When a crack is present in a material there is a critical stress that must be exceeded in order for the crack to grow.
The level of stress required is defined by the amount of work that must be done for crack growth. This level of work is referred to as the material toughness, Gc.
A high Gc indicates that crack growth in the material is difficult, such as in copper or plastic composites. A low Gc indicates that crack growth is easy, such as in glass or cement.