aluminium in commercial vehicles - European Aluminium Association

EUROPEAN ALUMINIUM ASSOCIATION ALUMINIUM IN COMMERCIAL VEHICLES CHAPTER VI 59
4. Aluminium versus Steel
Both steel and aluminium are
metals with relatively high
strength. Both materials are
incombustible and will not contribute
to a fire. For structural
purposes the main differences
are:
Elasticity: The modulus of elasticity
(E-modulus) of aluminium is
1/3 of that of steel. This means
that an aluminium beam with
the same cross-section and the
same loads as a steel beam will
have a deflection 3 times that of
the steel beam.
Weight: The density of aluminium
is 1/3 of that of steel. This
means that a steel beam will
weigh 3 times more than an aluminium
beam with the same
cross-section.
Welding: When welding a hardened
aluminium alloy some of
the hardening effects will be lost.
The strength in the heat affected
zone (HAZ) will be reduced. This
reduction depends on the alloy,
temper, type of product and
welding procedure. Ordinary
steel has no strength reduction
after welding.
Thermal elongation: The coefficient
of thermal elongation of
aluminium is twice that of steel.
This means that an aluminium
member will get twice the thermal
elongation as a similar steel
member with the same temperature
difference. Since the elastic
modulus of aluminium is 1/3 of
steel, the stresses in an aluminium
member with fixation are 2/3 of
that in a similar steel member.
Most of the structural aluminium
alloys have relatively high
“strength-to-E modulus” ratio.
This effect is especially clear
when the aluminium alloy is
strain-hardened or heat-treated.
Structural aluminium alloys have
roughly twice the “strength-to-E
modulus” ratio than standard steel.
However, when compared with
high strength steels, structural
aluminium alloys have about the
same “strength-to-E modulus”
ratio. It should also be noted that
the elastic modulus of an alloy
mainly depends on its parent
metal. In other words, all aluminium
alloys have very similar Emodulus,
but this is also valid for
steel alloys. Consequently, the so
called “high strength steels”
don’t have better elastic properties
than mild steel.
Steel designers often use the
strength of the material as governing
criteria when desig ning a
steel struc ture and check afterwards
whether the deflection is
within the requirement.
When designing an aluminium
structure, it will often be the
deflection criterion that will be
governing. For that reason, the
design procedure will start with
the deflection criterion and it will
be checked afterwards if the
stress or the resistance of the
structure is within the limits.
The deflection of members under
bending load depends on the
modulus of elasticity (E) and on
the moment of inertia (I) together
with the load and the span.
With the same span and load, it
will be the product E x I that will
determine the deflection.
To get the same deflection of
steel and aluminium beams in
bending, the moment of inertia
of the aluminium beam must be
three times that of steel. If the
increase in the moment of inertia
is to be done only by increasing
the thickness of the web and
flanges, the aluminium beam will
have the same weight as the
steel beam.