A DUCTILE material, like structural steel, is one which exhibits a clearly defined yield point followed by a large plastic strain, which allows the material to be bent or drawn without breaking and such material can absorb large amounts of strain energy prior to fracture. Examples include aluminium, copper, lead, bronze and nylon.
A BRITTLE material is one which fails in tension at low values of strain. Examples include concrete, stone, cast iron and glass.
PLASTICS can be ductile or brittle and their behaviour can be strongly dependent on temperature and may CREEP.
COMPRESSION
The stress-strain curves for materials in compression differ from the tensile test curves.
Ductile materials tend to have proportional limits close to the tensile limit but after yielding begins the behaviour is quite different. The specimen is flattened out and the resistance to compression grows. The cross-sectional area is increased and the true stress is therefore lower than the nominal stress.For brittle materials the ultimate stresses are much higher than in tension.
CREEP
When some materials are left under load for long periods of time they develop additional strain, called CREEP. This effect needs to be taken seriously in high temperature applications.
