The families of engineering materials

It is helpful to classify the materials of engineering into the six broad families
shown in Figure 3.1: metals, polymers, elastomers, ceramics, glasses, and
hybrids. The members of a family have certain features in common: similar
properties, similar processing routes, and, often, similar applications.
Metals have relatively high moduli. Most, when pure, are soft and easily
deformed. They can be made strong by alloying and by mechanical and heat
treatment, but they remain ductile, allowing them to be formed by deformation
processes. Certain high-strength alloys (spring steel, for instance) have ductil-
ities as low as 1 percent, but even this is enough to ensure that the material
yields before it fractures and that fracture, when it occurs, is of a tough, ductile
type. Partly because of their ductility, metals are prey to fatigue and of all the
classes of material, they are the least resistant to corrosion.
Ceramics too, have high moduli, but, unlike metals, they are brittle. Their
‘‘strength’’ in tension means the brittle fracture strength; in compression it is
the brittle crushing strength, which is about 15 times larger. And because
ceramics have no ductility, they have a low tolerance for stress concentrations
(like holes or cracks) or for high-contact stresses (at clamping points, for
instance). Ductile materials accommodate stress concentrations by deforming
in a way that redistributes the load more evenly, and because of this, they can
be used under static loads within a small margin of their yield strength.
Ceramics cannot. Brittle materials always have a wide scatter in strength and
the strength itself depends on the volume of material under load and the time
for which it is applied. So ceramics are not as easy to design with as metals.
Despite this, they have attractive features. They are stiff, hard, and abrasion-
resistant (hence their use for bearings and cutting tools); they retain their
strength to high temperatures; and they resist corrosion well.
Glasses are non-crystalline (‘‘amorphous’’) solids. The commonest are the
soda-lime and boro-silicate glasses familiar as bottles and ovenware, but there
are many more. Metals, too, can be made non-crystalline by cooling them
sufficiently quickly. The lack of crystal structure suppresses plasticity, so, like
ceramics, glasses are hard, brittle and vulnerable to stress concentrations.
Polymers are at the other end of the spectrum. They have moduli that are
low, roughly 50 times less than those of metals, but they can be strong—nearly
as strong as metals. A consequence of this is that elastic deflections can be large.
They creep, even at room temperature, meaning that a polymer component
under load may, with time, acquire a permanent set. And their properties
depend on temperature so that a polymer that is tough and flexible at 20
C
may be brittle at the 4
C of a household refrigerator, yet creep rapidly at the
100
C of boiling water. Few have useful strength above 200
C. If these aspects
are allowed for in the design, the advantages of polymers can be exploited.
And there are many. When combinations of properties, such as strength-
per-unit-weight, are important, polymers are as good as metals. They are easy
to shape: complicated parts performing several functions can be molded from
a polymer in a single operation. The large elastic deflections allow the design
of polymer components that snap together, making assembly fast and cheap.
And by accurately sizing the mold and pre-coloring the polymer, no finishing
operations are needed. Polymers are corrosion resistant and have low coeffi-
cients of friction. Good design exploits these properties.
Elastomers are long-chain polymers above their glass-transition temperature,
Tg. The covalent bonds that link the units of the polymer chain remain intact, but
theweaker Van derWaals and hydrogen bonds that, belowTg, bind the chains to
each other, have melted. This gives elastomers unique property profiles: Young’s
moduli as lowas 103
GPa (105
time less than that typical ofmetals) that increase
with temperature (all other solids show a decrease), and enormous elastic
extension. Their properties differ so much from those of other solids that special
tests have evolved to characterize them.This creates a problem: ifwewish to select
materials by prescribing a desired attribute profile (as we do later in this book),
then a prerequisite is a set of attributes common to allmaterials.To overcome this,
we settle on a common set for use in the first stage of design, estimating approxi-
mate values for anomalies like elastomers. Specialized attributes, representative of
one family only, are stored separately; they are for use in the later stages.
Hybrids are combinations of two or more materials in a pre-determined
configuration and scale. They combine the attractive properties of the other
families of materials while avoiding some of their drawbacks. Their design is
the subject of Chapters 13 and 14. The family of hybrids includes fiber and
particulate composites, sandwich structures, lattice structures, foams, cables,
and laminates. And almost all the materials of nature—wood, bone, skin,
leaf—are hybrids. Fiber-reinforced composites are, of course, the most
familiar. Most of those at present available to the engineer have a polymer
matrix reinforced by fibers of glass, carbon or Kevlar (an aramid). They are
light, stiff and strong, and they can be tough. They, and other hybrids using a
polymer as one component, cannot be used above 250
C because the polymer
softens, but at room temperature their performance can be outstanding.
Hybrid components are expensive and they are relatively difficult to form and
join. So despite their attractive properties the designer will use them only when
the added performance justifies the added cost. Today’s growing emphasis on
high performance and fuel efficiency provides increasing drivers for their use.

The material life cycle

The material life cycle. Ore and feedstock are mined and processed to yield a material. This is manufactured into a product that is used and, at the end of its life, discarded or recycled. Energy and materials are consumed in each phase, generating waste heat and solid, liquid and gaseous emissions.

Ore and feedstock, drawn from the earth’s resources, are processed to give materials; these are manufactured into products that are used and, at the end of their lives, discarded, a fraction perhaps entering a recycling loop, the rest committed to incineration or landfill. Energy and materials are consumed at each point in this cycle (we shall call them ‘phases’), with an associated penalty of CO2, SOx, NOx and other emissions—heat, and gaseous, liquid and solid waste, collectively called environmental ‘stressors’. These are assessed by the technique of life-cycle analysis (LCA). A rigorous LCA examines the life cycle of a product and assesses in detail the eco-impact created by one or more of its phases of life, cataloging and quantifying the stressors. This requires information for the life history of the product at a level of precision that is only available after the product has been manufactured and used. It is a tool for the evaluation and comparison of existing products, rather than one that guides the design of those that are new. A full LCA is time-consuming and expensive, and it cannot cope with the problem that 80% of the environmental burden of a product is determined in the early stages of design, when many decisions are still fluid. This has led to the development of more approximate ‘streamline’ LCA methods that seek to combine acceptable cost with sufficient accuracy to guide decision-making, the choice of materials being one of these decisions. But even then there is a problem: a designer, seeking to cope with many interdependent decisions that any design involves, inevitably finds it hard to know how best to use data of this type.

How are CO2 and SOx emissions to be balanced against resource depletion,
toxicity or ease of recycling?

How are CO2 and SOx emissions to be balanced against resource depletion, toxicity or ease of recycling? This perception has led to efforts to condense the eco-information about a material production into a single measure or indicator, normalizing and weighting each source of stress to give the designer a simple, numeric ranking. The use of a single-valued indicator is criticized by some. The grounds for criticism are that there is no agreement on normalization or weighting factors, and that the method is opaque since the indicator value has no simple physical significance.
But on one point there is international agreement: the Kyoto Protocol of 1997 committed the developed nations that signed it to progressively reduce carbon emissions, meaning CO2. At the national level the focus is more on reducing energy consumption, but since this and CO2 production are closely related, they are nearly equivalent. Thus, there is a certain logic in basing
design decisions on energy consumption or CO2 generation; they carry more conviction than the use of a more obscure indicator. We shall follow this route, using energy as our measure.Ore and feedstock, drawn from the earth’s resources, are processed to give materials; these are manufactured into products that are used and, at the end of their lives, discarded, a fraction perhaps entering a recycling loop, the rest committed to incineration or landfill. Energy and materials are consumed at each point in this cycle (we shall call them ‘phases’), with an associated penalty of CO2, SOx, NOx and other emissions—heat, and gaseous, liquid and solid waste, collectively called environmental ‘stressors’. These are assessed by the technique of life-cycle analysis (LCA). A rigorous LCA examines the life cycle of a product and assesses in detail the eco-impact created by one or more of its phases of life, cataloging and quantifying the stressors. This requires information for the life history of the product at a level of precision that is only available after the product has been manufactured and used. It is a tool for the evaluation and comparison of existing products, rather than one that guides the design of those that are new. A full LCA is time-consuming and expensive, and it cannot cope with the problem that 80% of the environmental burden of a product is determined in the early stages of design, when many decisions are still fluid. This has led to the development of more approximate ‘streamline’ LCA methods that seek to combine acceptable cost with sufficient accuracy to guide decision-making, the choice of materials being one of these decisions. But even then there is a problem: a designer, seeking to cope with many interdependent decisions that any design involves, inevitably finds it hard to know how best to use data of this type.