I remember some lugged titanium bikes back in the late 80’s. They did not have external lugs like the classic steel frames, but had internal lugs which were somehow bonded to the tubes. I suspect that that these were early attempts to build a titanium frame by companies without the equipment to weld their frames. I remember Fuji selling one. Am I getting old?
"the weak point, a tube will bend or buckle before the joint will fail
Having broken 4 frames in my younger years ,( dirt biking road bikes) they all failed at the joint line mostly bb seat tube & one head to down tube crack, But they were cheap tubing and brazed, not silver solder/brazing joints. It seems the stress is at the step from tube to edge of lug. I don’t know if you can duplicate bike frame loads on a test bench, maybe if your at a manufacture test lab, they seem to just do cycle units for / fiber / metal fatigue tests . I would like to build a metal lug -carbon tube glue job frame like the old specialized bikes. there should be a kit,