8:26 PM

Fatigue

Picture 349
I popped this lever in to remove a tire today and without inserting much strength the thing just snapped in two.  I’m not sure if this applies directly to plastics but it got me thinking about how everything has a fatigue limit. 
 
fatigue_SN_01
Here’s a graph showing the number of cycles it takes for a certain material to fail at a certain amplitude of force.  ie. if you cycle Aluminum 100000 times at about 250 MN/m^2 of force, it will fail.   Now as basic knowledge goes, most materials like aluminum fail if used or cycled enough times at any amount of stress.  That’s why the graph for aluminum points downward as such.   What this means in theory is that if you use that Aluminum bike for 100 years or whatever a long time is to you, it could take the smallest of forces like you sitting on the saddle to crack the frame.  This is not to worry anyone who’s thinking of buying an Aluminum bike though, they’re designed with fatigue limits beyond what most people’s power and enthusiasm for cycling can reach. 

I must also point out two very special materials that defy this common rule of material mortality.  As it turns out, Steel and Titanium have specific stress amplitudes.  These can be seen in the graph above for 1045 steel where the line levels off at about 320 MN/m^2.  What this virtually means is that no matter how many cycles you put it though, as long as you don’t apply a force over that limit, the material will not fail.  This is indicative of engineer’s choice to use Steel or Titanium as high cycled parts in machines such as connecting rods for engines.  What this means for you bikers is if you have a titanium or steel bike, it will last forever unless you crash it or a really fat dude sits on it.