Beam shear failure
OK so standard quoted materials properties would apply.
Shear stress = Force/area
So you can calculate a coarse failure load by multiplying the shear stength of your steel by the 1/8 sq inch.
However the forces on gear teeth are much more complex so this will only give you an upper bound. There are significant stress concentrations depending upon the shape of the teeth.
Also, what did the fracture surface look like? Can you describe it?
Was it crystalline and shiny or dull and smooth? or was it part crystalline and part dull?