If you have a neighbor who is a plumber, see if you can borrow some lead. It would not surprise me if some of your handles required over 300 pounds of static weight to break them. If the paddle shafts are well made the weight might even go to double that. A 5 gallon bucket of sand or pebbles is only going to be around 70 pounds.

You may want to borrow weightlifting weights ( measure them!)to pre load the test, and then add sand to the bucket only as the break point appears near.

Use a funnel when adding the sand. The restriction in the neck of the funnel lets the material flow out smoothly, with no jerky motion, and you can stop the flow quickly by capping the funnel's outlet with your hand. Use a large scoop or a measuring glass to add sand to the funnel.

Long before they break your paddle shafts will flex. It might be useful to record the amount of flexing with different weights. A paper ruler taped to the top of the paddle, and lined up with an unmoving point next to the paddle, can give you some indication of what's going on. This will also help determine how stiff the paddle will feel. A paddle that flexes might be loved by one person, and hated by another.

When you first start, after adding every few cups of sand, stop and weigh the bucket. This will give you some indication of how much single cup of sand weighs. You can use that along with your ruler to show the amount of flex at each weight.

Build up a platform under your bucket, set a bathroom scale on it, and try to keep this about 3 to 4 inches under the bottom of the bucket. If the bucket drops this far it should not spill much sand -- if it spills any at all.

By raising this platform you put the scale directly under the bucket. Keep raising the platform to lift the bucket free of the paddle shaft, and you can make weight measurements as you go. Gently lower the bucket and platform back to where the bucket hangs freely, and then go and add some more weight.

WEAR SAFETY GLASSES !

When that paddle shaft breaks you'll be looking at it intently, and probably your face will be a lot closer than it would normally be while paddling.
a splinter in your face is no fun. Even a tiny shred of fractured fiberglass in your eye is very serious -- and since your whole objective is to torture these things until they fracture you are generating potential risks. Stay protected.

A video camera set on a tripod can record the amount of deflection, and show the amount of added weight. It might be a nice addition to a science project, and would permit you to analyze any sudden fractures.

hope this helps.

PGJ

p.s. Gee, now your paddle building business has its own R&D department. Next you'll sell stock through an IPO.