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)
values for some common materials are shown below (in MPam1/2
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.