Lightweight Construction Possibilities of Lightweight Material Construction
Although lightweight construction helps to reduce weight, the rapid success of mere material substitution hides the fact that it's not that simple at all. Magnesium plays an important role in this context, as it is most suitable for castings.
Lightweight construction can be defined as the effort to change a design in such a way that the quotient of useful weight to dead weight improves without having a negative impact on functionality resulting from this change. One goal could therefore be to increase the payload of a vehicle without increasing its dead weight; or to reduce the weight of the vehicle to the same possible payload without compromising its function as a safety, comfort and assistance system.
Lightweight construction has been a hot topic for years. However, although lightweight construction usually results in higher weight reductions than a mere substitution of a material could achieve, in practice, designers try to use different materials in lightweight construction. But a simple material exchange is not a satisfying solution, because another material usually requires a different design (e.g. more installation space or different radii) and/or production, e.g. forming, original forming or joining technology.
From a technological point of view, a combination of lightweight construction and materials has the greatest potentials. Therefore, anyone who wants to reduce the weight of components must be familiar with the limitations of lightweight material construction.
Quality Indicators Facilitate Assessment
If one focuses on the mechanical properties for a start, it is customary in lightweight construction to consider the specific, density-related property values of a material. For simple analytically solvable stress cases such as tension/compression, bending, torsion, buckling of bars and buckling of plates, it is quite easy to show that the required mass depends on geometric and material parameters.
If the geometry is assumed to be given, which from a technological point of view often makes sense — for example the given length and the installation space of the construction — the final weight depends on specific material properties, which are referred to as quality characteristics for these load cases and can be found in the tables provided by lightweight literature.
These tables, however, often use average material properties to indicate an average quality index. But if these parameters are taken as a basis for material preselection, provided properties are not adequate since many materials have such a wide range of properties that the mean values are not suitable for a specific case. While at least in the case of metallic materials some values, such as modulus of elasticity or density hardly change with the alloy composition or heat treatment, this does not apply at all to other important values such as tensile strength.
Standardized to the Characteristics of Steel
At the top of figure 1 a table shows the quality indicators that reflect the range of property variance of a material. The table illustrates that the modulus of elasticity and density of composite materials such as wood or fiber-reinforced plastics, as well as their proportions (fiber density) and geometric orientation to the load (fiber orientation, layer structure with several layers) vary considerably in some cases. This is because fiber composites are not actually materials, but they are rather constructions consisting of fibers orientated in a matrix.
The table has been standardized in accordance with the properties of steel, the most commonly used material in mechanical engineering. Therefore, all table values directly indicate the weight differences of a geometrically similar construction made of a certain material compared to steel. For example, when a GRP strut is designed for strength, it can be 1.19 to 15.86 times lighter than a steel strut, depending on which alloys or fiber composite is used as a cover. However, if the strut has to be designed for rigidity, it is only 0.37 to 0.93 times lighter in GRP, i.e. at least 7 % heavier.
An interesting fact is that with regard to the quality index for longitudinal rigidity, all metallic construction materials are equally suitable. For this reason, a material substitution offers no advantage — only CFRP enables a lighter construction under these conditions.
Since the tables frequently used in literature are not very descriptive, the lower part of figure 1 again illustrates these correlations. The tables or corresponding diagrams, as shown in the lower part of figure 1, provide a clear classification of a material’s suitability for certain load cases. However, the mechanical property values, which can be defined by quality indicators, are not the only aspects that have to be considered during the selection of materials. Further technological characteristics that have to be considered as well include fatigue strength, elongation and toughness at break, thermal stability or coefficient of thermal expansion as well as other material properties such as corrosion resistance, formability, separability and joinability and others, which are beyond the scope of this article.
Lightweight Construction Must be Economical
In industrial terms, lightweight construction is not only the technically best compromise between weight and other technological requirements — it must also be economical. For cost reasons, most mechanical engineering products are made of metal. The most important materials are steel, aluminium and magnesium. Their differing material properties not only have an influence on the quality indicators but also on the processability of semi-finished and finished products and the associated costs.
Sheet Metal as Ideal Semi-Finished Product for Steel
Steel as the traditional mechanical engineering material can be excellently rolled and processed into plates and sheets. Thanks to these characteristics, it is available in an almost unmanageable variety and relatively inexpensive. It’s not a surprise that plates and sheets made of steel are used in many constructions. For this reason, engineers often try to keep this semi-finished product in the processing and manufacturing chain when materials are exchanged as part of lightweight construction efforts. But this approach doesn’t take the whole picture into consideration: other types of metals are also processed into semi-finished products and enable other manufacturing processes.
Figure 2 illustrates these possibilities. The matrix shows the three most widely used metallic construction materials and the three most important manufacturing processes (of semi-finished products). The suitability of the three materials for the individual processes is classified by means of a traffic-light system. The criteria for this assessment are given as keywords in the fields.
First of all, it is striking that for aluminium and magnesium there is a competing process for the production of profiles and tubes, i.e. longitudinally oriented components, which is not used industrially for steel: extrusion.
While sheet metal is the most superior semi-finished product of steel, this is not the case for the other two materials: a sheet metal construction made of aluminium is always directly competing with a thin-walled casting equipped with similar properties; extrusion is also the technologically superior manufacturing process for longitudinally oriented components and it is more cost-effective.
Magnesium Most Suitable for Castings
With regard to magnesium, the image familiar from steel is changing completely: casting (or injection molding for smaller components such as mobile phone, camera or laptop housings) is superior to the production of magnesium sheets in terms of technology and costs in large series production, because magnesium sheets are very complex and technologically difficult to produce: for example, complex thin-walled components can be produced at weight-related costs whereas magnesium sheets are hardly available as a semi-finished product.
Economical lightweight construction is therefore a task that requires the designer to have considerable knowledge in the areas of material properties, but also production technology and the costs involved. Companies and designers who want to be economically successful in this field must therefore close these gaps.
*Professor Dr.-Ing. Jochen Dörr is the head of the subject areas "lightweight construction, product development, design theory" at the university of applied sciences Ostwestfalen-Lippe.
This article was first published by konstruktionspraxis.