Q:

Will a tug-of-war battle damage my climbing rope

I plan to organize a big tug-of-war with a climbing rope (70m long) with 50-60 adults. I know the climbing rope is elastic. The rope resistance is 9 kN when a chock occurs.
Will the rope breaks or become unsafe after such a game ?

A:

Don't even think about this!! REALLY. The risk is probably low, but the outcome of a broken rope is pretty severe in climbing. Too severe for the sake of a tug of war game.
This is an advanced materials engineering question. 9kN is presumably the rope's rating, right? 50-60 grownups will easily impart this force onto the rope (30 at each end, each pulling with 300N). Even so, I should think that it would withstand the game in the sense that the rope would not break during the game.
However, how the game affects your rope's safety is an advanced materials engineering question. Crucial to your question is whether the rope leaves its elastic (linear) regime under the load and whether the fibres begin to deform. Unless the rope stays perfectly elastic, or very near thereto, its load bearing capacity will be compromised by the game.
I would advise buying a rope especially for your tug of war game. There are 60 grown ups to split the cost over, so anything else strikes me as a bit foolhardy.

