Rapiers are ineffective against armor. If you want to stab through joints in armor, you use a good, stout blade with a diamond-shaped cross-section, probably in a half-sword grip. With a rapier, you're going to be dead long, long before you manage to prize the blasted flexible thing through a joint. If you can.
The very word "rapier" is thought by many historians to come from "ropera," "robe," that is, a sword which is worn in civilian life for personal defense and duelling. It's fine for puncturing cloth and murdering some other fop over a prostitute, but it's too light and flexible and limited for real combat.
Gustavus Adolphus -- who, like all his contemporaries, used a broadsword on the battlefield (with a basket hilt, hence the confusion with the peacetime rapier) -- was badly injured at Lutzen by sword thrusts because he didn't wear armor. It weighed on his shoulder, where a musket ball acquired in Poland was permanently lodged in the muscle, close enough to an artery so they were afraid to remove it. The Imperial cuirassiers who had brought him down shot him in the head with a pistol when it looked it like Swedish troops were about to advance over that area of the battlefield.
That same Gustavus Adolphus, immediately before the battle, told his cavalry that it was "useless" to stab at the heavily-armored cuirassiers even with their broadswords (never mind the flabby rapiers). Instead, he recommended thrusting the blade into the chest of the cuirassier's horse and twisting with it to rip the wound open, causing the horse to fall on the cuirassier.
That tactic also didn't work, because the Imperial cavalry absolutely mauled the Swedish cavalry, and as an English eyewitness serving in the Swedish army wrote home to his father, ".... had not our foot [infantry] stood like a wall, not a man of us would have come off alive ...."
(See, in particular, "The Army of Gustavus Adolphus," books 1 & 2, by Richard Brzezinksi, and "Gustavus Adolphus" by Theodore Dodge.)
