Strength and toughness

Strength and toughness? Why both? What’s the difference?
Strength, when speaking of a material, is its resistance to plastic flow. Think of
a sample loaded in tension. Increase the stress until dislocations sweep right
across the section, meaning the sample just yields, and you measure the initial
yield strength. Strength generally increases with plastic strain because of work
hardening, reaching a maximum at the tensile strength. The area under the whole
stress–strain curve up to fracture is the work of fracture. We’ve been here
already—it was the subject of Chapter 5.
Toughness is the resistance of a material to the propagation of a crack. Suppose
that the sample of material contained a small, sharp crack, as in Figure 8.1(a).
The crack reduces the cross-section A and, since stress σ is F/A, it increases the
stress. But suppose the crack is small, hardly reducing the section, and the sample
is loaded as before. A tough material will yield, work harden and absorb
energy as before—the crack makes no significant difference. But if the material
is not tough (defined in a moment) then the unexpected happens; the crack suddenly
propagates and the sample fractures at a stress that can be far below the
yield strength. Design based on yield is common practice. The possibility of fracture
at stresses below the yield strength is really bad news. And it has happened, on
spectacular scales, causing boilers to burst, bridges to collapse, ships to break
in half, pipelines to split and aircraft to crash. We get to that in Chapter 10.
So what is the material property that measures the resistance to the propagation
of a crack? And just how concerned should you be if you read in the paper
that cracks have been detected in the track of the railway on which you commute
or in the pressure vessels of the nuclear reactor of the power station a few
miles away? If the materials are tough enough you can sleep in peace. But what
is ‘tough enough’?
This difference in material behavior, once pointed out, is only too familiar.
Buy a CD, a pack of transparent folders or even a toothbrush: all come in perfect
transparent packaging. Try to get them out by pulling and you have a problem:
the packaging is strong. But nick it with a knife or a key or your teeth and
suddenly it tears easily. That’s why the makers of shampoo sachets do the nick
for you. What they forget is that the polymer of the sachet becomes tougher
when wet, and that soapy fingers can’t transmit much force. But they had the
right idea.

Materials
Engineering, Science,
Processing and Design
Michael Ashby, Hugh Shercliff and David Cebon
University of Cambridge,
UK
AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD
PARIS • SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO
Butterworth-Heinemann is an imprint of Elsevier

Technology Push and Market Pull

Many radical product innovations seem to be based on new
technology. For example, pocket calculators, personal computers
and many other new electronics-based products were made possible
by the development of the microprocessor chip. However, as
we have seen in the success and failure stories, people's willingness
to buy new products is the ultimate deciding factor; if people do
not want the product then it fails. There are also many examples of
new product development that do not depend on new technology
but on recognizing what people want or need, whether that is
recyclable packaging, stacking hi-fi systems or dish washers, etc.
There are therefore two strong aspects to new product devel
opment: the push that comes from new technology and the pull of
market needs.
These two aspects are usually called technology push and
market pull. Technology itself, of course, does not do any pushing;
that comes from the developers and suppliers of the new technology,
and from the makers of the new products. In practice, a lot
of new product development is influenced by a combination of
both technology push and market pull.
Many companies prefer to work on the market-pull model,
using market research to identify customers' wants and needs. The
technology-push view, on the other hand, emphasizes that
innovations can create new demands and open up new markets.
Market research usually cannot identify demands for products that
do not yet exist.
This has been recognized particularly by those companies that
try to plan new product development in terms of both technological
seeds and customer needs; success depends on matching
seeds with needs. However, even when a market need and a
technology seed can be matched, and a new product concept
identified, there is no guarantee that a product will actually be
developed. It may require far too much financial investment, for
example, or a product champion may not emerge or be successful
within the company. Another reason is that some product
concepts are actually suppressed by companies and organizations
that have a strong vested interest in maintaining the markets for
their existing products. This is particularly true of industries with
a heavy capital investment in the continued production of a
particular product type. The motor industry, for example, failed to
support the development of alternative vehicles, such as electric
cars, until it began to see such innovations as potentially important
to its survival.
Some opportunities for new product development lie in the
region where an already-developed technology can meet an
undeveloped market, while others lie in the region where new
technology can be applied in an already developed market
(Figure 88). A third region, for the most radical (and risky) product
innovations, is where new technology and new market opportunities
might be developed together. The Sony Walkman and
Sinclair C5 were both examples of the latter.

Engineering Design Methods
Strategies for Product Design
THIRD EDITION
Nigel Cross
The Open University, Mi/ton Keynes, UK
JOHN WILEY & SONS, LTD
Chichester- New York. Weinheim • Brisbane. Singapore. Toronto