What weakens the J-hooks (rivets) when they have been used over several times, are stress fractures - similar to bending a piece of wire back & forth until it breaks.
I have re-used J-hooks and S-hooks (same problem exists with both) several times w/o disaster striking. It has been recommended that these hooks be closely examined before re-use. There is nothing wrong with this advise, but stress fractures can be very hard to see.
The best test for me to detect stress fractures and weakness in these hooks is their feel when opening or closing them. I can feel a difference in the resistance to opening or closing, and when one of these hooks goes, "soft", I immediately feel it and replace it. I also throw away, it right then! I fling it out in the beaver pond or a long way from my dirt set. I don't put it back in my pocket - that IS a disaster looking for a place to happen.