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
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