There are two types of strings: nylon and phospor bronze. And each type is found in the species E A D G B E.
Here is the evidence:
Nylon
Phosphor Bronze
End of my physics lesson.
Addendum
The latest research has shown that the type of steel still exists in a parallel universe. Strangely enough, also in the species E A D G B E:
Steel
but did you see a D A D G A D ?
These are different energy levels: less tension!
There are a gazilion types these days. For steel string acoustic, there is 80/20 bronze (my favourite, a bit brighter, still cheap), for classical there are carbon and other types, and for electric there are at least 3 variants.
Good ones, Kurt. Not much wear on those frets … 
My own favorites were Ernie Ball’s Hybrid Slinky with the 0.009" top …
I haven’t picked up a… physics book… in quite some time. I’ve kinda had the itch lately but my… physics books… really need some work done on them.
Well, I use D’Addario Nickel Wound 10/46, Philippe Bosset Phosphor Bronze 12/53. And I replaced the Savarez 510 ARJ Nylon with Knobloch 400ADC Double Silver C…Ca…Carb… very special Nylon.
I swore to myself to exercise more when I’m retired. But the first thing I’ve done was to injure my left hand - so I couldn’t play for more than three months. By now I try to de-clumsy my fingers.
Take this: www.gitarrenphysik.de. About 3000 pages information. Also for download. Unfortunately in German - but your browser of trust or deepl (or something else) may help you.
