Page 150 of the Trellis Craft book mentions using 1/2 inch rebar as an internal support for 1/2 inch copper pipe projects.

The Problems

Some book owners reported that they cannot get 1/2" rebar to fit inside 1/2" copper pipe, so I went back to the home centers to double-check the rebar, and found these problems:

1. Well made, uniform diameter 1/2" rebar does indeed fit inside 1/2" copper pipe. The problem is that 1/2" rebar does not seem to be manufactured to an exacting standard, so a lot of rebar has a slight elliptical or "squashed" shape, making it too big to fit inside 1/2" pipe.
2. Most rebar is cut with a shear, which distorts the cut ends. You may be able to cut off the distorted ends and use the rest of the rebar.
3. Straight lengths of rebar are apparently obtained from a large coil of rebar which is straightened and then sheared to length. The straightening may not be very precise. If you sight down the length of a piece of rebar from one end you will be able to tell how straight it is. Even slight back-and-forth bends in 1/2" rebar can prevent using it inside 1/2" copper pipe.
4. Some home centers also sell 5/8" rebar, which may be mixed in with the 1/2" rebar.

When going to buy rebar, it is a good idea to take a short section of 1/2" pipe with you. You can then use the pipe to check for any of the above problems before you buy the rebar.

Solution

The easiest and cheapest solution is to use 3/8" inch rebar inside 1/2" copper pipe. The same back-and-forth bending that is a problem with 1/2" pipe actually helps 3/8" pipe fit snugly inside 1/2" pipe.

Try to find pieces of cut 3/8" rebar that are straight overall. Then sight down the length of the rebar from one end and you will probably see that the rebar actually bends back and forth a bit. These bends will keep it in contact with the walls of 1/2" pipe and make a reasonably snug fit.

I have tested this by making several pot trellises from 1/2" pipe that are supported internally by 3/8" rebar. The end result is good support. There are some step-by-step instructions and photos for making these pot trellis supports here.

If you want to support freestanding trellises in your yard, it would probably be a good idea to use 3/8" rebar inside each leg of the trellis.