AOPA Pilot Magazine - February 2018 - 94

P&E
Stress
(PSI)

SAVVY MAINTENANCE
Proportional limit

Ultimate strength

Yield point

Stress
(PSI)

High-carbon steel

Rupture

E = Elastic limit
Y = Yield point
U = Ultimate limit

Mild steel
Y

U
E

Elastic zone
(Reversible)

Plastic zone
(Irreversible deformation)

A TYPICAL stress-strain diagram for metal.

SOFT, HARD, AND TOUGH

The simplest elemental metals such as iron
and aluminum and copper are relatively soft
and ductile. Their elastic limit is relatively
low, so it doesn't take much force to deform
them permanently. Where higher strength
is required, the elemental metal is usually
alloyed by adding a relatively small amount
of one or more additional elements that
strengthen the metal.
It doesn't take much. Adding just a few
percent (by weight) of carbon turns soft iron
into high carbon steel with 10 times the tensile strength. Adding about 4 percent copper
to commercially pure aluminum creates
2024 aluminum alloy (the most common
one used in aircraft) that's more than four
times as strong. These alloying elements
work their magic by invading the crystalline
structure of the metal in a way that makes
the resulting lattice of atoms much denser
and harder to dislodge.
The graph above shows what happens to
the stress-strain curve of iron when carbon
is added to form steel. Notice how the linear
portion of the curve gets much steeper, indicating that steel is much harder than iron-it
can handle much greater stress without permanent deformation, and a given amount of
stress causes far less strain.
But also notice that as more carbon
is added to create high carbon steel, the
well-defined "knee" at the yield point
that is seen in iron and mild steel tends to
94 | AOPA PILOT February 2018

Wrought iron

Strain
(% Elongation)

HERE'S WHAT HAPPENS to the stress-strain curve of iron when

a bit of carbon is added to form steel. In the elastic zone, deformation is reversible. After the yield point, plastic deformation is irreversible.

disappear. This means that high carbon
steel is more prone to sudden failure without warning-it's more brittle and can't
bend much before it breaks.
In some applications-such as cylinder
barrels, crankshaft journals, cam lobes, and
tappets-hardness is all-important for maximum service life. In other cases-wing spars
and landing gear legs, for example-toughness (the ability to bend without breaking)
is crucial. Metallurgists spend a lot of their
time coming up with the optimal tradeoff
between hardness and toughness for each
metal airplane part.
Iron also can be alloyed with chromium and/or nickel to form various flavors
of stainless steel. These generally are not
nearly as strong as carbon steel, but they're
much more resistant to heat and corrosion.
These chromium and nickel varieties are
often used in exhaust systems, heat shields,
and firewalls.
Aluminum can also be alloyed with
various elements to improve its structural
properties. Copper is added to create the
2000-series alloys most commonly found
in aircraft structures, providing excellent
strength-to-weight ratio and good fatigue
resistance. The biggest downside of this
alloy is that it is far more vulnerable to corrosion than pure aluminum. Consequently,
it is generally protected by an anodized finish or with a thin layer of pure aluminum
(Alclad).

Other aluminum alloys used in aircraft
include the 6000 series (containing magnesium and silicon) and the 7000 series
(containing zinc)-the former is highly
corrosion-resistant, while the latter is the
strongest of all aluminum alloys.
HEAT-TREATING

The strength, hardness, and toughness
of alloys like carbon steels and aluminum-copper alloys can also be profoundly
affected by heat-treating. Anyone who has
built model airplanes probably is already
familiar with this.
Suppose you want to make a springsteel landing gear for your model out of a
length of high carbon steel "piano wire"
available at any hobby shop. As it comes
from the store, piano wire is springy and
tough-perfect for a landing gear-but it's
also extremely difficult to bend and form
without breaking. The solution most modelers use is to heat the wire with a torch
until it becomes a bright cherry red (about
1,400 degrees Fahrenheit), and then let it
cool slowly. This is called annealing and
transforms the steel wire into a soft, nonspringy form that can easily be bent and
formed to the desired shape.
Of course, the annealed wire is way
too soft to be suitable landing gear material, but that can be easily rectified. Once
the gear is bent to shape, the next step is
to heat-treat the wire by heating it back

AOPA Pilot Magazine - February 2018

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