Its all to do with tensile strenght of and failure analysis of structures and materials like aluminium.
KSI stands for kilo-pound per square inch,
FTU is the Tensile Ultimate Strenght,
FTY is the Tensile Yield Strenght,
FSU is the Ultimate Shear Strenght,
E is the symbol for the Youngs Modulus (of elasticity) of a material....
G.......