For this week’s assignment we had to either review
the chapters assigned to up or Air Detective tip #14. I decided to choose
chapter 8 because it is relatively associated with my topic of structural
failures. When broken down to the materials utilized for building an aircraft,
engineers and designers have to take in the ideas of what materials to use and
specific areas that high temp materials will be utilized. Aircraft materials
that are commonly utilized with aircraft construction have certain melting
points and an overall temperature range that has to be adhered to. Below in
Figure 1, is a reference chart of these common metals.
Figure 1. Melting Points of Common Aircraft
Materials Utilized for Aircraft Construction. Adapted from “Fire
Investigation.” by R. H. Wood and R. W. Sweginnis,
2006, Aircraft accident
investigation, 2nd ed., p. 67. Copyright 2006 by Endeavor
Books.
Aircraft
Materials
As aircraft over
time have been developed to withstand the structural forces associated with
flight, a need for metals to withstand the speed and temperatures associated
and thus a melting point must be higher. The author Pol Duwez (1954) stated,
“This immediately imposes a drastic limit on the possibilities. Of the 92
natural elements, not more than 20 have melting points above 3,000 degrees” (p.
100). He continued to test his theories and concluded that the intrinsic
property of metal cannot be altered be any means, the elements melting point is
set. But, by creating chemical compounds, this could be achieved and oxides and
carbides would be used for their property includes a high melting point. Now
for aircraft construction theses chemical compositions would have to retain all
physical strength and be chemically inert to high temperatures (Duwez, 1954).
Composite
Materials
A composite
material is the end result, a combination of different materials
(non-homogenous) that make it stronger then separate individual materials. When
the materials are combined especially in any metallic alloys, each separate
material in the combination will retain its mechanical, chemical and physical
property making it stronger, stiffer and lighter. This combination becomes a
matrix and reinforcement. But composites due tend to leave engineers with
headaches because of their high costs (Campbell, 2010). With those high costs,
different attributes of materials to make up the composites that engineers are
picky over, such as; elasticity and overall stiffness. The overall shape and genetic makeup of the
composites utilizes does help save costs though because they are well suited
for certain environments and the end buyers (Michaels, 2013).
The
two main elements that make up current aircraft structure for composites are
carbon fiber and fiberglass. These two items can be utilized to make up a
sandwich construction of layered non metallic or metallic core. But, it can be
utilized in conjunction with each other or separately. When utilized, the
aircraft becomes stronger and is more susceptible to withstand higher
temperatures of fire, but at 1,200 degrees Fahrenheit it will melt. Regardless,
the actual reaction of carbon fiber to fire will be depended on the resin
(resin burn point is 1,100 degrees Fahrenheit or less) utilized to bond the
fibers. In accordance with the authors, Wood & Sweginnis, (2006) “The
temperature at which liberates the fibers or reduces their structural
integrity…a characteristic frequently considered proprietary among
manufacturers” (p. 66).Since it pure carbon, the fibers will melt but in
general it will not decompose further. To understand more of what leading
builders, Boeing and Airbus are doing to bring their jumbo jets into the
future, please see figure 2 (Wood & Sweginnis, 2006).
Figure 2. The picture depicts leading manufactures,
Boeing and Airbus with their new class of jumbo jets and the genetic makeup of
their fuselage (structure). Adapted from “Their New Materials: Using the latest
alloys and composites is only part of the answer for today’s manufacturers.” by
D. Michaels, 2013, The
wall street journal, Copyright 2013 by The Wall Street Journal.
Aluminum
Alloys
Most
of aircraft taking to the skies today, their structures are comprised of 95%
aluminum alloy and 5% zinc or copper and other small amounts of trace elements.
When thinking about the chemical properties of the aircraft structures, the
alloying elements, associated stress on the structure, exposure time, the
overall temperature and the configuration (cast structure/heavy forged vs. thin
layers of paneling) will determine how it will behave in a fire(Wood &
Sweginnis, 2006).
Initial heating. Aircraft strength can be severely
jeopardized when exposed to heat. Regardless if the fire was a slow burning one
that started in the cargo hold, thriving for one of the elements of fire
(oxygen, fuel and heat), or a violent high temperature fire, it’s only a
function of time. Time of exposure is only a guess of an assumption that
engineers can determine when generating the correct chemical composition for
aircraft structural materials. Testing the alloy for hardness and make
temperature calculations, allows for them to predict the function of hardness
loss over the specified alloy to be utilized (Wood & Sweginnis, 2006).
Eutectic melting. Eutectic melting is when any of the
alloying metals or other mixtures yield their lowest melting point, but some of
the chemicals in the metal mixture remain a solid while the rest become a
liquid (Eutectic, 2011). Once this point is reached and the alloy is extremely
stressed, the “broomstraw effect” takes place. This phenomenon will resemble a
green stick fracture of a bone because the fibers in the metal will show signs
of delimitation in the failed area along the grain boundaries. At the accident site, the investigator could
determine that the stress was result of the impact and heating was prevalent
from an in-flight fire. If a part or component is under a high stress load during
the flight, heated then the “broom straw” fracture could not be 100% presumed
by the investigator to prove that an in-flight fire occurred. 890 degrees
Fahrenheit is aluminum alloys eutectic melting point (Wood & Sweginnis,
2006).
Melting. During an accident investigation, the
investigator could be mislead by structural bending of aluminum alloys because
at 850 degrees Fahrenheit, it becomes plastic and will begin sagging at post
impact fires. If the fire is burning hot enough, around 1,175 degrees Fahrenheit,
aluminum alloy will liquefy and at this point the molten remnants will follow
either gravity or if the plane is still in the air, the slipstream, see Figure 3.
Investigators would be able to determine the casual factors of molten aluminum
in the slipstream because tiny droplets will impinge on the structure of the
aircraft. For gravity now, aluminum droplets will be larger than that of what
slipstreams do (Wood & Sweginnis, 2006).
Figure 3. Temperature Ranges of Metals and Aftermath
Appearance. Adapted from “Fire Investigation.” by R. H. Wood and R. W. Sweginnis, 2006, Aircraft accident investigation, 2nd ed., p.
68. Copyright 2006 by Endeavor Books.
References
Campbell, F. (2010). Structural
composite materials. (p. 1). Materials Park, OH: A S M International.
Retrieved from http://site.ebrary.com.ezproxy.libproxy.db.erau.edu/lib/
erau/docDetail.action?docID=10439480
Duwez,
P. (1954, September). High temperatures: Materials. Scientific American,
191(3),
98-106. Retrieved from http://www.nature.com.ezproxy.libproxy.db.erau.edu/
scientificamerican/journal/v191/n3/pdf/scientificamerican0954-98.pdf
Eutectic. (2011). In The
American Heritage Science Dictionary. Retrieved from http://
ezproxy.libproxy.db.erau.edu/login?url=http://search.credoreference.com/content
/entry/hmsciencedict/eutectic/0
Michaels, D. (2013, June 17). Their
new materials using the latest alloys and composites is only part of the answer
for toda'ys manufacturers. The Wall Street Journal, Retrieved from
http://online.wsj.com/news/articles/SB10001424127887323844804578530982555671760
Wood, R., & Sweginnis, R.
(2006). Aircraft accident investigation. (2nd ed., pp. 61-74).
Casper, WY: Endeavor Books.
