Place the two broken or cracked pieces of the arm together as nearly as possible to their original shape and position.
Hold these pieces in place as you wrap the break or crack with a heavy-duty tape such as Gorilla Tape or similar. Provide a good amount of tension when wrapping around the area to be repaired and continue the wrap an inch past this area on both sides.
Open a small hose clamp (usually a 1/2" to 3/4" clamp will work) all the way so that you slide it over the umbrella arm. Move the clamp directly over the break or crack area and fasten snugly.
Apply another 2-3 wraps of the repair tape over the hose clamp so it is completely covered by the tape.
Paint the wrap to match the color of the umbrella arm.