Sometimes it is important to look at more than one materials-selection map
and merit index. For instance on comparison of the use of steel and aluminium
as a material for aircraft, the merit index 
for loading a tie in tension shows that steel outperforms aluminium in this
context. (Of course, as shown on the maps, both steels and aluminium alloys
are families of materials with broad ranges of properties.) In fact, steel wires
were used by the Wright brothers and in World War I biplanes:
Wright brother's aeroplane.
However aluminium performs well under the elastic bending of beams and flat
panels, having a greater value than steel for the merit indices 
and 
,
and hence aluminium alloys are now used to make major parts of aircraft whereas
steel is not.
It may also be important for a material to have high values of more than one merit index. For example a material may be wanted that will be tough and also have a good value of Young’s modulus for a low relative cost per unit volume:
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Looking at the materials-selection maps, good materials that fit these requirements are lead alloys, zinc (Zn) alloys and steels.
Consider a comparison of biomaterials to man-made materials, specifically commonly used engineering materials such as steel, aluminium alloys, titanium alloys, alumina and polyethylene. It can be seen that alumina, aluminium alloys, steel and titanium alloys perform quite well in terms of Young’s modulus – density related merit indices. This is due to their high values of Young’s modulus, despite their high densities. Polyethylene compares poorly with biomaterials in this respect however, due to its low value of Young’s modulus.
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Alumina, aluminium alloys, steel and titanium alloys are more commonly used
in engineering applications than biomaterials. This is despite biomaterials,
particularly types of wood, outperforming these materials for some of the merit
indices on this materials-selection map. It is important to note that the distinction
between the materials of living systems and conventional engineering materials
is not absolute. Wood is more widely used than these engineering materials for
low-tech applications, and is in fact the world’s principal material for
building. This makes wood among the most common and important structural engineering
materials. Although wood has only a quarter the strength of steel, it has four
times the specific strength (
)
and is renewable, recyclable and requires only a low energy input to make. This
makes the price of wood 1/60th per tonne the price of steel with
109 tonnes being used annually worldwide, which is comparable to
the amount of iron and steel used globally. Wood has many uses such as the hubs
and rims of traditional wheels (for which elm and oak, respectively, are used),
longbows (made from yew) and cricket bats (made of willow). However wood is
strongly anisotropic, highly susceptible to water damage and unable survive
and to perform at high temperatures. There is also a large amount of variability
in wood, as different growing conditions for trees of the same species lead
to differing mechanical properties. It is possible to limit the anisotropy of
wood by using plywood, which involves creating layers of wood with orthogonal
“grains” (orientation of the tracheid/ fibre cell structure). The
other limitations cannot so easily be overcome, limiting wood’s use as
an engineering material with light specifications. (This is discussed in The
Structure and Mechanical Behaviour of Wood teaching and learning package).
Palm trees can survive very high winds, behaving well under bending moments.
Looking at the strength – density materials selection chart:
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Alumina clearly outperforms biomaterials when loading a tie under tension,
it has a high value of the merit index 
and performs well for all merit indices shown on the above map. But the applicability
of alumina is likely to be limited by its brittleness (low Kc)
and the difficulty of forming into components. Aluminium alloys, titanium alloys,
and particularly steel and polyethylene, have merit indices inferior to those
of many biomaterials. This is due to their greater density and in polyethylene’s
case relatively low strength. This doesn’t necessarily mean that biomaterials
will be chosen over common engineering materials however. Other factors to be
considered in choosing a material for an engineering application once merit
indices have been compared include:
- cost
- resistance to creep
- ease of manufacture
- availability
- resistance to corrosion
- ability to function at a required temperature
- energy efficiency
Note: This animation requires Adobe Flash Player 8 and later, which can be downloaded here.
Only when all these factors along with the merit indices and materials-selection maps have been considered, can the best material for a particular application be found. Materials-selection maps and merit indices can be used to compare broad ranges of different materials to discover roughly which few materials are most suitable. Each material would then need to be assessed in more detail to discover which material is truly best for a specific use.
© 2004-2012 University of Cambridge.