A block of material with a crack in it is subjected to tensile load. As the material is stretched, elastic energy is stored. Its relative magnitude is indicated by the color of the block. The crack propagates when the elastic energy released is equal to the energy necessary to form a new surface; this is called the Griffith criterion. Varying the stress ratio, surface energy, Young's modulus, or initial height changes the point at which the crack propagates. All units are arbitrary with the stress ratio equal to the stress divided by Young's modulus.