But be careful with ALLEXCEPT: if there is no DIRECT filter on NR_of_week and/or Material (code), it will remove ALL filtering from the EXPANDED version of Sheet (so, not only Sheet but all tables that are in a 1:many relationship with Sheet) and you might end up with something you won't be able to explain.
As long as you KNOW exactly what ALLEXCEPT in CALCULATE does, you're safe. And, of course, you should test.
DO NOT USE A FUNCTION IN YOUR CODE IF YOU DON'T KNOW PRECISELY HOW IT WORKS.
Let this statement above sink in. If you don't stick to it, you'll be sorry rather sooner than later. Please, before you start using something in your DAX code read about it. Here's a good source: www.daxguide.com.
Do you have any idea how ALLSELECTED works? No? That's what I thought.
ALLSELECTED is a very complex DAX function that works with something that's called "shadow filter context." To use it correctly you have to know quite a few things. Once again, please read about it.
Your measure should in all likelihood be this:
Qty total for Product (code) = SUM( 'Sheet'[Qty] )
I'd also urge you to read about the proper dimensional design.