat fluxes of 10, 25, and 50 kW/m
2

were applied to
a carbon reinforced epoxy laminate while it was
under constant compression load for a variety of
load forces. [5]



[5]

Structural Behavior of
Burned Composites


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
/m
2

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
/m
2

case heated the laminate above both the
epoxy viscous softening temperature and the matrix
decomposition temperature (300
-
450
°
C), but not above
the fiber oxidation temperature.


The 50 kW/m
2

case heated the laminate enough that it
reached the viscous softening temperature, the matrix
decomposition temperature and the fiber oxidation
temperature (>500
°
C). [5]


Structural Behavior of
Burned Composites


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]

Structural Behavior of
Burned Composites


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.

o
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 2
nd

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.
2024
-
2033

[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. 529
-
533

[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. 78
-
89

[7]
Burns, Lauren, “Fire
-
Under
-
Load Testing of Carbon Epoxy Composite,”
47
th

AIAA Aerospace Sciences Meeting
, Orlando, Florida,
2009

References

[8]
McGurn
, Matthew,
DesJardin
, Paul, “Modeling of Charring and
Burning Carbon
-
Epoxy Composites in Fire Environments,”
50
th

AIAA
Aerospace Sciences Meeting
, Nashville, Tennessee, 2012

[9]
Czarnecki
, G. J.,
Ripberger
, E. R.,
Meilunas
, R. J., Milan, W., “Thermal
Degradation of Composites,” 52
nd

AIAA Structures, Structural Dynamics
and Materials Conference, Denver, Colorado, 2011


Questions?