Specifically, in engineering and metallurgical terms, it is the ability of a material to resist the propagation of a (pre-existing) crack when load is applied. The toughness of a material can be altered in a number of ways depending on the material, but commonly by cold working or simply by a change in temperature.
The most commonly quoted measurement of toughness for a material is the plane strain fracture toughness, known as KIC. The I refers to the mode of loading when the sample is tested. If you imagine looking into a horizontal notch cleaved into a block of metal,
- Mode I is pulling the faces apart, one up; one down
- Mode II is pulling the top half towards you and pushing the bottom half away (or vice versa)
- Mode III is pushing the top half to the right and the bottom half to the left (or vice versa)
Typical
KIC values for some common materials are shown below (in MPam
1/2):
Generally speaking, materials with low fracture toughness fail by brittle fracture, whereas materials that are able to deform plastically, i.e. flow, when a shock load is applied are more able to absorb the energy without catastrophic failure. This explains the difference in fracture toughness between brittle ceramic alumina and ductile metal aluminium.