Originally Posted by
Matt Gonitzke
Kyle-
"The fatigue curves for steel appear to be headed towards a limiting value of about 40ksi. So under a 40ksi load, steel will never fail, regardless of the number of cycles." True, but if the structure had a stress higher than 40ksi, this would no longer be true. Both curves I posted have the same trend, but obviously different numbers because they are different materials.
"The fatigue curves for aluminum are still pretty steep at 10KSI. If you had the entire curve, it would limit out at zero, meaning that aluminum will eventually fail after only one fatigue cycle, with that cycle coming from the structure's own weight." You are misinterpreting the chart. The bottom edge of the vertical axis is 10ksi...there is no data down there, as the lowest run-out is at about 17ksi. One cycle is off the left end of the chart; notice it starts at 10^3 cycles. You have the entire curve...it does not 'limit out at zero', the numbers 'run out' below a certain stress level, meaning that the test specimen did not fail before the conclusion of the test.