I imagine the lift-off process itself puts a large amount of stress and strain on the rockets materials.
With the amount of debris up in space there is always the risk of rocks to collide with the craft. Depending on their speed and where they hit this could cause untold havoc, especially if it were to penetrate the craft.
I understand we have a question asking about how we could shield rockets in the future using a shielding method, but what i am interested in is how do they currently test the rockets on earth to ensure the parts can withstand a blow from space debris? Are any physical tests done or is it all theoretically calculated into the design of the rocket?