The standard is divided into specific test procedures to ensure material durability:
The 2017 revision added procedures for testing joint seams both in the lab and on construction sites. Where to Find the Full Standard
If you type that exact phrase into Google, you will encounter:
While the exact DIN 16726 PDF may not be free, world-class alternatives are freely available:
You can view real-world application data in the Dilip Buildcon Test Report on Scribd, which outlines how DIN 16726 is used for testing HDPE and PVC membranes.
To download the PDF using these platforms:
Some European countries have "Normenausschüsse" that archive withdrawn standards for public reference. Example: (Austria) or SNV (Switzerland) sometimes keep historical DIN standards. Visit their reading rooms.