how will this frame fail?
I've recently found a bubble/ripple in my head tube, just below the upper lug. It's accompanied by a slight bend in the headtube. Picture below. It's 531 from 1979 or so. There are no other bubbles/ripples/bends/cracks or any other sign of damage to any other tube.
I took this to two different shops. The mechanic at one shop told me that the tube would eventually crack, ruining the frame, but the failure would not be catastrophic in a crash-causing way. The mechanic at the second shop took one look at the bike and told me that the bubble has "nullified the strength of the lug" so that the top tube will completely separate from the head tube at any moment, pitching me onto my face. He told me I should not even ride the bike home. I asked him for some clarification, but couldn't find out anything more what he meant (the shop was near closing, and I think he didn't want to have a longer conversation). I don't really understand how the bend in the head tube would have "nullified" the lug - what would that mean?
I have no doubt that this tube will continue to fail, but my question is, what should I expect as it does? A sudden catastrophic failure in which the tubes separate, or a gradual bending and cracking?
Thanks for any insight you can give!