Automotive sheet metal stretcher strain defects

Historical case (pre-1960) of nitrogen-induced Lüders strains creating visible bands on painted automotive sheet metal. Resolved by improved steelmaking that removes nitrogen.

Old mild steel is the answer. In the old days, before 1960 let's say, when we didn't know how to control the steel chemistry so well, we had a lot of nitrogen in the steels, and we would end up with something called the Lüders strain. If you did a tensile test on mild steel, you'd have the upper yield point, the lower yield point, and the Lüders strain — which would actually be a bunch of hash marks oscillating, but it's basically a constant strain. Then it would start work hardening.