3.4 Brittle and ductile
Strength defines the stress conditions at which failure occurs but it is important to think
about how the material fails. If you bend a biscuit it will not deform very much but
it will suddenly snap. This is brittle behaviour. If you load warm butter by pressing it
with your hand it will gradually deform. This is ductile behaviour.
Concrete is brittle: in compression tests there are relatively small strains and it fails
with a bang. Many metals such as steel, copper and aluminium (but not cast iron)
are ductile. In compression tests they gradually deform and continue to deform at
relatively large strains. (In tension tests metal samples snap but that is because the area
decreases as the sample necks.)
Soils and rocks may be either brittle or ductile. Soft clay is ductile when it has a
relatively high water content but, if it is highly compressed, stiff clay becomes brittle.
Rocks are generally brittle when they are near the ground surface but they become
ductile if they are at great depth. We will see later that soils and rocks behave in
essentially the same way and what is important is the state, which is the combination
of confining stress and water content.
Structures made of brittle materials are inherently unsafe. They fail catastrophically
after very little deformation. Brick and masonry buildings fail in this way particularly
during earthquakes. Structures made of ductile materials such as steel and timber