Toughness

An expression of the rupture behaviour of a material in the presence of a notch.

This notion is widely used with isotropic materials (metals), but it is tricky to apply to composite materials because of their heterogeneity and anisotropy.

Damage mechanics is difficult to apply, but for these materials it is a means of accurately defining behaviour.

In general a tough material is not very fragile (though this is not true in all cases).