How to detect metal fatigue?

Giraffoto

The article on buying second hand bikes makes the sweeping statement that

With aluminium components under normal conditions, a maximum of five years’ use should be allowed for parts under stress, particularly the bar and stem. Under racing conditions, shorten that to two

Leaving aside that I got twenty five years out of an aluminium bar and stem, this did get me thinking. How would I be able to tell that metal fatigue has finally overtaken my (seven year old) handlebar or stem? Do fatigued items go gradually or catastrophically?

ai_1

Fatigue results in propagation of cracks from stress points. Once a crack has grown enough catastrophic failure can result. However 5 and 2 years seem fairly conservative figures to me. My bike's a 2009 aluminium frame. I have a replacement on the way but I don't intend to stop using the current one despite the fact that the frame, brake levers, cranks, etc are all original!
It shouldn't really be given in terms of years of age but rather hours of use anyway.

bill57

Load of bo**ocks. Most Boeing 747's have a service life of 25-30 years. Their daily use includes takeoff, landing and constant pressurization/depressurization - bit tougher than a bike race?

monkimark

How does the aluminium know if it's being raced or not?

desweller

bill57 wrote:

Load of bo**ocks. Most Boeing 747's have a service life of 25-30 years. Their daily use includes takeoff, landing and constant pressurization/depressurization - bit tougher than a bike race?

Er, apart from the rigorous crack detection schedule and frequent replacement of components.

Anyway, the fatigue life of aluminium parts depends entirely on how stressed they are in use.