 A. Griffith during World War I. Griffith looked at the energy required to create new surfaces by breaking material bonds versus the elastic strain energy of the stretched bonds released.By analyzing a rod under uniform tension Griffith determined an expression for the critical stress at which a favorably orientated crack will grow.The critical stress at fracture is given by,

formula_1

where γ = surface energy associated with broken bonds, E = Young's modulus, and a = half crack length.