Intro: Foam Knight's Sword
Broadsword with dagger made out of foam. Bit rough but it did the trick. They lasted through a kids party of a dozen young knights bashing the heck out of each other (and me) without any damage.
Made with foam, cardboard and an inner core of a hardwood dowel.
Having used the dowel successfully without any breaks or tear outs must have been beginners luck.
I think that in future I would use a smallish diameter pvc pipe instead. Sturdy enough not to break and can be safely capped at the end so that if a tear out of the foam occurs then the point would not be sharp.
Not hard to make just a bit fiddly. You want an instructable for it then let me know.