In reply to NottsRich:
> if you've just observed a broken wire on the outside of the rope then it's possible that there are many more inside that are also damaged.
Sorry, I'm going to call you up on that one, as everyone on this thread is taking about failure through bending the steel wire over an edge.
The highest bending moments and therefore the highest stresses always occur in the furthest fibre of any member. This means that although you can only see 20% of the cross section, you are seeing the worst affected fibres (in terms of bending).
However, I do agree that corrosion could play a big part in failure of internal fibres, thus don't rely on the fact that the outer fibres that you can see are all intact.