Crack growth resistance curve
In fracture mechanics, a crack growth resistance curve shows the energy required for crack extension as a function of crack length in a given material. For materials that can be modeled with linear elastic fracture mechanics (LEFM), crack extension occurs when the applied energy release rate G {\displaystyle G} exceeds the material's resistance to crack extension G R {\displaystyle G_{R}} .
Source: Wikipedia — Crack growth resistance curve (CC BY-SA 4.0)