Are service bodies best made from aluminium or steel?

For any equivalent quantity of metal, aluminium is lighter but steel is significantly stronger, so how do you decide which path to go down when choosing service bodies for your fleet? The answer is not as simple as you think.

Many factors will decide which metal is best suited to any given purpose - what it is used for, how it is manufactured, the environment in which it will be used, what life expectancy is required and many other real-world issues need to be taken into account. Weight, strength, hardness, stiffness and a range of other variables need to be considered.

Weight

As a reference point, water has a specific gravity (sg) of 1.0 g/mm². Aluminium has an sg of 2.6 g/mm² and steel one of 7.8 mm/g². Obviously, this means that steel is heavier, but that’s only half the story. It is the comparative strength-to-weight ratio of metals that is the critical factor in most applications, including the manufacture of service bodies.

Strength

Strength is commonly defined as the maximum load to which a material can be subjected without bending, or yielding. Most aluminium alloys used in general service body construction (6061-T6 series) will yield under a force of between 275 to 310 MPa.

Kilo for kilo, 6016-T6 is stronger than some steel alloys, yet not as strong as others. For every given high-strength alloy, there are higher strength steels that will outperform aluminium alloys in strength-to-weight comparisons.

Additional factors to strength and weight eventually dictate which material is a better choice for a particular application. Aluminium can be made to match the strength of steel through the addition of extra fabrication, but it requires almost three times the amount by volume to equal the strength of steel, thereby eliminating the weight savings offered by aluminium.

Stiffness

Stiffness refers to how much a material bends when a load is applied. Each metal’s stiffness is quantified by a parameter called the Modulus of Elasticity, usually measured in gigapascals (GPa). Aluminium’s modulus of elasticity is about 69 GPa, versus the significantly higher (around 3 times higher in fact) 200 GPa demonstrated by steel.

Regardless of the type of alloys used for either metal, the modulus of elasticity for each varies only slightly. High-strength aluminium alloys offer almost no corresponding increase in stiffness and steel still outperforms them all by a factor of nearly 3:1.

In practical terms, that means that the wall thickness of an aluminium tube or plate needs to be three times as thick as its equivalent in steel to achieve the same degree of stiffness that steel provides.

Hardness

The hardness of a material is defined by the relative resistance that its surface offers to penetration by a harder body. Because of their inherent lack of stiffness, aluminium panels have relatively low ‘bounceback’ resistance and will dent quite easily. Steel has considerably greater strength and resistance to denting and deformation than is commonly experienced in everyday working environments.

A panel that dents easily will often deform and twist as a result. Twisted panels that don’t close properly and are prone to leakage are the most common complaints about aluminium service bodies.

Reactivity

Aluminium is more inert than steel and resists corrosion in normal environments. It does, however, still require protective coatings in environments such as coastal areas where saltwater corrosion is commonplace.

Aluminium’s inert nature also makes it very difficult to securely apply protective coatings as the surface makes adherence difficult.

Welding

Aluminium welds well, but the area around the weld has significantly lower yield strength than the base material. The Alcoa Structural Handbook recommends that, without proper heat treatment after welding, the design strength for the aluminium adjacent to any weld should be taken at just 75 MPa - a decrease in the average strength of the base metal of around 70%.

With the exception of hardened and exotic grades, mild steel that is welded or brazed drops only a fraction of its strength at and around welding points.

Steel spot-welds very well, while aluminium requires large TIG welds or complex and expensive manual fastening to attain the necessary strength.

Weighing in

The low weight of aluminium comparative to steel may make it seem like a better option from a fleet manager’s point of view; less weight equals less fuel, right? With so many other influencing factors, it’s imperative to understand how the vehicles will be used, in what type of environment and for what expected timeframe?

Steel and aluminium both have distinct advantages and disadvantages, so it’s important to work with a manufacturer who understands the nature of both and can determine which material should be used where for the right reasons. Only then can fleet managers be sure they are minimising maintenance, downtime and replacement costs, while experiencing increased safety, reliability and long service life and the return on investment they demand.