In a game that relies on armour of one type or another you would have thought that there would be a poignard to partially counter it.

poignard (dagger designed to penetrate mail or plate armour joints)

With a reduction on the AC of victim relative to the strength of the user

Not important, just a passing thought

5E is more streamlined and weapons like this would complicate things so they are not included. You can, however, RP it so if you got a particularly good hit in with a dagger you can narrate it as penetrating a weak point in the armor. 

With a ST-based weapon, ST is part of the bonus to the roll, with AC setting the target. So, in a sense, with all ST-based attacks, a higher ST gets past a higher AC.

Quote from kenclary >>

With a ST-based weapon, ST is part of the bonus to the roll, with AC setting the target. So, in a sense, with all ST-based attacks, a higher ST gets past a higher AC.

This was just a passing thought, so the numbers aren't well thought out:

Say any strength above +2, the opposing armour class is reduced by the same amount so  for +3 strength, you reduce the opposing AC by 1 (would be easiest to handle by increasing the hit by +1 ) so addition to the hit would be Proficiency Bonus + Strength Bonus + reduction to AC

Would be a perfect assassins weapon :)

I hate it when a passing thought won't go away :)

Would have to exempt the addition from magical armour for realism

Yes its getting complicated...

I'll argue that all weapons are already supposedly designed to be effective. They are all intended to make it easier to score meaningful hits. Older rulesets may ave tried to discern between various modifers to AC more than 5E does, but in this edition AC is largely just AC. A weapon designed to defeat chinks in armour wouldn't offer any benefit vs a monk's or a barbarian's unarmoured ACs or vs Mage Armour or other magic-based AC bonuses. If you want to make this all less streamlined and start divvying up the different contributors to AC again you absolutely can, but I think it's going to be more complex than you think if you don't want to derail the system too much. Anything that's effectively an attack bonus is potentially very powerful in 5E, because in this edition those are a very limited commodity (the bounded accuracy principle).

Quote from pangurjan >>

I'll argue that all weapons are already supposedly designed to be effective. They are all intended to make it easier to score meaningful hits. Older rulesets may ave tried to discern between various modifers to AC more than 5E does, but in this edition AC is largely just AC. A weapon designed to defeat chinks in armour wouldn't offer any benefit vs a monk's or a barbarian's unarmoured ACs or vs Mage Armour or other magic-based AC bonuses. If you want to make this all less streamlined and start divvying up the different contributors to AC again you absolutely can, but I think it's going to be more complex than you think if you don't want to derail the system too much. Anything that's effectively an attack bonus is potentially very powerful in 5E, because in this edition those are a very limited commodity (the bounded accuracy principle).

 Yes I agree, just a passing thought. 

Before 3rd Edition, there were even modifiers to how effective armor was vs the type of damage of the attack: chainmail was much more effective against slashing damage than piercing damage, for example.

This was not a popular rule and was dropped due to it over-complicating combat without adding anything of real value.

if you're looking for a D&D weapon that best represents a poignard without changing anything, I'd suggest the shortsword is what you want. You can simply call yours a poignard in game too. The way the specific design of any given weapon translates into making it easier to score hits is arguably already represented by the proficiency bonus anyway - those qualities don't come into play if you don't know how to use them. 

5e has a poignard. It's a rogue with a dagger.

I'd actually say weapons like the poignard and the tactics behind them broadly fall into the area of weapons handled as "finesse" weapons.  Some who want much more detailed clashes of arms and armor may find that unsatisfactory and can try to reimport systems found in prior editions, but combat in 5e is painted in broad strokes.

If a "poignard" can defeat AC by design, why can't magic in general defeat armor period?