For such a small part of a frame, the little dangly derailleur hanger is involved in an inordinate number of repairs. It is a small piece of material, but it is vitally important, unfortunately located, and relatively fragile.
Most classic frames and some modern custom frames built for confident ballers have contiguous or "fixed" derailleur hangers that are part of the dropout itself. Because it is so vital and fragile, modern mass produced bikes generally have replaceable hangers that bolt on with smalls screws, allowing a failure mechanism that in most cases has no potential to ruin the entire frame. Regardless of the main frame material, these replaceable hangers are primarily aluminum or titanium.
A sensible and practical design feature, replaceable hangers can still be poorly executed. Photos below show one such design; a replaceable hanger made of steel on a high end titanium frame where the mounting holes on the dropout leave precious little material around the edge and have a countersink radius that cuts right through the heat affected zone of the chainstay weld.
The latter situation resolved itself in a crack through the weld, even though the hanger was ruined at the same time of the source impact. We can easily repair the crack, but the original flaws in the design still remain and would surely cause the same result in another crash.
As a solution Dave fabricated a new titanium hanger to match the interface and location of the original, and then welded it all together into a solid hanger far stronger than the original.
