The energy release rate, often denoted by G, is the amount of energy per unit area that is supplied by the elastic energy in the body and loading system in creating a new fracture surface area.
When used on composites, it is often called a fracture toughness.
One can define values for each of the 3 modes of loading: GIc GIIc GIIIc
It is possible to have mixtures of these loading regimes, e.g. Mode I/II, which result in a mixed-mode fracture energy such as GI/IIc. This parameter is used to characterize the delamination resistance of a composite and the expressions GIc, GIIc and GIIIc are called the interlaminar fractures toughnesses under Mode I, Mode II and Mode III loading respectively.