In this case y is the outside radius of the tube. Where M is the bending moment applied, I is the cross section moment of inertia(natural resistance to bending of the particular shape), and y is the distance from the neutral axis to the point being calculated. SO, step one is to look up the yield stress in tension of the type of aluminum you are using. The yield stress in tension is usually the critical (lower) value. Yield stress is the stress at which the metal begins to deform. Strength of material are given as stress levels in pounds per square inch (PSI). Then the bending stress will be the critical factor.
It's alot easier than trying to calculate critical buckling loads. That can be solved by filling the tube with the right size (tight fit) dowel rod.
