Because the first principal stress, that is, the maximum tensile stress, appears on the inclined plane at an angle of 45 degrees with the axis.

However, cast iron is a brittle material with weak tensile properties, so 45 helicoid fracture will occur.

That is to say, when cast iron is twisted, it is not cut, but pulled off!

Only by knowing the stress state can the failure mode of materials be determined.

This is why the stress state is always put together with the strength theory.