Can chain-slap cause damage to a chainstay that is beyond superficial? Yesterday a coworker insisted electrical tape on his chainstay was necessary, as "[he] gets so much chain slap [he's] literally snapped frames".

Now there's a couple dubious aspects to this--but my question is simply: can chain slap result in structural damage?
YES and NO

Depending on the type of frame and type of riding you are doing it could be a problem. 

YES (you might have a problem)


If you have a steel frame slapping of the chain can damage the paint which is very important in keeping oxidants like water and air away from the bare steel. Steel will rust and eventually fail catastrophically.
If you have a composite mountain bike frame, chain slap can damage the outer layers of fiber which generally carry the most load. Nicks made by the chain in the frame's resin could be propagation sites for cracks that can lead to delaminating and failure. If you are missing or have worn through the factory included sticker guards, I would apply some type of wear guard.


NO (It won't be a problem)


If you have aluminum, stainless steel or another relatively non-corrosive metallic frame, no amount of beating with a chain is going to cause the thing to break. Aluminum and stainless steel are ductile materials and are very hard to damage without hitting them very hard with a large mass or by using some type of lever to bend them.


Probably No...


If you have a composite road bike, chain slap can damage the outer layers of fiber or could cause cracks in the frame as well, however the likelyhood of this happening is very small. Composite frame manufacturers generally include one ply of weave fiber on the lower chain stays to increase the durability along with a plastic sticker which should be more than adequate for many years of riding. 


For what it’s worth I have &lt;14 lbm composite road bike and I would never think of adding electrical tape (I’m counting grams here) for something as silly as chain slap.