Fatigue

[sandras]

So I'm reading about this material fatigue thingy. The idea is, that there's a limit (not for iron or titanium), of how many times you can apply a certain force to a piece of material, before it breaks. You can apply a very small amount of force very many times, or you can apply a very big force just a few times, and the piece of material will break. Do I understand this correctly? If so, how come our aluminum blocks in our civics just don't go cracking everywhere? I mean, an engine makes so many revolutions, and the forces aren't that shabby either. One thought I had, that maybe the heat from the combustion "resets" the fatigue? Thinking of annealing. But our engine run at like 90 degrees celsius, far too low for annealing. This fatigue thing makes me anxious. Help me understand. Thanks!

[Stubby79]

relax. Your engine isn't flexing. Not appreciably anyway. You need to apply enough force to make it flex, otherwise it isn't going to cause any real fatigue. Stress from uneven expansion of the metal if it heats and cools too rapidly is much more likely to damage it than the combustion pressures.

[sandras]

Flexing! Flexing is the key! Thanks. It must seems silly, but I thought that whispering it compliments a certain amount of times will cause it to crack.

[redpoint5]

Aluminum is interesting in that it gets stronger and stronger as it bends until it finally breaks. Most other metals are opposite, getting weaker and weaker as it bends until it breaks.