Aluminium as a structural material

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Design properties of aluminium

Weight

The density r of pure aluminium compares as follows with steel [1]:

Pure aluminium ρ = 2,70 g/cm³
Structural steel ρ =7,9 g/cm³
The value for the alloys used for wrought products lies in the range 2,67-2,80 g/cm³.
A rounded value of 2,7 g/cm³ is normally used in design according to Eurocode 9 (EN 1999-1-1).

Figure 1 – Volume per unit weight of aluminium and other metals [2]

Elastic constants

Modulus of elasticity

Aluminium is a springy metal with a relatively low modulus of elasticity (E). For the pure metal at room temperature it compares with steel as follows [1]:

Pure aluminium E = 69 kN/mm²
Structural steel E = 205 kN/mm²
The value for wrought alloys lies in the range 69–72 kN/mm².
For design purposes Eurocode 9 (EN 1999-1-1) adopts the figure of E = 70 kN/mm².

Poisson’s ratio

For design purpose Eurocode 9 (EN 1999-1-1) adopts the figure of ν = 0,30.
The corresponding figure for the shear modulus (G), based on the above values of E and v, is 2,7.
The elastic modulus of aluminium is only a third of the modulus of steel (Figure 2). This has essential consequences for the geometry of the design, since deflections of beams, bearing capacity of columns, i.e. lateral buckling and local buckling directly depend on the elastic modulus.
If a steel section is to be replaced by aluminium and the stiffness is to be kept at the same level, a thickening of all parts by the factor 3 is not very efficient, since the relation of the specific weight of the two materials is also approximately 3 to 1.
For the design of beams a practical and proved rule says: increase all dimensions with exception of width by the factor 1,4 and you will arrive at a cross section with a moment of inertia about three times as large and hence a section of the same stiffness (E × I) and you will save about 50% in weight (Figure 3) [4].

Figure 2 – Plastic Yield Behaviour of Aluminium and Mild Steel [2]

Figure 3 – Comparison between four beams
which will give the same deflection [3]

Thermal properties

Thermal expansion

The coefficient of linear expansion a for pure aluminium at room temperature compares with steel as follows:

Pure aluminium α = 23,5 × 10^-6 per °C
Structural steel α = 12 × 10^-6 per °C
The value for the wrought alloys lying in the range 22,0–24,5 × 10^-6 per °C.
Eurocode 9 (EN 1999-1-1) adopts the rounded value of 23 × 10^-6 per °C for use in design.
The linear thermal expansion is twice as large as that of steel. This has to be taken into account for many structures, where free thermal expansion is necessary.
Where expansion is restricted the resulting stresses are due to the smaller E-modulus only 2/3 compared to steel [4].

Thermal constants

The thermal conductivity of the pure metal at room temperature being about four times the figure for steel.
The conductivity is much reduced by alloying, down 50% for some alloys.
The specific heat of pure aluminium at room temperature is about twice the steel.

Influence of elevated temperature

The strength of aluminium decreases with increasing temperature (Figure 4).
With temperatures up to 80 °C the drop in strength is negligible for all alloys and tempers [4, 5].
Over 80 °C some design situations could require creep effects to be considered [4, 5].
Heat-treatable alloys begin to loose strength at temperatures over 110 °C depending on time [4].
Non-heat-treatable alloys in work hardened tempers begin to loose strength at temperatures over 150 °C – whereby the loss of strength also depends on time [4].
In ‘O temper’ nonheat-treatable alloys no permanent loss in strength occurs [4].

Influence of heat of welding

The local melting in the vicinity of the weld often forms an important aspect of the verification of the design of a structure (Figure 6).
The non-heat-treatable alloys loose all strength gained by work hardening and return to the ‘O temper’.
The heat-treatable alloys in temper T6 have a loss of approximately 40% of their strength with the single exception of such alloys as 7005 or 7020, which looses only 20% [4].

Figure 4 – Tensile Strength of 2014-T6 Tested at Room Temperature
after Exposure at Elevated Temperature [2]

Figure 5 – Reduction of strength in the heat affected zone (HAZ) (typical for EN AW-6082) [4]

More comparison of aluminium with steel

Its mechanical properties tend to be inferior to those of steel, the stronger alloys being comparable in strength but less ductile.
The approach to structural design is much the same for the two metals, but there are some differences [1].

The advantages

Lightness

Aluminium is light, one third the weight of steel.

Non-rusting

Aluminium does not rust and can normally be used unpainted.
However, the strongest alloys will corrode in some hostile environments and may need protection.

High corrosion resistance is due the fact that aluminium and aluminium alloys react with oxygen and water vapour in the air to produce a thin, compact oxide film which protects the underlying metal from further attack. So aluminium and most of the copper free alloys prove to be very corrosion resistant if the pH-value of any contact liquid lies between 5 and 8; with this range the most existing atmospheric/environmental conditions are covered [4].

Extrusion process

This technique, the standard way of producing aluminium sections, is vastly more versatile than the rolling procedures in steel. It is a major feature in aluminium design.

Weldability

Most of the alloys can be arc welded as readily as steel, using gas shielded welding.
Welding speeds are faster.

Machinability

Milling can be an economic fabrication technique for aluminium, because of the high metal removal rates that are possible.

Glueing

The use of adhesive bonding is well established as a valid method for making structural joints in aluminium.

Low-temperature performance

Aluminium is eminently suitable for cryogenic applications, because it is not prone to brittle fracture at low temperature in the way that steel is.
Its mechanical properties steadily improve as the temperature goes down (Figure 6).

Figure 6 – Tensile Properties of 6061 Alloy Heat Treated, Artifically Aged [2]

The disadvantages

Cost

The metal cost for aluminium (sections, sheet, plate) is typically about 1,5 times that for structural steel volume for volume. For aircraft grade material, the differential is much more [1].
However, fabrication costs are lower because of easier handling, use of clever extrusions, easier cutting or machining, no painting, simpler erection.

Buckling

Because of the lower modulus, the failure load for an aluminium component due to buckling is lower than for a steel one of the same

Effect of temperature

Aluminium weakens more quickly than steel with increasing temperature.
Some alloys begin to lose strength when operating above 100°C.

HAZ softening at welds

There tends to be a serious local drop in strength in the heat-affected zone (HAZ) at welded joints in some alloys.

Fatigue

Aluminium components are more prone to failure by fatigue than are steel ones (Figure 7).

Thermal expansion

Aluminium expands and contracts with temperature twice as much as
However, because of the lower modulus, temperature stresses in a restrained member are only two-thirds those in steel.

Electrolytic corrosion

Serious corrosion of the aluminium may occur at joints with other metals, unless correct precautions are taken. This can apply even when using alloys that are otherwise highly durable.

Deflection

Because of the lower modulus, elastic deflection becomes more of a factor than it is in steel. This is often a consideration in beam design.

Figure 7 – Difference in Fatigue Behaviour between
Mild Steel and Aluminium Alloys [2]

Sources:

Aluminum Structures: A Guide to Their Specifications and Design – J. Randolph Kissell, Robert L. Ferry
TALAT Lecture 1501
TALAT Lecture 2204
R. Gitter Design of Aluminium structures: Selection of Structural Alloys, EUROCODES – Background and Applications – 2008
Eurocode 9 (EN 1999-1-1)