On the Unity program, when you are editing a part, you have to add a script called the Polygon Collider Script that you can change how strong it's into
4 values: Low,
Medium,
High and
Wheel. I only found this on the Placeholder RTG on the Legacy folder so I don't know if it works (should work) and you can't find that script on the other game parts, only on the 3 legacy parts that are on the legacy folder.
Also, I don't know at what velocity these parts can resist at an impact with each of those values, but my theory is that
Low is the same as the Solar Panels impact tollerance,
Medium is the same as the Fuel Tanks and Engines,
High is maybe the same as the Landing legs, and
Wheel, is the same impact tollerance as the,
Wheel.