How does it break?

[Maker]

I was just wondering how different metals break at the weld.
Take a bike frame for example, made out of mild steel, if a joint failed, would it just snap with no warning or crack then snap or just bend and would it be the weld or the tube that breaks? What about different materials like Cro-Mo, SS or alu, how do they fail?

 

[hotrodder]

Very generally crashing (massive overload)/design flaws aside things virtually always begin with fatigue cracks. http://books.google.co.uk/books?engineer to win should go to the right chapter (pg 101) which is a good passage on fatigue
With metals generally stronger flavours = harder = less ductile and more sensitive to stress risers. In the bike frame example stronger typically = less material as the goal is typically weight saving rather than higher strength. With thinner wall tube the crack doesn't have to grow as far before there isn't enough material left intact to survive the next stress cycle and it fails catastrophically.

With a bike frame what happens then depends some on which tube/joint has let go and what's happening at the time... as a youf i turned a 531 (similar/same carbon manganese steel as T45) road frame into a boomerang when a chainstay let go while climbing a hill out of the saddle. IIRC the chainstay didn't snap, it just collapsed when the crack grew large enough followed by the rest of the rear triangle. If it'd been say an ally MTB with a crack near the downtube/headtube joint then ripping the entire front end off is more likely. Virtually always the majority of the break will occur in the tube even if the origin of the crack was say a weld crater. The crack is gonna follow the path of least resistance so might follow the weld toes for a bit (undercut) or even venture out into the HAZ. Lugged and brazed or fillet brazed steel frames sometimes tend to go even further out from the joint usually when whoever fabricated it overheated things waaaaay too much and cooked the tube well past the butted ends. Fatigue always finds the weakest link in the chain
SS used for bike frames tend to be higher strength than 'basic' chromoly, Reynolds 953 for example which is a martensitic aging (maraging) steel and uses insanely thin wall tube. Dunno about 953 off the top of my head but generally the various flavours of steel used all tend to have higher % elongation (there are some exceptions) than the usual aluminium suspects so can be stretched (bent) further before letting go completely