>>1028823>hurr durr aluminum fatigue life is shitAlright this needs to fucking die.
Materials like steel and titanium have what is called a fatigue limit - if the applied stress/load is below that threshold, you can cyclically load the part an infinite amount of times without the part failing due to fatigue. Aluminum does not have a fatigue limit - even at low loads, the part will *eventually* fatigue.
However, no steel bicycle frame is loaded so lightly that it's within the fatigue limit of the material. ALL STEEL BIKE FRAMES WILL EVENTUALLY FATIGUE AND CRACK IF RIDEN LONG ENOUGH. Fact of life.
>But look at that graph you posted the aluminum line is below the steel line hurr durr durrThat's a graph of # of cycles vs. stress, not vs. load. Stress is proportional to area. Aluminum is roughly 1/3 as dense as steel, but also has roughly 1/3 the elastic modulus, so in order to build a frame that's not as floppy as a wet noodle, larger diameter tubes and thicker tube walls are used. More cross sectional area = less stress for a given load! Most aluminum bike frames have EXCELLENT fatigue properties; you'll really only see failures on shit frames (at the welds, usually).
See
http://www.sheldonbrown.com/rinard/frame_fatigue_test.htm for some real world frame fatigue testing.
>>1028847>continue to weaken until it's basically butter.Garbage. Fatigue does not change the stiffness/elastic modulus of a material in the slightest. Fatigue failure is due to microcracks forming and propagating until failure.
>>1028861>>1028884>>1028885>>10288916000 series aluminum is characterized by relatively high amounts of silicon and magnesium. 7000 series aluminum has high % of zinc in the alloy. All alloys in each family are pretty much the same; subtly different alloys for different purposes.
