After confirming the mechanical stability and strength, we applied uniaxial and biaxial tensile strain with the increment of 2% until the monolayer is deformed. The stress as a function of uniaxial and biaxial tensile strain is shown in Fig. . The stress increases linearly with an increase in tensile strain within the harmonic range. Finally, the stress curve reaches the maximum value called ultimate tensile strength (U). The value of stress to achieve the ultimate strength is called ultimate stress. This is the maximum stress that a material can withstand without damaging itself. The critical stress also helps to separate the elastic and plastic regions of the material, which is discussed in Section 3.5. Below the ultimate stress higher order elastic constants are determined.