Your math is incorrect. This is a common mistake. Tonnes is not a unit of force although many laymen believe it is. 90 GPa is 90x10^9 Pa which is 90x10^9 N/m

Your math is incorrect. This is a common mistake. Tonnes is not a unit of force although many laymen believe it is. 90 GPa is 90x10^9 Pa which is 90x10^9 N/m^2 90x10^9 N/m^2 x 1m^2/1^6mm^2 = 90x10^3 N/mm^2 90x10^3 N/mm^2 is the equivalent to weight of 9,174 kg at sea level (9x10^10 / 9.81 = 9.17x10^9 kg/m^2) which is equivalent to 9.174 tonnes per square millimeter. As an aside, to make this more relatable: 90GPa is equivalent to ~3x10^7 feet worth of water pressure or roughly 5700 miles.