Making swords that are both durable and able to pass con rules is tricky, especailly if you are making them full size and not using tiny plastic kids toys.
Firstly, I would say do not use corregated card, it is too weak for the job. There are other materials better suited, many of which I have used in the past with pretty good results. Thankfully, they are also relatively cheap.
Balsa wood and paper:
For my Raiden katana I used a combination of thick cartridge paper laminated to balsa wood. There are pics of it in my profile, here
http://www.cosplayisland.co.uk/costume/view/7889 This passed both Minamicon and Ayacon's prop inspectors. If you want to know more on the construction I can go through this in greater detail?
Foam board:
I used foam board to make the tira ring, pics of which are here,
http://www.cosplayisland.co.uk/costume/view/1147 The foam board is light but is more susceptible to being dented, as can be seen in a couple of the pics if you look closely (battle damage?!). I am currently investigating ways to prevent this. I have no idea how this would hold up as a longer sword shape, however.
Mount board:
If you are to go down the card route then I would suggest using mounting board from any art shop. It is like a really thick piece of card. Create a template of the design using paper and then cut out two or three of these from the mounting board. Glue them together and the glue gives structural strength. Also, because it's layered, it is possible to sand an edge on both sides to give the blade effect. To make the guard (the circle bit at the hilt), I would cut it again out of car and make a slit in the centre just big enough to fit the blade through.
I hope that is of some use.
Following on from this, I have noticed recently that more cosplayers are starting to venture in to build props of their own that are able to survive a con but also pass regulations. Understandably, not everyone has the space, experience, confidence, or the money to use some of the more technical materials such as fibre glass or resin. However, this doesn't mean that decent props can not be built. I have been asked on occassion about the possibility of giving a panel at an event focusing on basic prop building, designed for people starting out or those on a budget. If this is something people would be interested in then I could look in to it further?