A 1 m $\times$1m mild steel sheet of 1 mm thickness is stretched in its own plane by stress $\sigma _{X}$= 20 MPa and $\sigma _{Y}$ = 30 MPa as shown in figure below. Point O is centre of the plate. OC and OD are two mutually perpendicular lines inclined at 450 each to x and y directions respectively before application of stress. Determine the change in angle (in degrees) between OC and OD after application of stresses. Take modulus of rigidity of plate material G = 80 GPa.