where P {\displaystyle P} is the applied load, t {\displaystyle t} is the thickness of the plate, u {\displaystyle u} is the displacement at the point of application of the load due to crack growth, and a {\displaystyle a} is the crack length for edge cracks or 2 a {\displaystyle 2a} is the crack length for plane cracks. The crack is expected to propagate when the strain energy release rate exceeds a critical value G I c {\displaystyle G_{\rm {Ic}}} - called the critical strain energy release rate.