Q:

Does driving at top speed damage a car or stress the engine?

Lots of manufactures like Vauxhall/Opel and Ford seem to test their new models of cars by driving it at top speed for hours on end.
What is the point of this? 
Does it stress an engine?

A:

I've posted this graphic before, but I think it's pertinent here as well:

This is an image of a graph which Carroll Smith put in his book, Engineer to Win. In the book, one of the sections talks about metal fatigue. In this specific graph, he's talking about how stress affects any given metal part: how much will a part take before it fails. The graph describes that a part only has a finite number of cycles in them before they let go. That number, if the part is treated nicely, may be 10,000,000 (or some other stratospheric number). When you put more stress on the part, the parts lifetime is decreased. The effect of the stress is accumulative: the part will not heal itself. As you put the higher stress on any given part, its overall lifetime is diminished. The more stress you put on the part, the sooner it will fail. This is even if you keep it within its engineered limits.
I believe the manufacturer's main reason for testing the engine at the higher stress levels is to try to get it to fail. In doing the testing this way, if it doesn't fail, they know they've done the engineering well. They know the customer will never stress a car this way. If nothing fails in testing, they can be pretty sure the car won't fail when on the road under normal use.

