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,聽Gc.
A high聽Gc聽indicates that crack growth in the material is difficult, such as in copper or plastic composites. A low聽Gc聽indicates that crack growth is easy, such as in glass or cement.