than other flaws in a specimen. The experiments showed that the product of the square root of the flaw length () and the stress at fracture () was nearly constant, which is expressed by the equation: An explanation of this relation in terms of linear elasticity theory is problematic. Linear elasticity theory predicts that stress (and hence the strain) at the tip of a sharp flaw in a linear elastic material is infinite. To avoid that problem, Griffith developed a thermodynamic approach to explain the relation that he observed. The growth of a crack, the extension of the surfaces on either side of the crack, requires an increase in the surface energy. Griffith found an expression for the constant in terms of the surface energy of the crack by solving the elasticity problem of a finite crack in an elastic plate. Briefly, the approach was: Compute the potential energy stored in a perfect specimen under a uniaxial tensile load. Fix the boundary so that the applied load does no work and then introduce a crack into the specimen. The crack relaxes the stress and hence reduces the elastic energy near the crack faces. On the other hand, the crack increases the total surface energy of the specimen. Compute the change in the free energy (surface energy − elastic energy) as a function of the crack length. Failure occurs when the free energy attains a peak value at a critical crack length, beyond which the free energy decreases as the crack length increases, i.e. by causing fracture. Using this procedure, Griffith found that where is the Young's modulus of the material and is the surface energy density of the material. Assuming and gives excellent agreement of Griffith's predicted fracture stress with experimental results for glass. For the simple case of a thin rectangular plate with a crack perpendicular to the load, the energy release rate, , becomes: where is the applied stress, is half the crack length, and is the Young’s modulus, which for the case of plane strain should be divided by the plate stiffness factor . The strain energy release rate can physically be understood as: the rate at which energy is absorbed by growth of the crack. However, we also have that: If ≥ , this is the criterion for which the crack will begin to propagate. For materials highly deformed before crack propagation, the linear elastic fracture