How many years an aluminium frame lasts in a touring bike? How long does an aluminium frame lasts in a touring bike?

[Removed extra questions to be clear.]



Background

So far I preferred aluminium vs. steel for many reasons, but if steel lasts significantly longer, for my next touring bike I will go for steel.

My Cannondale Badboy 2004 has a crack in the alu-frame. I was surprised by this, I thought my bike will last 20+ years easily. The workshop they said this is a high quality, strong frame, but after 10 years alu frames can get tired. Especially if I use my bike both in winter and summer, as the heat change wear alu faster. They say it can be welded, which will cost significantly less vs. a new frame (if it is available), but I should be prepared for other cracks to appear. The workshop contracts a welder specialized in bike frame welding and repair.

I also found a scientific research article on this topic. If someone has access to ScienceDirect (Cicero et al. (2011): Analysis of the cracking causes in an aluminium alloy bike frame), I'd be interested in the conclusions.

I'm interested in touring bikes, not road bikes, city bikes, etc.

I'm happy to update/clarify my question if needed, make a comment.



Topics I have researched:


Maintenance on an Aluminium frame
Which type of frame is better for a touring bike, steel or aluminium?
Is it possible to repair a dent on an aluminium frame?
What are the pros and cons between an aluminium and carbon frame?
Is torque relevant for steel/aluminium? Where do I find torque specs?
non-steel frames and longevity
http://forums.mtbr.com/frame-building/aluminum-welding-cracked-frame-134600.html
http://singletrackworld.com/forum/topic/repairing-a-cracked-frame


Crack in the alu frame near the rear axe (the bike is upside down):
There are a lot of question so I will settle on the one in the title.  

How many years will an current aluminum frame last of a touring bike?  

Depends:


Don't know what aluminum frame
Construction is a larger factor than material 
Don't know the use
Use is a larger factor than material
Don't know how you are going to care for the bike
Care/maintenance is larger factor than material 


Aluminum has not changed in the 10 years 

All we know is your last aluminum, high quality, strong frame bike lasted about 10 years   

The best guess is your next aluminum, high quality, strong frame will also last 10 years
Really that is the best guess  

That is as vague as how long will a current pair of sneakers last me?
And the best guess for how long my next pair of sneakers will last is how long did my last pair last.  

Aluminum fatigues and steel does not fatigue. 
As stated in a comment steel does fatigue - well not always. Mild steel will not normally admit fatigue crack growth if the applied stresses are below about 10% of the strength of the material. Materials, such as aluminium alloys, do not have any such fatigue limit. If a cyclic load is applied, aluminium alloys will always fatigue.  For the same strength a steel bike is typically going to be heavier than aluminum.  But you get a bike that is much much less susceptible to fatigue.  With steel you also get a bike that will take a ding (dent) and not compromise structural strength to the extent of aluminum. Not exactly the same thing but at the molecular level they are related. Frame Materials 

Found a quoted number.  Just because it was on the Internet does not make it right but here is a 5-10 year quote.
Bike Frame: Aluminum Vs. Steel 


  Aluminum frames possess the shortest fatigue life of any material used
  to manufacture bicycle frames. The typical aluminum frame possesses a
  life expectancy of five to 10 years. The fatigue life of steel is much
  longer, but the material requires more maintenance. To prevent rust
  formation steel frames must be cleaned and polished regularly and
  periodically coated with rust stop on the interior of the frame.


If you are discounting steel because it will rust then you are not caring for the bike properly.  A properly maintained and stored steel bike should not rust.  And for sure it should not rust out in your life time.   You stated bikes from you fathers era lasted 40-50 years - pretty sure they were steel bikes.

If you want fatigue free and rust free then get titanium.