where E {\displaystyle E} is the Young's modulus of the material, γ {\displaystyle \gamma } is the surface energy per unit area of the crack, and a {\displaystyle a} is the crack length for edge cracks or 2 a {\displaystyle 2a} is the crack length for plane cracks. The quantity σ π a {\displaystyle \sigma {\sqrt {\pi a}}} is postulated as a material parameter called the fracture toughness. The mode I fracture toughness for plane strain is defined as