Originally Posted by mtbikerinpa

THere may have ben some element of truth to the core of the statement.
The process that makes a frame crack is not in fact a weakening but a hardening. 'Work hardening' is the techincal term. The more a metal is cycled, or pounded the harder it gets. The side effect of this of course is brittleness, the tendency to crack rather than bend. The bigger problem is that this effect concentrates at the points of highest stress, welds. This is why if it is a crack in the front triangle, I will treat it with major caution, unlike a rear brake mount or rack mount(which I reattach). When something large and uniform such as a top tube weld begins to go, it is likely that the rest of the triangle is not far off.
In aviation, where we use aluminum extensively, we will sometimes counteract this effect by 'anealing' the metal as a re-heat treatment. This process will bring the frame to a uniform strength, essentially what it was mint.