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:

G_Ic

G_IIc

G_IIIc

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 G_I/_IIc. This parameter is used to characterize the delamination resistance of a composite and the expressions G_Ic, G_IIc and G_IIIc are called the interlaminar fractures toughnesses under Mode I, Mode II and Mode III loading respectively.