diplomech said:
I understand it's pretty rare to get an instance when fatigue doesn't come into play because most things go through some sort of stress cycle...
Not at all, it all depends on the application. In my line of work, I very rarely have a design that requires fatigue anlysis. For "static" applications, I usually try keep things at about 50-75% of the material's yield strength (if not far less) just to be safe. For designs where every gram counts, I might go right up to the yield strength of the material; but this depends heavily on application and intended use, plus consequences if it fails... and of course deflection comes into play also. It could be you are not in a stress-limited application but rather a deflection limited application, where limiting deflection means stresses are negligeble.
Of course there are also applications where trying to minimize material is cost-prohibitive, as in it's easier to do two machining operations and call it good since you don't really care about weight much. It is VERY common for us to make our products out of Aluminum 6061 however. It's cheap, easy to machine, strong for its weight, and lots of suppliers have it readily available. "Default" designs for me will default on Al 6061, and when I need more strength, hardness, or other properties, then I may consider a stainless steel or perhaps brasses or ceramics.
diplomech said:
...but what if cycles were expected to be <10,000, and rarely at high stresses? I was reading on the wiki link that typical values for fatigue strength in steels are 0.5 UTS, so could I use that as a general rule of thumb for bog-standard designs that aren't safety critical?
It's better to design based on the material's yield strength, so that if it does deflect, it will at least return to its previous shape. Ultimate strength should only be used for predicting failure modes (e.g. it will probably break here first), rather than a design parameter for stress analysis IMO.
diplomech said:
More importantly, there must be a thought process/procedure engineers go through to determine what level of stress they will design something to operate in nominal conditions, OR a reference on factors of safety?
Depending on application of course, I would say that Yield Strength and Fatigue Strength are the two most commonly used material stress properties used in my line of work. Safety factors are largely a personal preference for my field, but safety-related applications like in Aerospace or Automotive will even have minimum safety factors called out I think.
diplomech said:
I'd be very interested to find out more about this, because often, you know the material you want to use, because of the wear properties, chemical stability, heat properties or how pretty the colours are etc. but not how to size it! Materials are getting more expensive and if there are ways I can reduce that cost and still be safe, by going down to the next diameter steel or something, then I'd really like to know how.
Cheers!
This is a mechanical engineer's bread and butter- designing parts and sizing components based on expected loads and cycles. Bolts, shafts, beams, etc. are all sized using basic stress analysis and deciding how big is "big enough."
If you're interested in learning more, you could buy a few textbooks on the topics of
https://www.amazon.com/dp/0534553974/?tag=pfamazon01-20 is a staple book for any mechanical engineer looking for quick answers to stresses in simple (or not-so-simple) geometries.