I dropped my smartphone and the screen was cracked. However, you must tap at that location. Therefore, we decided to evaluate whether it is okay to continue using the product even if it has cracks based on the energy balance of crack growth. For simplicity, assume that the material and the shape and fracture shape of the crack are as follows.

$$Young$\text{'}$s modulus of glass E$=72$Gpa

$$Crack length: $3\mathrm{.}5$cm

$$Destruction mode: Mode I

$$Glass surface energy: $1\mathrm{.}0$Nm$/$m$^2$Griffith$$

$\sqrt[\phantom{2}]{\frac{2E}{a}}$